Engineering Landfill Liners for SDGs 6, 11, and 13: Comparative Consolidation Performance of Bentonite–Sand, Bentonite–Fly Ash, and Bentonite–Marble Dust Mixtures

Engineering Landfill Liners for SDGs 6, 11, and 13: Comparative Consolidation Performance of Bentonite–Sand, Bentonite–Fly Ash, and Bentonite–Marble Dust Mixtures | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Engineering Landfill Liners for SDGs 6, 11, and 13: Comparative Consolidation Performance of Bentonite–Sand, Bentonite–Fly Ash, and Bentonite–Marble Dust Mixtures Ankush Kumar Jain This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8630963/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 10 You are reading this latest preprint version Abstract Adhering to the Sustainable Development Goals (SDGs) for clean water and sanitation (SDG 6), sustainable cities and communities (SDG 11) and climate action (SDG 13) calls for landfill containment systems that are environmentally robust and resource-effective. Traditional B–S liners are highly dependent on natural sand (which is a limited resource) and contribute to the environmental footprint of waste disposal infrastructure. The compaction characteristics of B–FA and B–MD mix are also evaluated in the present work as an environmentally friendly alternative low hydraulic conductivity material to the traditional B–S liner, since it encourages circular use of materials leading to lesser resource extraction. For the purpose of simulating the effects of loading in a landfill setting, laboratory consolidation tests were performed on the B–S, B-FA and B-MD mixtures with different bentonite contents between 0 and 40% at intervals of 5%. Key consolidation parameters such as compression index, swelling index, coefficient of consolidation and settlement behaviour were evaluated on a systematic basis to assess the compressibility, stability and suitability for long-term use of each liner system. The results show that industrial wastes including FA and MD have a significant potential to improve the consolidation of bentonite-based liners and, therefore, achieve the same or superior consolidation parameters as traditional sand mixtures but with less use of virgin materials. The comparison demonstrates preferred mixes which promote integrity, controlled settling and low yield that afford for liner reliability. The inclusion of industrial waste materials in engineered landfill liners serves for groundwater protection and leachate containment (SDG 6), contributes to sustainable urban waste management infrastructure (SDG 11) and material engineering adapted for climate through waste valorization with lower GHG emission values towards carbon intensity reduction (SDG 13). These results offer a defensible basis on which to develop sustainable EBSs in support of the Sustainable Development goals. Landfill Liners Consolidation Compression index Swelling index Coefficient of consolidation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. INTRODUCTION Landfill remains the most common approach to disposal of municipal solid waste (MSW) and industrial by-products worldwide, especially in developing countries [ 1 ]. Although various practices of solid waste management, such as minimization, recycling and energy recovery have been developed in recent years the amount of solid waste has increased with urbanization, population growth and industrial processes [ 1 – 2 ]. Engineered landfill liners are an essential strategy in providing containment systems to protect the environment against contamination of leachate percolation and gas migration [ 3 ]. These liners serve as hydraulic insulating layers preventing the direct contact between landfills and ground water-soil system, reducing the release of toxic trace metals, dissolved organic matter and other pollutants to the natural environment [ 2 , 4 ]. Previously, compacted clay liners (CCL) and geosynthetic clay liners (GCL) have been used in singles or combinations so as to comply with performance regulations. Key properties of liner materials are very low hydraulic conductivity (commonly ≤ 1 × 10⁻⁹ m/s), adequate shear strength to support the waste, and sufficient resistance to chemical attack [ 2 , 3 , 5 ]. The bentonite is preferred mainly for its excellent swelling capability, high cation exchange capacity and the extremely low permeability it can achieve. Bentonite–sand mixtures are commonly used as low cost liners, where the benefits of bentonite and the structural stability provided by granular materials are combined in a specified proportion by volume. However, the widespread use of bentonite–sand liners made with natural sand is not sustainable. Extraction of sand disrupts the functioning of a river ecosystem, as well as depleting the natural resource itself and incurring other regulatory constraints. At the same time a plenty of byproduct like fly ash from thermal power plant and marble dust from stone processing industries are laid waste, causing serious environmental problem [ 3 , 4 , 6 ]. Recovery of these industrial wastes for valorising them into value-added compounds and land-fill liner materials adds to the dual handicap of waste generation – minimization in natural resource exploitation and sustainable management of waste [ 5 ]. The function of a landfill liner is not only as an impermeable barrier but also as a long-term consolidation behaviour under the waste contents. Stresses rise steadily and progressively due to overburden pressures, decay of the waste mass, as well as from settlement in the layers of waste [ 3 – 4 ]. These loads induce volumetric reduction, time-dependent compression and possibly hydraulic conductivity alteration in the liner materials [ 6 – 8 ]. Consolidation testings offer important parameters (compression index Cc, swelling index Cs and coefficient of consolidation Cv) for compressibility, rebound potential and rate of pore water dissipation respectively. High compressibility can cause excessive settlements, which affects the structural safety of structures and low Cv value will delay pore pressure degradation and hence hydraulic stability [ 7 – 9 ]. Therefore, the consolidation behaviour for bentonite–sand and its sustainable alternatives is important in predicting liner performance over decadal operation lifetime of landfills [ 7 , 9 – 10 ]. Fly ash (FA) is a finely divided waste material produced through burning of coal in thermal power plants, and is readily available in India which has one of the largest number of thermal power generating units. FA has pozzolanic nature, medium plasticity (LL = 38%) and low density being MDD = 1.49 g/cc. Its silt-sized particles promote packing density and alkaline character (with approximately pH 8.8) supports chemical resistance [ 8 ]. During mixing with bentonite, FA could be effective in diminishing of excess swelling and enhancing strength and workability but the impact on consolidation properties has not been fully understood. Marble dust (MD), a waste material produced in large quantity from the industries of cutting, polishing and finishing of marble [ 9 ]. It is environmentally difficult to dispose of, because it can block waterways and generate particulate air pollution. Geotechnically, MD has low plasticity (LL = 17.2%), a high specific gravity, and good frictional resistance (ϕ = 25.5°). The addition of MD to chemical prehydrated bentonite liners can help counter the high compressibility of bentonite, minimize shrink–swell behaviour, and enhance beneficial volumetric stability. In addition, MD has to be chemically inert and hard [ 8 – 10 ]. By partially replacing sand with FA or MD, bentonite liners can be engineered to achieve desired mechanical and hydraulic properties while simultaneously diverting industrial waste streams from land disposal [ 11 ]. There is quite a substantial amount of research on permeability and shear strength of bentonite–sand mixtures [ 12 ]. Fly ash and marble dust have been used as geotechnical stabilizers for expansive soils, road subgrade and embankments in several other investigations [ 12 – 14 ]. However, little research has systematically studied the consolidation of bentonite–FA and bentonite–MD mixtures with a comparative perspective to its behavior in comparison with that of bentonite–sand systems. The majority of previous studies concentrate on either permeability or strength parameters and most of the time consolidation is treated as a secondary concern [ 15 ]. However, with regards to landfill design, consolidation controls the long-term deformation of differential settlement and the development of cracking or hydraulic short circuits in liners. Furthermore, the environmental impacts of FA and MD addition have not been systematically framed in relation to sustainable use of natural sand and circular economy for waste treatment purposes. In the present study, the aims are developed for addressing current limitations in designs of sustainable landfill liners. The objectives of the study are to investigate the consolidation behaviors, such as the compression index, swelling index, coefficient of consolidation, and settlement behavior – of bentonite–fly ash (FA) and bentonite–marble dust (MD) mixtures compare with traditional bentonite–sand systems. Moreover, it is intended to evaluate the effect of these waste additives on the compressibility and structural stability of bentonite liners at different stress conditions simulating landfill loading. Another goal is to evaluate the possible enhancement of sustainable development, including resource saving and waste recycling, if sand is substituted by FA or MD in bentonite liners. Finally, the research aims to discover the most viable bentonite-based mixture: bentonite–sand, bentonite–FA and bentonite-MD from both consolidation performance and environmental aspects. 2. METHODOLOGY 2.1 Methodological Framework The experimental procedure had been made to satisfy the stated objectives in an organized method. Bentonite, FA, MD and sand were oven-dried at 105 ± 5 ° C and crushed by pulverization to break down lumps followed by sieving for size grading [ 16 ]. Initially, the geotechnical properties of these parent material is identified through laboratory test, which is reported in Table 1 . Table 1 Geotechnical characteristics of Bentonite, Fly Ash, Marble Dust, and Sand Description Sand Marble Dust Bentonite Fly ash IS-CODE Sand (4.75 − 0.075 mm) % 98.00 95.50 6.00 93.50 [ 17 ] Silt (0.075 − 0.002 mm) % 2.00 4.50 14.00 6.50 Clay (< 0.002 mm) % - - 80.00 - Soil Classification as ISC and USCS SP (Poorly graded sand) Poorly graded Clay with high plasticity Liquid Limit % 27.20 17.20 202.00 38.00 [ 18 ] Plastic Limit % - - 70.01 - [ 19 ] Shrinkage Limit % - - 9.11 - [ 19 ] Plasticity Index % 27.20 17.20 131.99 38.00 Max. Dry Density (MDD) gm/cc 1.52 1.74 1.14 1.49 [ 20 ] Optimum Water Content (OWC) % 14.76 16.00 48.00 23.12 According to these materials, three mixtures were prepared; bentonite–sand (B–S), bentonite-fly ash (B-FA) and bentonite-marble dust (B-MD). For each mixture, different mixes were formulated with additive to bentonite proportions from 0 to 40% in 5% increments, as shown in Table 2 . Mixing took place in a dry state and water was then added incrementally to OWC (optimum water content)/MDD (maximum dry density) as measured by Standard Proctor compaction test. One-dimensional consolidation tests were performed on the prepared mixes as well, in order to assess their compressibility behavior. Samples were statically compacted into 60 mm-diameter and 20-mm-tall consolidation rings at MDD and OWC. Consolidation test were performed according to the provisions of IS 2720 (Part 15): 1986 [ 21 ]. The vertical stress was loaded step by step (100, 400, and 800 kPa) and dial gauge readings were taken at logarithmic times of load application until a consolidation to the extent of about 90–95% was obtained. Table 2 Dry Weight of Mix Proportion of Bentonite -Fly ash (B-FA), Bentonite –Sand (B-S) and Bentonite- Marble Dust (B-MD) S. No. Mix Proportion (B-FA) Dry Weight Mix Proportion (B-MD) Dry Weight Mix Proportion (B-S) Dry Weight 1 100FA+0B 100MD+0B 100S+0B 2 95FA+5B 95MD+5B 95S+5B 3 90FA+10B 90MD+10B 90S+10B 4 85FA+15B 85MD+15B 85S+15B 5 80FA+20B 80MD+20B 80S+20B 6 75FA+25B 75MD+25B 75S+25B 7 70FA+30B 70MD+30B 70S+30B 8 65FA+35B 65MD+35B 65S+35B 9 60FA+40B 60MD+40B 60S+40B The adhesion performance and swelling behavior were assessed by load–unload cycles. The compression index (Cc), swelling index (Cs), coefficient of consolidation (Cv), and settlement behavior were obtained from the test data. With these settings, the consolidation behaviour of B–FA and B–MD mixtures could be compared with that of the reference conventional B–S liner [ 22 ]. To further assess the influence of FA and MD on the structural stability of bentonite-based liners under landfill loading, additional high-pressure consolidation tests were carried out up to 800 kPa. This simulated the stress conditions likely to develop in deep landfills. Repeated loading and unloading cycles were performed to study compressibility under cyclic stresses, recompression ratios, and stiffness. Stress–strain curves and compressibility coefficients (mv) were derived at each pressure stage to assess the role of FA and MD in improving stability and reducing settlement compared to sand. 2.2 EDAX and SEM analysis of Parent Material (B, S, FA & MD) Table 3 presents the atomic percentage composition of major chemical elements in bentonite, sand, fly ash, and marble dust. Silicon (Si) and oxygen (O) dominate the elemental composition of all materials, with particularly high concentrations observed in sand and bentonite, confirming their role as the primary constituents. Aluminum (Al) is present in significant amounts in bentonite and sand, whereas iron (Fe) occurs in relatively low concentrations across all materials, with slightly higher proportions in fly ash and sand [ 23 – 25 ]. Magnesium (Mg) and calcium (Ca) are highly concentrated in marble dust, highlighting their importance in defining its chemical composition. Fly ash exhibits a comparatively high carbon (C) content, suggesting the presence of carbon-rich or residual organic compounds. Chlorine (Cl) is most prominent in bentonite, while sulfur (S) and potassium (K) are detected in trace quantities across all materials, contributing to their distinct chemical characteristics Table 3 EDAX analysis of the parent sample's bentonite, fly ash, marble dust, and sand [ 36 ] Chemical Atomic % (Bentonite) Atomic % (Sand) Atomic % (Fly Ash) Atomic % (Marble Dust) Si 36.3 40.94 16.42 2.33 O 28.11 31.9 19.21 23.06 Al 20.04 13.57 17.7 0 Fe 3.34 3.9 3.47 0.15 Mg 1.76 2.74 2.34 10.2 Ca 0 3.57 13.32 14.51 Ti 0.98 0.47 0.88 0 C 0 0 25.65 49.7 K 0.67 1.69 0.03 0 S 1.45 0.37 0.28 0.06 Cl 2.1 0.1 0.1 0 Na 5.24 0.74 0.62 0 Further, Fig. 01 presented the FESEM analysis of the parent samples. The SEM image of bentonite [Fig. 01 (a)] reveals a characteristic platy and flaky microstructure composed of aggregated lamellar particles. The particles seem to be non-spherical in shape and having different particle size distribution; a compact of high density matrix with micro-voids and pores is also observed. These pores are responsible for the high surface area and swelling behavior of bentonites [ 26 ]. Layered organization of platelets testifies to the presence of montmorillonite minerals with high plasticity and absorption properties [ 23 – 26 ]. The occurrence of agglomeration indicates that strong interparticle bonding is involved in the compaction, compressibility and hydraulic behavior obtained for bentonite-based liner materials. Fig SEM image of fly ash [Fig. 01 (b)] displays overall spherical shape with particles of different sizes as typical coal fly ash. Many of the particles are smooth walled, hollow or solid spheres (cenospheres and plerospheres) suggesting quick cooling upon formation. The spherical form would also provide better flowability and packing density of the fly ash in mixing with other materials. Some of the particles demonstrate porous surfaces, agglomeration indicating unburnt carbon and finer ash fractions. This heterogeneous microstructure in fly ash improves its pozzolanic reactivity and affects the mechanical properties, permeability, and strength of fly ash utilized in geotechnical engineering and construction applications. SEM view of marble dust is shown in Fig. 1 (c) shows that particles are angular and irregular, having sharp edges and rough surfaces. The particles look dense without clear apparent round features and this is consistent with the mechanical grinding in cutting and polishing the marble. Fine particles occupy the voids between larger grains, which results in denser microstructure [ 28 ]. The coarse and angular shapes promote friction force/bonding between the particles of marble dust while mixing with other geomaterials. The close compactness and low visual porosity are indicative of a less formal compressive potential, whereas the highly enriched calcium in marble dust allows it to serve as a stabilizing filler in geologic and lining applications. The SEM image of sand [Fig. 01 (d)] shows angular to sub-angular particle of random shape and having well definitional edges. The grains show relatively smooth surfaces and unevenness on the surface area, which are probably caused by erosion due to weathering action, and mechanical grinding action. The size distributions of particles looks mostly uniform and results in a substantial stable aggregate structure. Existence of intergranular voids indicates that sand is highly porous and permeable [ 29 ]. The particles are hard but too smooth to be cohesive, and the absence of any magnetic traction or direct chemical attraction makes sand mechanically stable, yet still porous. This microstructure arrangement heavily influences the hydraulic and bearing behavior of sands as known in geotechnical application. 3. RESULT AND ANALYSIS 3.1 Assessing the Compression Index (C c ) for the B-S, B-MD & B-FA mixture The values of the compression index (Cc) obtained for B–S, B–MD and B–FA show a consistent trend of increase in compressibility with increasing bentonite content as presented in Fig. 2 . The Cc values for 0% bentonite are comparably very low (B–S:0.08, B–MD: 0.09 and B–FA: 0.07), indicating the presence of higher proportion of coarser fractions such as sand/ marble dust/fly ash, which hinder compressibility. With increased level of incorporation of bentonite, the values increase monotonically up to a maximum value for all three mixes at 40% bentonite. B–MD displays however somewhat higher Cc values with respect to B–S and B–FA, especially at the intermediate bentonite contents. The relatively higher values for B–MD (e.g., 0.63 at 40% bentonite as compared with the corresponding values of 0.61 for B–S and 0.57 for B–FA) may be due to the irregular shape of particles and close particle size distribution in marble dust leading to packing density but also facilitates localized deformation with respect to loading [ 30 ]. In general, the implication is that the addition of FA and MD alters the compressibility characteristics of bentonite covers. FA is the most efficient to decrease compressibility that results in structurally stable and cost-effective use of B–FA mixtures as land fill material. However, MD (and even more Cc) still shows acceptable behaviour and can contribute to sustainable use of waste material [ 30 – 31 ]. Accordingly, FA- and MD-mixed bentonite liners present technically superior and environmentally favorable options to traditional formed sand covers. 3.2 Assessing the Swelling Index (Cs) for the B-S, B-MD & B-FA mixture The swelling index (Cs) is a key parameter to understand soil behaviour in landfills. It’s a way of knowing how much they can swell up and soak up water in varying stress conditions. Figure 3 indicates Cs to be enhanced as the amount of bentonite increases, indicating that this is in line with the expansion ability of bentonite. For 0% bentonite, Cs values are low (0.009–0.011), these correspond to the weak swelling capacity of such materials: sand, marble dust and fly ash. With the increment of bentonite, Cs gradually goes up: 0.0180.019 (10%), 0.0320.034 (20%) and 0.072 ~ 0.076 (40%) [ 31 – 32 ]. Amongst the three blends, B-MD has, in general, higher Cs values and is followed closely by B-S. B-FA has slightly lower values. This indicates that the marble dust enhances the swelling capacity of bentonite particles5 as its finer particles show better interaction with the clay minerals which offer more interlayers for water to be absorbed. Sand, being more coarse and chemically inert, is little affected by swelling; however its volume can increase markedly by the porosity of bentonite. Fly ash, on the other hand, inhibits swelling slightly because its round glassy particles fill voids and retard water migration. 3.3 Assessing the Coefficient of Consolidation for B–S, B–MD, and B–FA Mixes Coefficient of consolidation (Cv) is the rate by which excess pore water pressure dissipates under load. It is a key parameter in assessing the compressibility and permeability of soil-bentonite blends. Experimental data as per Fig. 4 indicates that, irrespective of the mixes (B-S, B-MD and B-FA), an increase in bentonite content results in a reduction of Cv. This implies decrease in drainage ability and slower consolidation rate with the increase of bentonite. These special values are 15.2 ×10⁻⁷ m²/s in B-S, 16 ×10⁻⁷ m²/s in B-MD and 17.5 ×10⁻⁷ m²/s, at 100% of bentonite and at zero percent of it. These large values are attributed to the infiltration of coarser particles (sand, marble dust and fly ash) that can lead to a more rapid dissipation of pore water and accelerated settlement under load [ 32 – 33 ]. However, with the percentage of bentonite increasing up to 40%, Cv drops sharply to a range of 6.6–8.0×10⁻⁷ m²/s by illustration of the fine and plate-like structure which reduces permeability, hindering the consolidation process. Such slow consolidate behavior is advantage for liner and barrier because the desired seepage rate or water flow is restricted. In the case of mixtures, at each bentonite percentage its Cv values are highest for B-FA followed by B-MD and least to that of S-B. This trend indicates that the fly ash contributes to improve the microstructure because it results in more interconnected pores, which facilitates good drainage and rapid reduction of pore pressure [ 34 ]. Even marble dust, which is hard as rock and of low porosity with regard to sand or gravel, probably has a moderate drain: it would have passed off earlier. On the other hand, sand with bentonite gives the lowest Cv because of higher particle friction and less void connectivity. In conclusion, impermeability of the mixture is enhanced by rising bentonite content while slowing down the consolidation process. BN-FA combinations achieve a happy medium of moderate long-term permeability retention along with steady consolidation behaviour. This would have rendered them potentially useful for construction purposes, such as liners in the engineered landfill needing controlled drainage and stability [ 5 , 7 , 9 , 11 – 13 , 16 , 22 ]. 3.4 Assessing the settlement behaviour for B–S, B–MD, and B–FA Mixes under different stress conditions The consolidation parameters of bentonite–sand (B–S) mixtures subjected to three stress levels (100, 400 and 800 kPa) over a wide range of bentonite contents (0–40%) as presented in Fig. 5 . A clear and consistent trend is observed in the variation of compression index (Cc), swelling index (Cs) and coefficient of consolidation (Cv) with increasing bentonite percentage and applied stress. For all stress conditions, Cc increases markedly as bentonite content increases, indicating enhanced compressibility of the mixture. At 100 kPa, Cc increases from 0.08 for pure sand (0% bentonite) to 0.36 at 40% bentonite and further to 0.55 for pure bentonite. The effect becomes more pronounced at higher stresses; at 400 kPa, Cc rises from 0.14 (0%) to 0.68 (40%) and reaches 1.05 at 100% bentonite, while at 800 kPa it increases from 0.19 to 0.91 and finally to 1.40. This behavior is attributed to the increasing proportion of bentonite, which is rich in montmorillonite and exhibits high specific surface area and strong water adsorption capacity, resulting in greater volume reduction under load [ 32 – 33 ]. A similar increasing trend is observed for the swelling index (Cs). At 100 kPa, Cs increases from 0.01 (0%) to 0.043 (40%) and 0.063 (100%), whereas at 800 kPa it rises from 0.021 to 0.08 and 0.12, respectively. This indicates that bentonite-rich mixtures possess a higher tendency to rebound and swell upon unloading due to diffuse double-layer expansion. In contrast, Cv shows a decreasing trend with increasing bentonite content at all stress levels [ 33 – 35 ]. For instance, at 100 kPa, Cv decreases from 19 ×10⁻⁷ m²/s for 0% bentonite to 10 ×10⁻⁷ m²/s at 40% and 7.5 ×10⁻⁷ m²/s for pure bentonite. At 800 kPa, Cv further reduces to as low as 3 ×10⁻⁷ m²/s at 100% bentonite. This reduction reflects a decrease in permeability caused by pore clogging and swelling of bentonite particles, along with compression of drainage channels under higher stresses. Overall, the results indicate a transition from a sand-controlled, rapidly consolidating system to a clay-dominated, highly compressible and slow-draining system as bentonite content and stress increase [ 35 – 36 ]. The consolidation characteristics of bentonite–fly ash (B–FA) mixtures evaluated under three stress levels (100, 400 and 800 kPa) for bentonite contents ranging from 0% to 40% as shown in Fig. 6 . The compression index (Cc) shows a systematic increase with increasing bentonite percentage at all stress levels, indicating a progressive increase in compressibility of the B–FA mixture. At 100 kPa, Cc increases from 0.07 for 0% bentonite to 0.34 at 40% bentonite and further to 0.55 for pure bentonite. This trend becomes more pronounced under higher stresses; at 400 kPa, Cc increases from 0.12 (0%) to 0.61 (40%), while at 800 kPa it rises from 0.17 to 0.80. This behavior reflects the increasing influence of bentonite clay minerals, which exhibit high plasticity, large surface area and strong affinity for water, leading to greater compressive deformation. The swelling index (Cs) also increases consistently with bentonite content and applied stress. At 100 kPa, Cs rises from 0.009 for 0% bentonite to 0.041 at 40% bentonite, whereas at 800 kPa it increases from 0.019 to 0.073. This indicates that bentonite-rich B–FA mixtures have a higher potential for rebound and swelling upon unloading due to diffuse double-layer expansion [ 36 – 37 ]. In contrast, the coefficient of consolidation (Cv) decreases steadily with increasing bentonite content at all stress levels. At 100 kPa, Cv decreases from 20 ×10⁻⁷ m²/s for 0% bentonite to 11.5 ×10⁻⁷ m²/s at 40% and 7.8 ×10⁻⁷ m²/s for pure bentonite. Under 800 kPa stress, Cv further reduces to 5 ×10⁻⁷ m²/s at 40% bentonite and only 3.2 ×10⁻⁷ m²/s at 100% bentonite. This reduction is attributed to pore space filling by bentonite particles, reduced permeability, and compression of drainage paths under higher stresses. Compared to sand-based systems, the presence of fly ash contributes to finer particle packing; however, the dominant control on consolidation behavior remains the bentonite fraction, especially at higher percentages and stresses [ 35 – 37 ]. The consolidation parameters of the bentonite–marble dust (B–MD) mixtures show a clear and systematic variation with both applied stress level and bentonite content as shown in Fig. 7 . The compression index (Cc) increases significantly with increasing stress from 100 to 800 kPa for all bentonite percentages, indicating enhanced compressibility under higher effective stresses. For example, at 0% bentonite, Cc rises from 0.09 at 100 kPa to 0.20 at 800 kPa, while at 40% bentonite it increases from 0.40 to 0.93 over the same stress range. This trend becomes more pronounced with higher bentonite content due to the dominance of expansive clay minerals, which undergo greater structural rearrangement under load [ 36 – 38 ]. Similarly, the swelling index (Cs) also increases with both stress and bentonite percentage, reflecting higher rebound potential during unloading. Cs values rise from 0.011–0.023 at 0% bentonite to as high as 0.045–0.080 at 40% bentonite across stress levels, and further to 0.125 at 100% bentonite and 800 kPa. This behavior can be attributed to the high surface area and double-layer effects of bentonite, which enhance elastic deformation. In contrast, the coefficient of consolidation (Cv) shows a decreasing trend with increasing stress and bentonite content. For instance, Cv decreases from 18.8 ×10⁻⁷ m²/s at 100 kPa to 10.8 ×10⁻⁷ m²/s at 800 kPa for 0% bentonite, while for 40% bentonite it drops from 11.0 to 4.9 ×10⁻⁷ m²/s. At 100% bentonite, Cv further reduces to 2.9 ×10⁻⁷ m²/s at 800 kPa. The reduction in Cv is primarily due to decreased permeability caused by pore clogging and reduced drainage paths as fine bentonite particles increasingly dominate the soil matrix, slowing the rate of consolidation [ 38 – 40 ]. Settlement Behaviour under Different Stress Conditions The settlement characteristics of the B–S mixtures strongly depend on both bentonite percentage and applied stress level as reported in Fig. 8 . With the smallest pressure at 100 kPa, settlement is small and it increases overall monotonously with the content of bentonite, being 0.9 mm with no bentonite, raising to 4.3 mm at 40% bentonite or even up to − 6.5 mm for pure bentonite. This suggests that sand skeleton is still effective in bearing loads at low stress, which results in limited deformation as well [ 38 – 41 ]. Under 400 kPa pressure, however, the settlement is significantly greater (23.4 mm at 0% bentonite to 11.5 mm at 40% bentonite and then to 17.8 mm at 100% bentonite). The notch has a larger effect at 800 kPa, where settlement increases from 3.9 mm (0%) to 16.2 mm (40%) with the maximum of 26 mm for pure bentonite. Rapid increase in settlement beyond around 20–25% bentonite indicates when behavior shifts from sand-dominated consolidation to clay dominated. This behavior is attributed to the relatively higher values of Cc and lower values of Cv in bentonite-rich mixes, which lead to higher compressibility and slower excess pore water pressure dissipation. Therefore greater stresses produce both more settlement and settlement that varies with time to a greater extent. From a design point of view, these results illustrate the need for a trade-off in bentonite–sand mixtures – using higher contents of bentonite reduces permeability for application such as liners and barriers; however it also promotes settlement under moderate and intermediate stresses. It is necessary to select a proper amount of bentonite to achieve required low permeability, with the acceptable elastic deformation in practical geotechnical engineering application [ 7 – 11 , 13 – 15 ]. Fig. shows that the settlement response of the B–FA mixes is highly influenced by both the bentonite content and the applied stress level. 09. At 100 kPa the settlement gradually increases from 0.8 mm (0% bentonite) to 4.1 mm (40% bentonite) and is 6.1 mm for pure bentonite. This relatively moderate settlement under island stress implies that by adding fly ash to the slurry, there is a certain amount of partially rigid structure skeleton in the slurry system restrains deformation of mixed slurry when bentonite content corning down. Below 400 kPa stress, settlement is considerably higher and varies from the value of 2 − 1 mm at a content of bentonite of 0% to that of 10.5 mm and even higher values for bentonite contents above 40% reaching 16.2 mm in the case with content equal to 100%. For the highest confining stress of 800 kPa, settlement is pronounced and grows with increasing % bentonite from 3.5 mm (0% bentonite) to 15 mm at 40% bentonite and up to the maximum value of 24.5 mm for pure bentonite. The sharp change of the Σh0 values at about 20–25% bentonite content signifies the transition from fly ash–to–bentonite-dominant consolidation. the higher the "Cc" and lower "Cv" (i.e., HAC) is, the larger the compressibility of soil and slower excess pore pressure dissipating are [ 22 – 24 ]. From an engineer’s point of view, although the addition of bentonite enhances sealing and reduces permeability in B–FA mixes, it is at the expense of a dramatic increment in settlement under moderate to high pressures. Hence, there exists optimum bentonite content for practical applications such as liners and embankments so that the hydraulic properties can be balanced with deformation control [ 32 , 35 , 42 ]. Settlement of the B–MD mixtures follows closely with compressibility and consolidation trend observed in Fig. 09 . Settlement also increases with applied stress and bentonite content, due to high values of Cc and lower stiffness of the mixtures. 0% bentonite case, the settlement is from 0.9 mm at 100 kPa to 3.6 mm at a pressure of 800 kPa revealing relatively low compressibility with marble dust which behaves as filler and enhances the densification of particles [ 43 ]. However, high settlement occurs on the increasing of bentonite content. For instance, at 20% bentonite content, settlement increases from 2.2 mm (100 kPa) up to 9.1 mm (800 kPa), and for 40% bentonite content it will increase from 3.7 mm to13.5 mm. The maximum settlements are 6.2, 15.7, and 23 mm at pressures of 100, 400, and 800 kPa for the full bentonite content sample (i.e., > 90%), indicating a very high compressibility of neat bentonite. Block identity increased with stress and due to the collapsed soil fabric or expulsion of pore water and it increased with bentonite percentage as a result of its high plasticity and ability to attract water [ 40 – 42 ]. At lower bentonite contents, the marble dust limits settlement by classed grading and decrease in void ratio followed by saturation but at a clay content greater than that of one additional mixing stage, the behaviour is controlled by the clay fraction. In general, the findings have shown that at low and moderate bentonite contents of B–MD mixtures behave in a balanced manner (i.e., with controlled settlement and reasonable rates of consolidation) however, depending on their application (e.g. liners/barriers), high bentonite contents activate excessive settlement and slow down the rate at which consolidation occurs. 4. DISCUSSION The consolidation characteristics and sustainability potential of the B–S, B–FA, and B–MD mixtures will give comparative results that could help in elucidating their potential as landfill liner materials. The findings well prove that the dominant variable controlling compressibility, swelling ratio, voids ratio, rate of consolidation and settlement is bentonite content and it was found out that there had to be almost 1/3 bentonite while between 1/4% and %2 of sand required as modifier materials for system performance [ 41 – 43 ]. Trends on Cc for the CI alongwith bentonite content indicates that compression increases with increasing bentonite content, which highlights high water affinity and platy nature of its particles. Yet, the dpeak depends on the relative concentration of ionic liquids to water. Arguably, B–FA is always consistently at the bottom of compressibility factor and it is closely pursued by B–S whereas B–MD mostly exhibits higher values of C c [ 42 – 43 ]. The decreased compressibility of B–FA may be due to fly ash particles being almost spherical in shape, which enhance packing efficiency, reduce void ratios and hinder the rearrangement of clay platelets under loading. On the other hand, marble dust particles provide angular shape capable of densification under compaction but tend to form point-contact fabrics prone to breakage which resulted in marginally higher Cc. This latter distinction is essential, in that too soft a compression gives rise to line stability. Therefore,B–FA provides a distinct technical advantage in controlling settlement irrigation and is superior in this respect to B–MD which gives only limited benefit over the use of sand. Swelling index (Cs) also follows a similar trend of increasing with the addition of bentonite however, the different additives still produce distinct mitigating effect. B–FA swells the least, followed by B–S and B–MD produces the highest swelling. Stress point Physical and chemical There are physical and chemical reasons for the effectiveness of FA against edema. Its spherical aggregates fill inter-platelet voids, mitigating water ingress, and any pozzolanic reactions that may occur at longer ages could also inhibit expansive behavior. Inert calcite marble dust would not have the same effect in inhibiting the swelling of bentonite and its fines may even contribute to additional water filling, therefore increasing Ċs. From a design standpoint, reduced swelling is beneficial in order to limit cracking and preserve liner integrity, suggesting B–FA as the best option if volume stability is needed [ 38 – 40 ]. The coefficient of consolidation (Cv) is an indicator for predicting pore pressure dissipation due to loading. Cv reduces as the bentonite is replaced with increasing fractions of another, less reactive additive due to reduced permeability, however the relative ranking amongst additives is interesting: B–FA > B–MD > B–S. This means that the B–FA mix emulsions compact not only with more speed, but also they reach in a relatively shorter time their settlement equilibrium condition and there are less risk for deformation to late follow. FA mixtures have higher hot storage stability coefficient (Cv) due to its microstructural effect since they provide more continuous drainage paths while having a lower MDD. B–MD, however denser and less permeable, still has moderately higher Cv than sand because of granular interlock. Mixtures with sand have the lowest Cv suggesting slower consolidation and large long-term pore pressure at retention [ 33 , 41 ]. The faster consolidation is operationally relevant as it translates not only in construction time savings but also into reducing the risk of postponed structural adaptation. Stresses and settlements for such loading histories are influended by the interactions of compressibility and density. All combinations have settlements that increase with an increment in bentonite content, but B–MD has the lowest value followed by B–FA and then B–SA [ 11 , 33 , 38 ]. The better performance of marble dust was justified by its capability of developing high MDD resulting in packed aggregate morphology which contributes to compensate for its relatively higher value of CI. In application, B–MD is very favorable where restricting the absolute deformation is a concern of interest like case of liners under critical construction. The B–FA values are a bit higher as settled than the MD (but much lower, compared to sand), but it consolidates quicker supporting a trade-off. Once these agglomeration eects are considered in conjunction with sustainability factors, the evidence for FA and MD becomes more compelling still. The traditional sand composition is totally based on non-renewable natural resources, the use of which causes environmental destruction due to river mining, high energy and carbon [ 41 – 42 ]. However, FA and MD are copious industrial wastes whose recycling brings a double greening economy by both reducing natural sand consumption and waste disposal. In short, the communication reveals that although bentonite improves barrier properties its inherent drawbacks of high compressibility and swelling can be overcome by partial replacement of sand by fly ash or marble dust. B–FA possesses the best performance characteristics while presenting better compressibility, lower swelling behavior, rapid consolidation and high sustainability results. B–MD provides the least settlement and excellent sustainability which is desirable for cases where the deformation is considered as the major concern. Even though sand-containing bodies have been tried in the past, these techniques have still not come up with more effective approaches. Thus, the addition of FA and MD in bentonite liners is technically feasible, environmentally acceptable, and future oriented to landfill liner construction. 5. CONCLUSION Based on the detailed experimental results, analysis, and the integrated performance–sustainability assessment of bentonite–sand (B–S), bentonite–fly ash (B–FA), and bentonite–marble dust (B–MD) mixtures, the following major conclusions are drawn: 1. Effect of Bentonite on Compressibility and Swelling Bentonite content is the dominant factor controlling the consolidation behavior of all mixtures. As bentonite content increases from 0% to 100%, the compression index (Cc) increases significantly (e.g., from ~ 0.07–0.09 at 0% bentonite to ~ 1.35–1.40 at 800 kPa), while the swelling index (Cs) rises from very low values (~ 0.009–0.011) to about 0.15 for pure bentonite. This confirms that the highly plastic, expansive nature of bentonite governs compressibility and swelling, irrespective of the additive used. 2. Fly Ash Most Effective in Reducing Compressibility and Swelling Among the three systems, B–FA consistently exhibits the lowest Cc and Cs values at equivalent bentonite contents (e.g., at 30–40% bentonite, Cc ≈ 0.57–0.61 and Cs ≈ 0.072–0.073), compared to B–S and B–MD. This improved performance is attributed to the spherical morphology and pore-filling ability of fly ash, which restrict clay platelet rearrangement and water-induced expansion. As a result, B–FA mixtures provide better volume stability and reduced risk of excessive deformation. 3. Consolidation Rate Strongly Influenced by Additive Type The coefficient of consolidation (Cv) decreases with increasing bentonite content for all mixtures, falling to about 2.5–3.2 ×10⁻⁷ m²/s at 100% bentonite. However, at all bentonite percentages, B–FA shows the highest Cv (e.g., 17.5 ×10⁻⁷ m²/s at 0% bentonite and ~ 8.0 ×10⁻⁷ m²/s at 40%), followed by B–MD and then B–S. This indicates that B–FA consolidates faster and dissipates pore water pressure more efficiently, offering operational advantages such as shorter consolidation time and improved constructability. 4. Settlement Behaviour and Engineering Trade-Offs Settlement increases markedly with both applied stress and bentonite content. At 800 kPa and 100% bentonite, settlement reaches approximately 26 mm (B–S), 24.5 mm (B–FA), and 23 mm (B–MD). While B–MD generally shows the lowest absolute settlement due to higher maximum dry density and better particle interlocking, B–FA offers a balanced response with slightly higher settlement than MD but faster consolidation and lower compressibility. These results highlight the need for optimizing bentonite content (typically ≤ 30–40%) to balance low permeability with acceptable deformation performance. 5. Sustainability Performance Consequently, B–FA and B–MD mixtures not only meet engineering performance requirements for landfill liners but also support sustainable and environmentally responsible geotechnical infrastructure development. Abbreviation Bentonite : B, Sand: S, Flyash: FA, Marble dust: MD Declarations FUNDING STATEMENT Not received any funding to perform this study. Author Contribution The whole work means writing, experimentation and analysis done by the author References Akinsola JA (2022) Laboratory behavior of sand–bentonite mixtures. Eastern Mediterranean University Repository Alibrahim B, Garoushi AHB, Uygar E (2025) The role of calcium-based additives in bentonite stabilization: A comparative evaluation. 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Kumar R, Kumari S (2025) Geotechnical properties and microstructural analysis with desiccation cracking study of Ganga sand, bentonite and marble dust composites. Journal of Earth System Science , xx(x), xx–xx Lu K, Chen X, Zhang J, Chen J, Liu Z, Chen L (2025) Comparison of artificial neural network and response surface methodology prediction in key performance of two-component grout material in shield tunneling. Case Studies in Construction Materials , xx, Article xxxx Norouzi A, Uygar E, Nalbantoglu Z (2023) Compacted clay liner formed with beach sand, Na-bentonite and waste materials. Research Square . https://doi.org/xxxxx Öncü Ş, Bilsel H (2018) Utilization of waste marble to enhance volume change and strength characteristics of sand-stabilized expansive soil. Environ Earth Sci 77:692 Roy R, Mishra AK (2024) Organoclay amended bentonite for enhanced barrier performance under lead (II) permeation. 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Measurement 164:108061. https://doi.org/10.1016/j.measurement.2020.108061 Di Sante M, Khan MK, Calò L, Fratalocchi E, Mazzieri F (2025) The combined use of fly ash and lime to stabilize a clayey soil: A sustainable and promising approach. Geosciences , 15(x), Article xxxx. https://doi.org/xxxxx Dutta RK, Sharma SS, Parti R (2018) Geotechnical characterization of fly ash–red mud mix stabilized with marble dust. Journal of Civil Engineering (IEB) , xx(x), xx–xx Galetakis M, Soultana A (2016) A review on the utilisation of quarry and ornamental stone industry fine by-products in the construction sector. Constr Build Mater 102:769–781. https://doi.org/10.1016/j.conbuildmat.2015.10.204 Luso E, Lourenço PB (2017) Experimental laboratory design of lime-based grouts for masonry consolidation. Int J Architectural Herit 11(7):987–1005. https://doi.org/10.1080/15583058.2017.1307025 Rout S, Singh SP (2017) Assessing the suitability of compacted bentonite–pond ash mixes as landfill liner. In Proceedings of the International Congress and Exhibition on Sustainable Civil Engineering (pp. xx–xx). Springer Turan C, Javadi AA, Vinai R, Zali B, R (2022) Geotechnical characteristics of fine-grained soils stabilized with fly ash: A review. Sustainability 14(9):5255. https://doi.org/10.3390/su14095255 Yıldız Ö, Çabalar AF (2025) Predictive modeling and experimental analysis of cyclic shear behavior in sand–fly ash mixtures. Applied Sciences , 15(x), Article xxxx. https://doi.org/xxxxx Ahmad J, Kontoleon KJ, Al-Mulali MZ, Shaik S (2022) Partial substitution of binding material by bentonite clay (BC) in concrete: A review. Buildings 12(5):634. https://doi.org/10.3390/buildings12050634 Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8630963","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":585112947,"identity":"171cf457-b867-4940-b99e-8c4f1665088b","order_by":0,"name":"Ankush Kumar 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11:38:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8630963/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8630963/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101948336,"identity":"28de7d73-5651-491d-ac23-3c8ea7fda9af","added_by":"auto","created_at":"2026-02-05 10:12:35","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":973610,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFESEM Analysis of (a) Bentonite (b) Fly ash (c) Marble dust (d) Sand\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/6dd1f31691c10c7fea0cd99e.png"},{"id":101948326,"identity":"c6932ff3-bae0-4571-8f20-e152e744951a","added_by":"auto","created_at":"2026-02-05 10:12:26","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":75391,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCompression Index (C\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003ec\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) for the B-S, B-MD \u0026amp; B-FA mixture\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/43d465e99254b0c9ba0dce65.png"},{"id":101948211,"identity":"698670af-3d9b-4856-882f-5d8ecab51b7a","added_by":"auto","created_at":"2026-02-05 10:11:59","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":78432,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSwelling Index (C\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003es\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) for the B-S, B-MD \u0026amp; B-FA mixture\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/db1a75d08889a8548b8106db.png"},{"id":101948327,"identity":"bc3ff333-3f26-4e01-92cb-45838b7c7abb","added_by":"auto","created_at":"2026-02-05 10:12:26","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":75456,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCoefficient of Consolidation for B–S, B–MD, and B–FA Mixes\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/3e7b8425b72839ad56ad3c97.png"},{"id":102298194,"identity":"57ede6f4-c79a-42e2-9ba9-5d613dc500c8","added_by":"auto","created_at":"2026-02-10 10:33:40","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":225107,"visible":true,"origin":"","legend":"\u003cp\u003eConsolidation characteristics of B–S mixtures under three stress levels (100, 400 and 800 kPa)\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/2966dc9884b2f1e0b3ea2f8b.png"},{"id":102298781,"identity":"68f3844a-bc56-457a-9990-6c4a70ac5296","added_by":"auto","created_at":"2026-02-10 11:00:22","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":193544,"visible":true,"origin":"","legend":"\u003cp\u003eConsolidation characteristics of B–FA mixtures under three stress levels (100, 400 and 800 kPa)\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/d45ec70f78addc2331707d22.png"},{"id":101948269,"identity":"43195b3b-7f47-4793-8fa9-c1216cdc6456","added_by":"auto","created_at":"2026-02-05 10:12:03","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":226692,"visible":true,"origin":"","legend":"\u003cp\u003eConsolidation characteristics of B–MD mixtures under three stress levels (100, 400 and 800 kPa)\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/40da7366bac098e55a90458b.png"},{"id":101948311,"identity":"ead53696-b4e1-44e0-8fd3-19cef21af619","added_by":"auto","created_at":"2026-02-05 10:12:19","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":110532,"visible":true,"origin":"","legend":"\u003cp\u003eSettlement behaviour of B–S mixtures under three stress levels (100, 400 and 800 kPa)\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/00b8cabac289519f49e683e9.png"},{"id":101948347,"identity":"8523b3f4-10a3-4cb6-99a7-06300979e7d8","added_by":"auto","created_at":"2026-02-05 10:12:44","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":136299,"visible":true,"origin":"","legend":"\u003cp\u003eSettlement behaviour of B–FA mixtures under three stress levels (100, 400 and 800 kPa)\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/87b2ca7fda45d4eac503e070.png"},{"id":101948315,"identity":"e33f4c1b-a02e-4833-be44-e3d249455f0d","added_by":"auto","created_at":"2026-02-05 10:12:21","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":134562,"visible":true,"origin":"","legend":"\u003cp\u003eFig. 09 Settlement behaviour of B–MD mixtures under three stress levels (100, 400 and 800 kPa)\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/9a05bd2f05a2f4bf3567ac23.png"},{"id":102301206,"identity":"ec04ab3a-fc2d-42d6-a72a-9d3e3f0e4920","added_by":"auto","created_at":"2026-02-10 11:20:26","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3141207,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8630963/v1/f80af8c0-ca95-457e-8cd6-539020675e8c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Engineering Landfill Liners for SDGs 6, 11, and 13: Comparative Consolidation Performance of Bentonite–Sand, Bentonite–Fly Ash, and Bentonite–Marble Dust Mixtures","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eLandfill remains the most common approach to disposal of municipal solid waste (MSW) and industrial by-products worldwide, especially in developing countries [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Although various practices of solid waste management, such as minimization, recycling and energy recovery have been developed in recent years the amount of solid waste has increased with urbanization, population growth and industrial processes [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Engineered landfill liners are an essential strategy in providing containment systems to protect the environment against contamination of leachate percolation and gas migration [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. These liners serve as hydraulic insulating layers preventing the direct contact between landfills and ground water-soil system, reducing the release of toxic trace metals, dissolved organic matter and other pollutants to the natural environment [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Previously, compacted clay liners (CCL) and geosynthetic clay liners (GCL) have been used in singles or combinations so as to comply with performance regulations. Key properties of liner materials are very low hydraulic conductivity (commonly\u0026thinsp;\u0026le;\u0026thinsp;1 \u0026times; 10⁻⁹ m/s), adequate shear strength to support the waste, and sufficient resistance to chemical attack [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The bentonite is preferred mainly for its excellent swelling capability, high cation exchange capacity and the extremely low permeability it can achieve. Bentonite\u0026ndash;sand mixtures are commonly used as low cost liners, where the benefits of bentonite and the structural stability provided by granular materials are combined in a specified proportion by volume. However, the widespread use of bentonite\u0026ndash;sand liners made with natural sand is not sustainable. Extraction of sand disrupts the functioning of a river ecosystem, as well as depleting the natural resource itself and incurring other regulatory constraints. At the same time a plenty of byproduct like fly ash from thermal power plant and marble dust from stone processing industries are laid waste, causing serious environmental problem [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Recovery of these industrial wastes for valorising them into value-added compounds and land-fill liner materials adds to the dual handicap of waste generation \u0026ndash; minimization in natural resource exploitation and sustainable management of waste [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe function of a landfill liner is not only as an impermeable barrier but also as a long-term consolidation behaviour under the waste contents. Stresses rise steadily and progressively due to overburden pressures, decay of the waste mass, as well as from settlement in the layers of waste [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. These loads induce volumetric reduction, time-dependent compression and possibly hydraulic conductivity alteration in the liner materials [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Consolidation testings offer important parameters (compression index Cc, swelling index Cs and coefficient of consolidation Cv) for compressibility, rebound potential and rate of pore water dissipation respectively. High compressibility can cause excessive settlements, which affects the structural safety of structures and low Cv value will delay pore pressure degradation and hence hydraulic stability [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Therefore, the consolidation behaviour for bentonite\u0026ndash;sand and its sustainable alternatives is important in predicting liner performance over decadal operation lifetime of landfills [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFly ash (FA) is a finely divided waste material produced through burning of coal in thermal power plants, and is readily available in India which has one of the largest number of thermal power generating units. FA has pozzolanic nature, medium plasticity (LL\u0026thinsp;=\u0026thinsp;38%) and low density being MDD\u0026thinsp;=\u0026thinsp;1.49 g/cc. Its silt-sized particles promote packing density and alkaline character (with approximately pH 8.8) supports chemical resistance [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. During mixing with bentonite, FA could be effective in diminishing of excess swelling and enhancing strength and workability but the impact on consolidation properties has not been fully understood. Marble dust (MD), a waste material produced in large quantity from the industries of cutting, polishing and finishing of marble [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. It is environmentally difficult to dispose of, because it can block waterways and generate particulate air pollution. Geotechnically, MD has low plasticity (LL\u0026thinsp;=\u0026thinsp;17.2%), a high specific gravity, and good frictional resistance (ϕ\u0026thinsp;=\u0026thinsp;25.5\u0026deg;). The addition of MD to chemical prehydrated bentonite liners can help counter the high compressibility of bentonite, minimize shrink\u0026ndash;swell behaviour, and enhance beneficial volumetric stability. In addition, MD has to be chemically inert and hard [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. By partially replacing sand with FA or MD, bentonite liners can be engineered to achieve desired mechanical and hydraulic properties while simultaneously diverting industrial waste streams from land disposal [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThere is quite a substantial amount of research on permeability and shear strength of bentonite\u0026ndash;sand mixtures [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Fly ash and marble dust have been used as geotechnical stabilizers for expansive soils, road subgrade and embankments in several other investigations [\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. However, little research has systematically studied the consolidation of bentonite\u0026ndash;FA and bentonite\u0026ndash;MD mixtures with a comparative perspective to its behavior in comparison with that of bentonite\u0026ndash;sand systems. The majority of previous studies concentrate on either permeability or strength parameters and most of the time consolidation is treated as a secondary concern [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. However, with regards to landfill design, consolidation controls the long-term deformation of differential settlement and the development of cracking or hydraulic short circuits in liners. Furthermore, the environmental impacts of FA and MD addition have not been systematically framed in relation to sustainable use of natural sand and circular economy for waste treatment purposes.\u003c/p\u003e \u003cp\u003eIn the present study, the aims are developed for addressing current limitations in designs of sustainable landfill liners. The objectives of the study are to investigate the consolidation behaviors, such as the compression index, swelling index, coefficient of consolidation, and settlement behavior \u0026ndash; of bentonite\u0026ndash;fly ash (FA) and bentonite\u0026ndash;marble dust (MD) mixtures compare with traditional bentonite\u0026ndash;sand systems. Moreover, it is intended to evaluate the effect of these waste additives on the compressibility and structural stability of bentonite liners at different stress conditions simulating landfill loading. Another goal is to evaluate the possible enhancement of sustainable development, including resource saving and waste recycling, if sand is substituted by FA or MD in bentonite liners. Finally, the research aims to discover the most viable bentonite-based mixture: bentonite\u0026ndash;sand, bentonite\u0026ndash;FA and bentonite-MD from both consolidation performance and environmental aspects.\u003c/p\u003e"},{"header":"2. METHODOLOGY","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Methodological Framework\u003c/h2\u003e \u003cp\u003eThe experimental procedure had been made to satisfy the stated objectives in an organized method. Bentonite, FA, MD and sand were oven-dried at 105\u0026thinsp;\u0026plusmn;\u0026thinsp;5 \u0026deg; C and crushed by pulverization to break down lumps followed by sieving for size grading [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Initially, the geotechnical properties of these parent material is identified through laboratory test, which is reported in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eGeotechnical characteristics of Bentonite, Fly Ash, Marble Dust, and Sand\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDescription\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSand\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMarble Dust\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBentonite\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFly ash\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eIS-CODE\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSand (4.75\u0026thinsp;\u0026minus;\u0026thinsp;0.075 mm) %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e98.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e95.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e93.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSilt (0.075\u0026thinsp;\u0026minus;\u0026thinsp;0.002 mm) %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6.50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eClay (\u0026lt;\u0026thinsp;0.002 mm) %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e80.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSoil Classification as ISC and USCS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSP (Poorly graded sand)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePoorly graded\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eClay with high plasticity\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLiquid Limit %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e27.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e202.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e38.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlastic Limit %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e70.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eShrinkage Limit %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlasticity Index %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e27.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e131.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e38.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMax. Dry Density (MDD) gm/cc\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOptimum Water Content (OWC) %\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e14.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e48.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e23.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAccording to these materials, three mixtures were prepared; bentonite\u0026ndash;sand (B\u0026ndash;S), bentonite-fly ash (B-FA) and bentonite-marble dust (B-MD). For each mixture, different mixes were formulated with additive to bentonite proportions from 0 to 40% in 5% increments, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Mixing took place in a dry state and water was then added incrementally to OWC (optimum water content)/MDD (maximum dry density) as measured by Standard Proctor compaction test. One-dimensional consolidation tests were performed on the prepared mixes as well, in order to assess their compressibility behavior. Samples were statically compacted into 60 mm-diameter and 20-mm-tall consolidation rings at MDD and OWC. Consolidation test were performed according to the provisions of IS 2720 (Part 15): 1986 [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The vertical stress was loaded step by step (100, 400, and 800 kPa) and dial gauge readings were taken at logarithmic times of load application until a consolidation to the extent of about 90\u0026ndash;95% was obtained.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDry Weight of Mix Proportion of Bentonite -Fly ash (B-FA), Bentonite \u0026ndash;Sand (B-S) and Bentonite- Marble Dust (B-MD)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS. No.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMix Proportion\u003c/p\u003e \u003cp\u003e(B-FA) Dry Weight\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMix Proportion\u003c/p\u003e \u003cp\u003e(B-MD) Dry Weight\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMix Proportion\u003c/p\u003e \u003cp\u003e(B-S) Dry Weight\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e100FA+0B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100MD+0B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100S+0B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e95FA+5B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e95MD+5B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e95S+5B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e90FA+10B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e90MD+10B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e90S+10B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e85FA+15B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e85MD+15B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e85S+15B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e80FA+20B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80MD+20B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e80S+20B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e75FA+25B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e75MD+25B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e75S+25B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e70FA+30B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e70MD+30B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e70S+30B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e65FA+35B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e65MD+35B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e65S+35B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e60FA+40B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e60MD+40B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e60S+40B\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe adhesion performance and swelling behavior were assessed by load\u0026ndash;unload cycles. The compression index (Cc), swelling index (Cs), coefficient of consolidation (Cv), and settlement behavior were obtained from the test data. With these settings, the consolidation behaviour of B\u0026ndash;FA and B\u0026ndash;MD mixtures could be compared with that of the reference conventional B\u0026ndash;S liner [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. To further assess the influence of FA and MD on the structural stability of bentonite-based liners under landfill loading, additional high-pressure consolidation tests were carried out up to 800 kPa. This simulated the stress conditions likely to develop in deep landfills. Repeated loading and unloading cycles were performed to study compressibility under cyclic stresses, recompression ratios, and stiffness. Stress\u0026ndash;strain curves and compressibility coefficients (mv) were derived at each pressure stage to assess the role of FA and MD in improving stability and reducing settlement compared to sand.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 EDAX and SEM analysis of Parent Material (B, S, FA \u0026amp; MD)\u003c/h2\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the atomic percentage composition of major chemical elements in bentonite, sand, fly ash, and marble dust. Silicon (Si) and oxygen (O) dominate the elemental composition of all materials, with particularly high concentrations observed in sand and bentonite, confirming their role as the primary constituents. Aluminum (Al) is present in significant amounts in bentonite and sand, whereas iron (Fe) occurs in relatively low concentrations across all materials, with slightly higher proportions in fly ash and sand [\u003cspan additionalcitationids=\"CR24\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Magnesium (Mg) and calcium (Ca) are highly concentrated in marble dust, highlighting their importance in defining its chemical composition. Fly ash exhibits a comparatively high carbon (C) content, suggesting the presence of carbon-rich or residual organic compounds. Chlorine (Cl) is most prominent in bentonite, while sulfur (S) and potassium (K) are detected in trace quantities across all materials, contributing to their distinct chemical characteristics\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEDAX analysis of the parent sample's bentonite, fly ash, marble dust, and sand [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChemical\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAtomic % (Bentonite)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAtomic % (Sand)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAtomic % (Fly Ash)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAtomic % (Marble Dust)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e36.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e40.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e16.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e28.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e31.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e19.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e23.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e13.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e17.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e13.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e14.51\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e25.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e49.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFurther, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e01\u003c/span\u003e presented the FESEM analysis of the parent samples. The SEM image of bentonite [Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e01\u003c/span\u003e (a)] reveals a characteristic platy and flaky microstructure composed of aggregated lamellar particles.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe particles seem to be non-spherical in shape and having different particle size distribution; a compact of high density\u0026ensp;matrix with micro-voids and pores is also observed. These pores are responsible for the high surface area and swelling behavior\u0026ensp;of bentonites [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Layered organization of platelets testifies to the presence of montmorillonite minerals with high plasticity\u0026ensp;and absorption properties [\u003cspan additionalcitationids=\"CR24 CR25\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. The occurrence\u0026ensp;of agglomeration indicates that strong interparticle bonding is involved in the compaction, compressibility and hydraulic behavior obtained for bentonite-based liner materials. Fig SEM\u0026ensp;image of fly ash [Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e01\u003c/span\u003e (b)] displays overall spherical shape with\u0026ensp;particles of different sizes as typical coal fly ash. Many of the particles are smooth walled, hollow or solid spheres (cenospheres and plerospheres) suggesting quick\u0026ensp;cooling upon formation. The spherical form would also provide better flowability and packing density of the fly ash in mixing with\u0026ensp;other materials. Some of the particles demonstrate porous surfaces, agglomeration indicating unburnt carbon and finer\u0026ensp;ash fractions. This heterogeneous microstructure in fly ash improves its pozzolanic reactivity and affects the mechanical\u0026ensp;properties, permeability, and strength of fly ash utilized in geotechnical engineering and construction applications. SEM view of marble dust is\u0026ensp;shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (c) shows that particles are angular\u0026ensp;and irregular, having sharp edges and rough surfaces. The particles look dense without clear apparent round features and this is consistent with the mechanical grinding in cutting and\u0026ensp;polishing the marble. Fine particles occupy the voids between larger grains, which results in denser microstructure\u0026ensp;[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The coarse and angular shapes promote friction force/bonding between the particles of\u0026ensp;marble dust while mixing with other geomaterials. The close compactness and low visual porosity are indicative of a less formal compressive potential, whereas the highly enriched calcium in marble dust allows it to serve as a stabilizing filler\u0026ensp;in geologic and lining applications. The SEM image of sand [Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e01\u003c/span\u003e(d)] shows angular to sub-angular particle of random shape and having well\u0026ensp;definitional edges. The grains show relatively smooth surfaces and unevenness on the surface area, which are probably caused by erosion due to weathering action, and mechanical grinding\u0026ensp;action. The size distributions of\u0026ensp;particles looks mostly uniform and results in a substantial stable aggregate structure. Existence of intergranular voids indicates that sand is highly porous and\u0026ensp;permeable [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The particles are hard but too smooth to be cohesive, and the absence of any magnetic\u0026ensp;traction or direct chemical attraction makes sand mechanically stable, yet still porous. This\u0026ensp;microstructure arrangement heavily influences the hydraulic and bearing behavior of sands as known in geotechnical application.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULT AND ANALYSIS","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Assessing the Compression Index (C\u003csub\u003ec\u003c/sub\u003e) for the B-S, B-MD \u0026amp; B-FA mixture\u003c/h2\u003e \u003cp\u003eThe values of the compression index (Cc) obtained for B\u0026ndash;S, B\u0026ndash;MD and B\u0026ndash;FA show a consistent trend of increase in compressibility with increasing bentonite content as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The Cc values for 0% bentonite are comparably very low (B\u0026ndash;S:0.08, B\u0026ndash;MD: 0.09 and B\u0026ndash;FA: 0.07), indicating the presence of higher proportion of coarser fractions such as sand/ marble dust/fly ash, which hinder compressibility. With increased level of incorporation of bentonite, the values increase monotonically up to a maximum value for all three mixes at 40% bentonite.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eB\u0026ndash;MD displays however somewhat higher Cc values with respect to B\u0026ndash;S and B\u0026ndash;FA, especially at the intermediate\u0026ensp;bentonite contents. The relatively higher values for B\u0026ndash;MD (e.g., 0.63 at 40% bentonite as compared with the corresponding values of 0.61 for B\u0026ndash;S and 0.57 for B\u0026ndash;FA) may be due to the irregular shape of particles and close particle size distribution in marble dust leading to packing density but also facilitates localized deformation with respect\u0026ensp;to loading [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn general, the implication is that the addition of FA and MD alters\u0026ensp;the compressibility characteristics of bentonite covers. FA is the most efficient to\u0026ensp;decrease compressibility that results in structurally stable and cost-effective use of B\u0026ndash;FA mixtures as land fill material. However, MD (and even more Cc) still shows acceptable behaviour and can contribute to sustainable use of waste material\u0026ensp;[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Accordingly, FA- and MD-mixed bentonite liners present technically superior and\u0026ensp;environmentally favorable options to traditional formed sand covers.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Assessing the Swelling Index (Cs) for the B-S, B-MD \u0026amp; B-FA mixture\u003c/h2\u003e \u003cp\u003eThe\u0026ensp;swelling index (Cs) is a key parameter to understand soil behaviour in landfills. It\u0026rsquo;s a way of knowing how much they can swell up and soak up water in varying stress\u0026ensp;conditions. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e indicates Cs to be\u0026ensp;enhanced as the amount of bentonite increases, indicating that this is in line with the expansion ability of bentonite.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor 0% bentonite, Cs\u0026ensp;values are low (0.009\u0026ndash;0.011), these correspond to the weak swelling capacity of such materials: sand, marble dust and fly ash. With the increment of bentonite, Cs gradually goes up: 0.0180.019 (10%), 0.0320.034 (20%) and 0.072\u0026thinsp;~\u0026thinsp;0.076 (40%)\u0026ensp;[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Amongst the three blends, B-MD has, in general, higher Cs values and\u0026ensp;is followed closely by B-S. B-FA has slightly lower values. This indicates that the marble dust enhances the swelling capacity of bentonite particles5 as its finer particles show better interaction with the clay minerals which offer more interlayers for water to\u0026ensp;be absorbed. Sand, being more coarse and chemically inert, is little affected by swelling; however its volume can increase markedly by\u0026ensp;the porosity of bentonite. Fly ash, on the other hand,\u0026ensp;inhibits swelling slightly because its round glassy particles fill voids and retard water migration.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Assessing the Coefficient of Consolidation for B\u0026ndash;S, B\u0026ndash;MD, and B\u0026ndash;FA Mixes\u003c/h2\u003e \u003cp\u003eCoefficient of consolidation\u0026ensp;(Cv) is the rate by which excess pore water pressure dissipates under load. It is a key parameter\u0026ensp;in assessing the compressibility and permeability of soil-bentonite blends. Experimental data as per Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e indicates that, irrespective of the mixes (B-S, B-MD and B-FA), an\u0026ensp;increase in bentonite content results in a reduction of Cv.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis implies decrease in drainage ability and slower consolidation rate with\u0026ensp;the increase of bentonite. These special values are 15.2 \u0026times;10⁻⁷ m\u0026sup2;/s in B-S, 16 \u0026times;10⁻⁷ m\u0026sup2;/s in B-MD and 17.5 \u0026times;10⁻⁷ m\u0026sup2;/s, at 100%\u0026ensp;of bentonite and at zero percent of it. These large values are attributed to the infiltration of coarser particles (sand, marble dust and fly ash) that can lead to a more rapid dissipation of pore water\u0026ensp;and accelerated settlement under load [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. However, with the percentage\u0026ensp;of bentonite increasing up to 40%, Cv drops sharply to a range of 6.6\u0026ndash;8.0\u0026times;10⁻⁷ m\u0026sup2;/s by illustration of the fine and plate-like structure which reduces permeability, hindering the consolidation process. Such slow consolidate\u0026ensp;behavior is advantage for liner and barrier because the desired seepage rate or water flow is restricted. In the case of mixtures, at each bentonite percentage its Cv values are highest for B-FA followed by B-MD and\u0026ensp;least to that of S-B. This trend indicates that the fly ash contributes to improve the microstructure because it results in more interconnected pores, which facilitates good\u0026ensp;drainage and rapid reduction of pore pressure [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Even marble dust, which is hard as rock and of low\u0026ensp;porosity with regard to sand or gravel, probably has a moderate drain: it would have passed off earlier. On the other hand, sand with bentonite gives the lowest\u0026ensp;Cv because of higher particle friction and less void connectivity. In conclusion, impermeability of the mixture is enhanced by rising bentonite content\u0026ensp;while slowing down the consolidation process. BN-FA combinations achieve a happy medium of moderate long-term permeability retention along\u0026ensp;with steady consolidation behaviour. This would have\u0026ensp;rendered them potentially useful for construction purposes, such as liners in the engineered landfill needing controlled drainage and stability [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Assessing the settlement behaviour for B\u0026ndash;S, B\u0026ndash;MD, and B\u0026ndash;FA Mixes under different stress conditions\u003c/h2\u003e \u003cp\u003eThe consolidation parameters of bentonite\u0026ndash;sand (B\u0026ndash;S) mixtures subjected to three stress levels (100, 400 and 800 kPa) over a wide range of bentonite contents (0\u0026ndash;40%) as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. A clear and consistent trend is observed in the variation of compression index (Cc), swelling index (Cs) and coefficient of consolidation (Cv) with increasing bentonite percentage and applied stress. For all stress conditions, Cc increases markedly as bentonite content increases, indicating enhanced compressibility of the mixture. At 100 kPa, Cc increases from 0.08 for pure sand (0% bentonite) to 0.36 at 40% bentonite and further to 0.55 for pure bentonite. The effect becomes more pronounced at higher stresses; at 400 kPa, Cc rises from 0.14 (0%) to 0.68 (40%) and reaches 1.05 at 100% bentonite, while at 800 kPa it increases from 0.19 to 0.91 and finally to 1.40. This behavior is attributed to the increasing proportion of bentonite, which is rich in montmorillonite and exhibits high specific surface area and strong water adsorption capacity, resulting in greater volume reduction under load [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA similar increasing trend is observed for the swelling index (Cs). At 100 kPa, Cs increases from 0.01 (0%) to 0.043 (40%) and 0.063 (100%), whereas at 800 kPa it rises from 0.021 to 0.08 and 0.12, respectively. This indicates that bentonite-rich mixtures possess a higher tendency to rebound and swell upon unloading due to diffuse double-layer expansion. In contrast, Cv shows a decreasing trend with increasing bentonite content at all stress levels [\u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. For instance, at 100 kPa, Cv decreases from 19 \u0026times;10⁻⁷ m\u0026sup2;/s for 0% bentonite to 10 \u0026times;10⁻⁷ m\u0026sup2;/s at 40% and 7.5 \u0026times;10⁻⁷ m\u0026sup2;/s for pure bentonite. At 800 kPa, Cv further reduces to as low as 3 \u0026times;10⁻⁷ m\u0026sup2;/s at 100% bentonite. This reduction reflects a decrease in permeability caused by pore clogging and swelling of bentonite particles, along with compression of drainage channels under higher stresses. Overall, the results indicate a transition from a sand-controlled, rapidly consolidating system to a clay-dominated, highly compressible and slow-draining system as bentonite content and stress increase [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe consolidation characteristics of bentonite\u0026ndash;fly ash (B\u0026ndash;FA) mixtures evaluated under three stress levels (100, 400 and 800 kPa) for bentonite contents ranging from 0% to 40% as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe compression index (Cc) shows a systematic increase with increasing bentonite percentage at all stress levels, indicating a progressive increase in compressibility of the B\u0026ndash;FA mixture. At 100 kPa, Cc increases from 0.07 for 0% bentonite to 0.34 at 40% bentonite and further to 0.55 for pure bentonite. This trend becomes more pronounced under higher stresses; at 400 kPa, Cc increases from 0.12 (0%) to 0.61 (40%), while at 800 kPa it rises from 0.17 to 0.80. This behavior reflects the increasing influence of bentonite clay minerals, which exhibit high plasticity, large surface area and strong affinity for water, leading to greater compressive deformation. The swelling index (Cs) also increases consistently with bentonite content and applied stress. At 100 kPa, Cs rises from 0.009 for 0% bentonite to 0.041 at 40% bentonite, whereas at 800 kPa it increases from 0.019 to 0.073. This indicates that bentonite-rich B\u0026ndash;FA mixtures have a higher potential for rebound and swelling upon unloading due to diffuse double-layer expansion [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. In contrast, the coefficient of consolidation (Cv) decreases steadily with increasing bentonite content at all stress levels. At 100 kPa, Cv decreases from 20 \u0026times;10⁻⁷ m\u0026sup2;/s for 0% bentonite to 11.5 \u0026times;10⁻⁷ m\u0026sup2;/s at 40% and 7.8 \u0026times;10⁻⁷ m\u0026sup2;/s for pure bentonite. Under 800 kPa stress, Cv further reduces to 5 \u0026times;10⁻⁷ m\u0026sup2;/s at 40% bentonite and only 3.2 \u0026times;10⁻⁷ m\u0026sup2;/s at 100% bentonite. This reduction is attributed to pore space filling by bentonite particles, reduced permeability, and compression of drainage paths under higher stresses. Compared to sand-based systems, the presence of fly ash contributes to finer particle packing; however, the dominant control on consolidation behavior remains the bentonite fraction, especially at higher percentages and stresses [\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe consolidation parameters of the bentonite\u0026ndash;marble dust (B\u0026ndash;MD) mixtures show a clear and systematic variation with both applied stress level and bentonite content as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The compression index (Cc) increases significantly with increasing stress from 100 to 800 kPa for all bentonite percentages, indicating enhanced compressibility under higher effective stresses.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor example, at 0% bentonite, Cc rises from 0.09 at 100 kPa to 0.20 at 800 kPa, while at 40% bentonite it increases from 0.40 to 0.93 over the same stress range. This trend becomes more pronounced with higher bentonite content due to the dominance of expansive clay minerals, which undergo greater structural rearrangement under load [\u003cspan additionalcitationids=\"CR37\" citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Similarly, the swelling index (Cs) also increases with both stress and bentonite percentage, reflecting higher rebound potential during unloading. Cs values rise from 0.011\u0026ndash;0.023 at 0% bentonite to as high as 0.045\u0026ndash;0.080 at 40% bentonite across stress levels, and further to 0.125 at 100% bentonite and 800 kPa. This behavior can be attributed to the high surface area and double-layer effects of bentonite, which enhance elastic deformation. In contrast, the coefficient of consolidation (Cv) shows a decreasing trend with increasing stress and bentonite content. For instance, Cv decreases from 18.8 \u0026times;10⁻⁷ m\u0026sup2;/s at 100 kPa to 10.8 \u0026times;10⁻⁷ m\u0026sup2;/s at 800 kPa for 0% bentonite, while for 40% bentonite it drops from 11.0 to 4.9 \u0026times;10⁻⁷ m\u0026sup2;/s. At 100% bentonite, Cv further reduces to 2.9 \u0026times;10⁻⁷ m\u0026sup2;/s at 800 kPa. The reduction in Cv is primarily due to decreased permeability caused by pore clogging and reduced drainage paths as fine bentonite particles increasingly dominate the soil matrix, slowing the rate of consolidation [\u003cspan additionalcitationids=\"CR39\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eSettlement Behaviour under Different Stress Conditions\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe settlement characteristics of the B\u0026ndash;S mixtures strongly depend on both bentonite percentage and applied stress level as reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWith the smallest pressure at 100 kPa, settlement is small and it increases overall monotonously with the content of bentonite, being 0.9 mm with no bentonite, raising to 4.3 mm at\u0026ensp;40% bentonite or even up to \u0026minus;\u0026thinsp;6.5 mm for pure bentonite. This suggests that sand skeleton is still effective in bearing loads at low stress, which results in limited\u0026ensp;deformation as well [\u003cspan additionalcitationids=\"CR39 CR40\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Under 400 kPa pressure, however, the settlement is significantly greater (23.4\u0026ensp;mm at 0% bentonite to 11.5 mm at 40% bentonite and then to 17.8 mm at 100% bentonite). The notch has a larger effect at 800 kPa, where settlement increases from 3.9 mm (0%) to 16.2 mm (40%) with the maximum of 26 mm\u0026ensp;for pure bentonite. Rapid increase in settlement beyond around 20\u0026ndash;25% bentonite indicates when behavior shifts from sand-dominated consolidation to clay\u0026ensp;dominated. This behavior is attributed to the relatively higher values of Cc and lower values of Cv\u0026ensp;in bentonite-rich mixes, which lead to higher compressibility and slower excess pore water pressure dissipation. Therefore greater stresses produce both more settlement and settlement that varies with time to a greater\u0026ensp;extent. From a design point of view, these results illustrate the need for a trade-off in bentonite\u0026ndash;sand mixtures \u0026ndash; using higher contents of bentonite reduces permeability for application such as liners and barriers;\u0026ensp;however it also promotes settlement under moderate and intermediate stresses. It is necessary to select a proper amount of bentonite to achieve required low permeability, with\u0026ensp;the acceptable elastic deformation in practical geotechnical engineering application [\u003cspan additionalcitationids=\"CR8 CR9 CR10\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFig. shows that the settlement response of the B\u0026ndash;FA mixes is highly influenced by both\u0026ensp;the bentonite content and the applied stress level. 09. At 100 kPa the settlement gradually increases from\u0026ensp;0.8 mm (0% bentonite) to 4.1 mm (40% bentonite) and is 6.1 mm for pure bentonite. This relatively moderate settlement under island stress implies that by adding fly ash to the slurry, there is a certain amount of partially rigid structure skeleton in the slurry system restrains deformation of mixed slurry when\u0026ensp;bentonite content corning down. Below\u0026ensp;400 kPa stress, settlement is considerably higher and varies from the value of 2\u0026thinsp;\u0026minus;\u0026thinsp;1 mm at a content of bentonite of 0% to that of 10.5 mm and even higher values for bentonite contents above 40% reaching 16.2 mm in the case with content equal to 100%. For the highest confining stress of 800 kPa,\u0026ensp;settlement is pronounced and grows with increasing % bentonite from 3.5 mm (0% bentonite) to 15 mm at 40% bentonite and up to the maximum value of 24.5 mm for pure bentonite. The sharp change of\u0026ensp;the Σh0 values at about 20\u0026ndash;25% bentonite content signifies the transition from fly ash\u0026ndash;to\u0026ndash;bentonite-dominant consolidation. the higher the \"Cc\" and lower \"Cv\" (i.e., HAC)\u0026ensp;is, the larger the compressibility of soil and slower excess pore pressure dissipating are [\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. From an engineer\u0026rsquo;s point of view, although the addition of bentonite enhances sealing and reduces permeability\u0026ensp;in B\u0026ndash;FA mixes, it is at the expense of a dramatic increment in settlement under moderate to high pressures. Hence, there exists optimum bentonite content for practical applications such as liners and embankments so that the hydraulic properties can be balanced with deformation control\u0026ensp;[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSettlement of the B\u0026ndash;MD mixtures follows closely with compressibility and consolidation trend observed\u0026ensp;in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e09\u003c/span\u003e. Settlement\u0026ensp;also increases with applied stress and bentonite content, due to high values of Cc and lower stiffness of the mixtures. 0% bentonite case, the settlement is from 0.9 mm at 100 kPa to 3.6 mm at a pressure of 800 kPa revealing relatively low compressibility with marble dust which behaves as filler and enhances the\u0026ensp;densification of particles [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. However, high\u0026ensp;settlement occurs on the increasing of bentonite content. For instance, at 20% bentonite content, settlement increases from 2.2 mm (100 kPa) up to 9.1 mm (800 kPa), and for\u0026ensp;40% bentonite content it will increase from 3.7 mm to13.5 mm. The maximum settlements are 6.2, 15.7, and 23 mm at pressures of 100, 400, and 800 kPa for the full bentonite content sample (i.e., \u0026gt;\u0026thinsp;90%), indicating a very high compressibility\u0026ensp;of neat bentonite.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBlock identity increased with stress and due to the\u0026ensp;collapsed soil fabric or expulsion of pore water and it increased with bentonite percentage as a result of its high plasticity and ability to attract water [\u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. At lower bentonite contents, the marble dust limits settlement by classed grading and decrease in void ratio followed by saturation but at\u0026ensp;a clay content greater than that of one additional mixing stage, the behaviour is controlled by the clay fraction. In general, the findings have shown that at low and moderate bentonite contents of B\u0026ndash;MD mixtures behave in a balanced manner (i.e., with controlled settlement and reasonable rates of consolidation) however, depending on their application (e.g. liners/barriers), high bentonite\u0026ensp;contents activate excessive settlement and slow down the rate at which consolidation occurs.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. DISCUSSION","content":"\u003cp\u003eThe consolidation characteristics and sustainability potential of the B\u0026ndash;S, B\u0026ndash;FA, and B\u0026ndash;MD\u0026ensp;mixtures will give comparative results that could help in elucidating their potential as landfill liner materials. The findings well prove that the dominant variable controlling compressibility, swelling ratio, voids ratio, rate of consolidation and settlement is bentonite content and it was found out that there had to be almost 1/3 bentonite while between 1/4% and %2\u0026ensp;of sand required as modifier materials for system performance [\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTrends on\u0026ensp;Cc for the CI alongwith bentonite content indicates that compression increases with increasing bentonite content, which highlights high water affinity and platy nature of its particles. Yet, the dpeak depends on the relative concentration of\u0026ensp;ionic liquids to water. Arguably, B\u0026ndash;FA is always consistently\u0026ensp;at the bottom of compressibility factor and it is closely pursued by B\u0026ndash;S whereas B\u0026ndash;MD mostly exhibits higher values of C c [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. The decreased compressibility of B\u0026ndash;FA may be due to fly ash particles being almost spherical in shape, which enhance packing efficiency,\u0026ensp;reduce void ratios and hinder the rearrangement of clay platelets under loading. On the other hand, marble dust particles provide angular shape capable of densification under compaction but tend to\u0026ensp;form point-contact fabrics prone to breakage which resulted in marginally higher Cc. This latter distinction is essential, in that too soft a compression\u0026ensp;gives rise to line stability. Therefore,B\u0026ndash;FA provides a distinct technical advantage in controlling settlement irrigation and is superior in this respect to B\u0026ndash;MD which gives only limited benefit over the\u0026ensp;use of sand.\u003c/p\u003e \u003cp\u003eSwelling index (Cs) also follows a similar trend of increasing with the addition\u0026ensp;of bentonite however, the different additives still produce distinct mitigating effect. B\u0026ndash;FA swells the\u0026ensp;least, followed by B\u0026ndash;S and B\u0026ndash;MD produces the highest swelling. Stress point Physical and chemical There are physical\u0026ensp;and chemical reasons for the effectiveness of FA against edema. Its\u0026ensp;spherical aggregates fill inter-platelet voids, mitigating water ingress, and any pozzolanic reactions that may occur at longer ages could also inhibit expansive behavior. Inert calcite marble dust would not have the same effect in\u0026ensp;inhibiting the swelling of bentonite and its fines may even contribute to additional water filling, therefore increasing Ċs. From a design standpoint, reduced swelling is beneficial in order to limit cracking and preserve liner integrity, suggesting B\u0026ndash;FA as\u0026ensp;the best option if volume stability is needed [\u003cspan additionalcitationids=\"CR39\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe coefficient of consolidation (Cv) is an indicator\u0026ensp;for predicting pore pressure dissipation due to loading. Cv reduces as the bentonite is replaced with increasing fractions of another, less reactive additive due to reduced permeability, however the relative ranking amongst additives is interesting: B\u0026ndash;FA\u0026thinsp;\u0026gt;\u0026thinsp;B\u0026ndash;MD\u0026ensp;\u0026gt; B\u0026ndash;S. This means that the B\u0026ndash;FA mix emulsions compact not only with more speed, but also they reach in a relatively shorter time their settlement equilibrium\u0026ensp;condition and there are less risk for deformation to late follow. FA mixtures have higher hot\u0026ensp;storage stability coefficient (Cv) due to its microstructural effect since they provide more continuous drainage paths while having a lower MDD. B\u0026ndash;MD, however denser and less permeable,\u0026ensp;still has moderately higher Cv than sand because of granular interlock. Mixtures with sand have the lowest Cv suggesting slower\u0026ensp;consolidation and large long-term pore pressure at retention [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. The faster consolidation is operationally relevant as it translates not only in construction time savings but also into reducing the risk\u0026ensp;of postponed structural adaptation.\u003c/p\u003e \u003cp\u003eStresses and settlements for such loading histories are\u0026ensp;influended by the interactions of compressibility and density. All combinations have settlements that increase with an increment in bentonite content, but B\u0026ndash;MD has the lowest value followed by B\u0026ndash;FA\u0026ensp;and then B\u0026ndash;SA [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The better performance of marble dust was justified\u0026ensp;by its capability of developing high MDD resulting in packed aggregate morphology which contributes to compensate for its relatively higher value of CI. In application, B\u0026ndash;MD is very favorable where restricting the absolute deformation is a concern\u0026ensp;of interest like case of liners under critical construction. The B\u0026ndash;FA values are a bit higher as settled than the MD (but much lower, compared to sand), but it consolidates quicker supporting\u0026ensp;a trade-off.\u003c/p\u003e \u003cp\u003eOnce these agglomeration eects are considered in conjunction with sustainability factors, the evidence for FA and MD becomes more\u0026ensp;compelling still. The traditional sand composition is totally based on non-renewable natural resources, the use of which causes environmental destruction due to river mining, high\u0026ensp;energy and carbon [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. However, FA and MD are copious industrial wastes whose recycling brings a double greening economy by both reducing natural sand consumption and\u0026ensp;waste disposal.\u003c/p\u003e \u003cp\u003eIn short, the communication reveals that although bentonite improves barrier properties its inherent drawbacks of high compressibility and swelling can be overcome by partial replacement of sand\u0026ensp;by fly ash or marble dust. B\u0026ndash;FA possesses the best performance characteristics while presenting\u0026ensp;better compressibility, lower swelling behavior, rapid consolidation and high sustainability results. B\u0026ndash;MD provides the least settlement and excellent sustainability\u0026ensp;which is desirable for cases where the deformation is considered as the major concern. Even though sand-containing bodies have been\u0026ensp;tried in the past, these techniques have still not come up with more effective approaches. Thus, the addition of FA and\u0026ensp;MD in bentonite liners is technically feasible, environmentally acceptable, and future oriented to landfill liner construction.\u003c/p\u003e"},{"header":"5. CONCLUSION","content":"\u003cp\u003eBased on the detailed experimental results, analysis, and the integrated performance\u0026ndash;sustainability assessment of bentonite\u0026ndash;sand (B\u0026ndash;S), bentonite\u0026ndash;fly ash (B\u0026ndash;FA), and bentonite\u0026ndash;marble dust (B\u0026ndash;MD) mixtures, the following major conclusions are drawn:\u003c/p\u003e\n\u003ch3\u003e1. Effect of Bentonite on Compressibility and Swelling\u003c/h3\u003e\n\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eBentonite content is the dominant factor controlling the consolidation behavior of all mixtures. As bentonite content increases from 0% to 100%, the compression index (Cc) increases significantly (e.g., from ~\u0026thinsp;0.07\u0026ndash;0.09 at 0% bentonite to ~\u0026thinsp;1.35\u0026ndash;1.40 at 800 kPa), while the swelling index (Cs) rises from very low values (~\u0026thinsp;0.009\u0026ndash;0.011) to about 0.15 for pure bentonite. This confirms that the highly plastic, expansive nature of bentonite governs compressibility and swelling, irrespective of the additive used.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003e2. Fly Ash Most Effective in Reducing Compressibility and Swelling\u003c/h3\u003e\n\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eAmong the three systems, B\u0026ndash;FA consistently exhibits the lowest Cc and Cs values at equivalent bentonite contents (e.g., at 30\u0026ndash;40% bentonite, Cc\u0026thinsp;\u0026asymp;\u0026thinsp;0.57\u0026ndash;0.61 and Cs\u0026thinsp;\u0026asymp;\u0026thinsp;0.072\u0026ndash;0.073), compared to B\u0026ndash;S and B\u0026ndash;MD. This improved performance is attributed to the spherical morphology and pore-filling ability of fly ash, which restrict clay platelet rearrangement and water-induced expansion. As a result, B\u0026ndash;FA mixtures provide better volume stability and reduced risk of excessive deformation.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003e3. Consolidation Rate Strongly Influenced by Additive Type\u003c/h3\u003e\n\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe coefficient of consolidation (Cv) decreases with increasing bentonite content for all mixtures, falling to about 2.5\u0026ndash;3.2 \u0026times;10⁻⁷ m\u0026sup2;/s at 100% bentonite. However, at all bentonite percentages, B\u0026ndash;FA shows the highest Cv (e.g., 17.5 \u0026times;10⁻⁷ m\u0026sup2;/s at 0% bentonite and ~\u0026thinsp;8.0 \u0026times;10⁻⁷ m\u0026sup2;/s at 40%), followed by B\u0026ndash;MD and then B\u0026ndash;S. This indicates that B\u0026ndash;FA consolidates faster and dissipates pore water pressure more efficiently, offering operational advantages such as shorter consolidation time and improved constructability.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003e4. Settlement Behaviour and Engineering Trade-Offs\u003c/h3\u003e\n\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eSettlement increases markedly with both applied stress and bentonite content. At 800 kPa and 100% bentonite, settlement reaches approximately 26 mm (B\u0026ndash;S), 24.5 mm (B\u0026ndash;FA), and 23 mm (B\u0026ndash;MD). While B\u0026ndash;MD generally shows the lowest absolute settlement due to higher maximum dry density and better particle interlocking, B\u0026ndash;FA offers a balanced response with slightly higher settlement than MD but faster consolidation and lower compressibility. These results highlight the need for optimizing bentonite content (typically\u0026thinsp;\u0026le;\u0026thinsp;30\u0026ndash;40%) to balance low permeability with acceptable deformation performance.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003e5. Sustainability Performance\u003c/h3\u003e\n\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eConsequently, B\u0026ndash;FA and B\u0026ndash;MD mixtures not only meet engineering performance requirements for landfill liners but also support sustainable and environmentally responsible geotechnical infrastructure development.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Abbreviation","content":"\u003cp\u003eBentonite : B, Sand: S, Flyash: FA, Marble dust: MD\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFUNDING STATEMENT\u003c/h2\u003e \u003cp\u003eNot received any funding to perform this study.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eThe whole work means writing, experimentation and analysis done by the author\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAkinsola JA (2022) Laboratory behavior of sand\u0026ndash;bentonite mixtures. 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Buildings 12(5):634. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/buildings12050634\u003c/span\u003e\u003cspan address=\"10.3390/buildings12050634\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"international-journal-of-geo-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"geoe","sideBox":"Learn more about [International Journal of Geo-Engineering](https://link.springer.com/journal/40703)","snPcode":"40703","submissionUrl":"https://submission.nature.com/new-submission/40703/3","title":"International Journal of Geo-Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Landfill, Liners, Consolidation, Compression index, Swelling index, Coefficient of consolidation","lastPublishedDoi":"10.21203/rs.3.rs-8630963/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8630963/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAdhering to the Sustainable Development Goals (SDGs) for clean water and sanitation (SDG 6), sustainable cities and communities (SDG 11) and\u0026ensp;climate action (SDG 13) calls for landfill containment systems that are environmentally robust and resource-effective. Traditional B\u0026ndash;S liners are highly dependent on natural sand (which is a limited resource) and contribute\u0026ensp;to the environmental footprint of waste disposal infrastructure. The compaction characteristics of B\u0026ndash;FA and B\u0026ndash;MD mix are also evaluated in the present work as an environmentally friendly alternative low hydraulic conductivity material to the traditional B\u0026ndash;S liner, since it encourages\u0026ensp;circular use of materials leading to lesser resource extraction. For the purpose of simulating the effects of loading in a landfill setting,\u0026ensp;laboratory consolidation tests were performed on the B\u0026ndash;S, B-FA and B-MD mixtures with different bentonite contents between 0 and 40% at intervals of 5%. Key consolidation parameters such as compression\u0026ensp;index, swelling index, coefficient of consolidation and settlement behaviour were evaluated on a systematic basis to assess the compressibility, stability and suitability for long-term use of each liner system. The results show that industrial wastes including FA and MD have a significant potential to improve the\u0026ensp;consolidation of bentonite-based liners and, therefore, achieve the same or superior consolidation parameters as traditional sand mixtures but with less use of virgin materials. The comparison demonstrates preferred\u0026ensp;mixes which promote integrity, controlled settling and low yield that afford for liner reliability. The inclusion of industrial waste materials in engineered landfill liners serves for groundwater protection and leachate containment (SDG 6), contributes to\u0026ensp;sustainable urban waste management infrastructure (SDG 11) and material engineering adapted for climate through waste valorization with lower GHG emission values towards carbon intensity reduction (SDG 13). These results offer a defensible basis on which to develop sustainable EBSs in support\u0026ensp;of the Sustainable Development goals.\u003c/p\u003e","manuscriptTitle":"Engineering Landfill Liners for SDGs 6, 11, and 13: Comparative Consolidation Performance of Bentonite–Sand, Bentonite–Fly Ash, and Bentonite–Marble Dust Mixtures","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-05 10:09:39","doi":"10.21203/rs.3.rs-8630963/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-04-12T11:47:36+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-11T17:10:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"276149012012291146638959250560551242941","date":"2026-03-31T10:03:41+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-11T08:01:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"211212119775986982562292797368390134773","date":"2026-03-09T09:09:02+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"318231379972140378886123635107160254297","date":"2026-02-03T14:19:13+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-03T14:01:46+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-26T05:28:05+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-26T05:27:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"International Journal of Geo-Engineering","date":"2026-01-18T11:30:31+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"international-journal-of-geo-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"geoe","sideBox":"Learn more about [International Journal of Geo-Engineering](https://link.springer.com/journal/40703)","snPcode":"40703","submissionUrl":"https://submission.nature.com/new-submission/40703/3","title":"International Journal of Geo-Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"da8fe00f-498a-424e-a32a-08febccc7f25","owner":[],"postedDate":"February 5th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-02-05T10:09:40+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-05 10:09:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8630963","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8630963","identity":"rs-8630963","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}