Seasonal Rainfall Effects on Shear Strength and CBR in Volcanic Ash Soils

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Samples were collected during low, intermediate, and high rainfall periods and tested for water content , plasticity , unconfined compressive strength , and California Bearing Ratio (CBR) . Results show that moisture content and Atterberg limits increase with rainfall, while CBR decreases exponentially , with values dropping by up to 42% in wet conditions. An empirical correlation (R² = 0.88) between water content and CBR was developed. Unconfined strength also decreased by 43% in dry conditions. Pavement reliability (Zr) declined significantly under saturated conditions, emphasizing the importance of accounting for seasonal variability in design. The findings highlight the need for drainage solutions , conservative designs , and site-specific assessments when building on volcanic ash soils. Seasonal rainfall variability volcanic ash soils Shear strength California Bearing Ratio (CBR) Pavement Reliability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1 INTRODUCTION Geotechnical engineering involves the identification of the physical and geological properties of soil layers for the design of foundations, earth-retaining structures, and the assessment of slope stability. Characterization of soils through laboratory and field testing allows engineers to evaluate mechanical behavior and detect problematic soils, such as those that are collapsible, liquefiable, or expansive. Among these, volcanic soils are often considered particularly challenging due to their high compressibility and susceptibility to moisture-related changes. Volcanic ash soils are formed from weathered volcanic materials transported by wind and influenced by local geomorphology. These soils cover approximately 0.84% of the Earth's surface, with about 60% located in tropical regions(Betancur et al., 2013 ; Nanzyo et al., 1993 ) This distribution often coincides with areas of high population density and significant economic development(Arnalds et al., 2007 ; Lizcano et al., 2006 ; Picarelli et al., 2007 ). In Colombia, volcanic ash soils occupy roughly 11.6% of the national territory, with notable concentrations in Eastern Antioquia and the southern Aburrá Valley (Rendón et al., 2020 ). These soils typically exhibit high natural water content, elevated liquid limits, low unit weights, and high void ratios. As a result, they are associated with a range of engineering challenges, including erosion, compressibility, collapsibility, slope instability, liquefaction, and inadequate compaction behavior (Lizcano et al., 2006 ; Matsumura et al., 2015 ; Picarelli et al., 2007 ; Terlien, 1997 ). The unique behavior of volcanic ash soils is strongly influenced by their geologic origins and the pronounced wet and dry seasonal cycles typical of tropical climates. Since rainfall can substantially alter the moisture content of these soils, understanding how seasonal variations affect their geotechnical properties is critical for reliable infrastructure design. Recent research has also demonstrated how heavy rainfall significantly affects the moisture dynamics and resilient modulus of pavement foundation layers, underlining the importance of accounting for seasonal hydrological cycles in pavement design (Jibon et al., 2024 ) This study is a continuation of previously published research titled "Chemical, Mineralogical and Geotechnical Index Properties Characterization of Volcanic Ash Soils" by Rendón et al., ( 2020 ), which established a comprehensive baseline of the soil’s physical and compositional characteristics. During post-publication analysis, significant variations were observed in the California Bearing Ratio (CBR) and shear strength values of samples obtained from the same site but at different times of the year. A subsequent review of rainfall data revealed that the sampling campaigns coincided with distinct precipitation conditions: approximately 100 mm in April 2016 (A1, intermediate rainfall), 10 mm in February 2017 (A2, low rainfall), and 192 mm in May 2017 (A3, high rainfall). These differences, initially detected by chance, suggest a strong correlation between seasonal moisture content and soil mechanical behavior. Accordingly, this paper investigates the influence of accumulated rainfall on the shear strength and CBR values of volcanic ash soils. The findings underscore the importance of considering seasonal climatic variations in geotechnical assessments, particularly when working with water content - sensitive materials such as volcanic ash. 2 VOLCANIC ASH DESCRIPTION Andesitic eruptions from stratovolcanoes produce significant quantities of tephra, a term used to describe all solid volcanic materials expelled during an eruption. Tephra includes pyroclastic fragments such as ash, lapilli, and volcanic fragments (Hermelín, 1984 ; Toro & Hermelín, 2012 ). Among these, volcanic ash is the finest component and is transported primarily by wind, leading to its classification as an aeolian (wind-borne) deposit (Lizcano et al., 2006 ). These deposits form volcanic ash layers with considerable thickness variability—ranging from a few centimeters to several meters—and typically exhibit minimal internal structure. The morphology and texture of volcanic ash particles undergo significant alteration during their expulsion, transport, and deposition (Hürlimann et al., 2001 ). Finer particles, depending on characteristics such as shape, sphericity, surface roughness, and specific surface area, may be deposited at distances of hundreds of kilometers from their volcanic source (Betancur et al., 2013 ; Hürlimann et al., 2001 ). This deposition process is geologically rapid, with the transformation from eruption to soil genesis occurring in less than 20,000 years (Toro & Hermelín, 2012 ). The mineralogical composition of volcanic ash is typically dominated by silicates, volcanic glass, crystals, and lithic fragments, followed by feldspars, quartz, hornblende, hypersthene, augite, and magnetite in decreasing order of abundance (Hürlimann et al., 2001 ; Lizcano et al., 2006 ; Verdugo, 2008 ). Once deposited, volcanic ash undergoes weathering due to geophysical and environmental factors, resulting in the dissolution of primary minerals and the formation of secondary clay minerals. This process leads to the formation of volcanic ash soils, characterized by the presence of minerals such as allophane, halloysite, imogolite, and montmorillonite (García-Leal & Colmenares, 2011 ; Hürlimann et al., 2001 ; Lizcano et al., 2006 ; Verdugo, 2008 ; Wesley, 1973 ). These distinctive clay minerals confer unique chemical and physical properties to volcanic ash soils, differentiating them from other sedimentary soils. In the department of Antioquia (Colombia), volcanic ash soils are believed to originate from the Ruiz–Tolima volcanic massif, which includes volcanoes such as Cerro Bravo, Nevado del Ruiz, Olleta, Santa Isabel, Tolima, and Machín. While erosional processes have removed these deposits from steep-sloped areas, they remain well-preserved in regions with gentler slopes (Hermelín, 1984 ). 2.1 Geotechnical Characterization The geotechnical characterization of volcanic ash soils involves determining their index and mechanical properties through field and laboratory testing. These results allow engineers to assess soil behavior, which is often complex due to the unique origin and mineral composition of volcanic materials. Volcanic ash soils are known for their high liquid limit (LL) values, which increase with water content. However, their plasticity index (PI) is lower than that of sedimentary clays with similar LL values (Gonzalez et al., 1981 ). This is attributed to the dominance of amorphous minerals such as allophane and imogolite, commonly found in the clay fraction of these soils (Lizcano et al., 2006 ; Molina et al., 2012 ; Rao, 1996 ) In the previous study by (Rendón et al., 2020 ), volcanic ash samples collected in Caldas, Antioquia, were subjected to a series of index tests. The results showed: Water content (w): Ranging from 103–205%, with an average of 156%. Liquid limit (LL): Between 158% and 257% (avg. 204%). Plastic limit (PL): Between 127% and 184% (avg. 150%). Void ratio (e): Ranging from 3 to 7, with an average of 5. Dry unit weight (γ d ): Between 0.36 and 0.62 g/cm³, indicating extremely loose soils. Bulk unit weight (γ): Ranging from 1.02 to 1.23 g/cm³. Saturation (S_r): Between 58% and 87%, with an average of 78%. These soils were classified as high plasticity silts (MH) using the Unified Soil Classification System (USCS), with over 70% passing the No. 200 sieve. Their particle size distribution showed low variability and a predominance of fine particles smaller than 0.075 mm. The SEM analysis revealed a weakly cemented microstructure with high porosity, which helps explain the relatively low shear strength observed. X-ray diffraction (XRD) and fluorescence (XRF) tests confirmed the presence of quartz, tridymite, and brucite, along with high concentrations of SiO₂ and Al₂O₃, characteristic of aluminum-silicate minerals. Notably, natural water contents exceeded the liquid limit in many cases, placing the soils in a near-liquid or plastic state under field conditions. This sensitivity to moisture leads to strong drying hysteresis, meaning Atterberg limits can vary dramatically depending on test conditions. Drying (air or oven) was shown to reduce plasticity, confirming findings from previous studies (Lizcano et al., 2006 ; Verdugo, 2008 ). Although some samples showed moderate shear strength in their undisturbed state, this was lost upon remolding, as the structure collapsed and plastic behavior became dominant (Betancur et al., 2013 ; Gonzalez et al., 1981 ). Furthermore, SPT N-values were consistently low—between 4 and 8—reinforcing the classification of these soils as very loose and highly problematic for engineering applications. In summary, the previous study confirmed that volcanic ash soils in the Aburrá Valley present high variability in water-sensitive properties, strong moisture dependency, and significant microstructural fragility. These characteristics warrant a deeper exploration of how seasonal changes in moisture, particularly due to rainfall, affect their mechanical performance—an issue addressed in the current research. 3 SITE DESCRIPTION The characterization of the volcanic ash soils was carried out for soil samples taken in the municipality of Caldas, Antioquia, at 22 km south of the city of Medellin, as shown in Fig. 1 . The site is located at an approximate altitude of 1.750m above sea level and has an average temperature of 19°C throughout the year. 3.1 Volcanic ash characteristics in situ At the study site, volcanic ash soils were found to extend from the surface down to approximately 1.5 meters in depth. These deposits were observed beneath a thin surficial layer (approximately 0.1 meters thick) composed of dark soil with high organic matter content. The volcanic ash layer was underlain by a reddish residual soil mantle, indicative of more weathered and older geological materials as is shown in Fig. 2 . The stratigraphy at the site reflects a typical profile of air-fall volcanic ash deposits, where the upper organic-rich layer likely formed due to vegetation accumulation over time, while the underlying residual soil marks the transition to more stable and less transport-influenced horizons. The volcanic ash horizon itself was visually homogeneous, with a light gray to whitish coloration and loose consistency. This configuration confirms that the studied volcanic ash soils are relatively recent in geological terms, loosely packed, and susceptible to environmental changes—particularly fluctuations in water content. Their position within the soil profile also supports their classification as surface deposits, prone to direct climatic and hydrological interactions, which are central to the present study. 4 Rainfall records To investigate the influence of seasonal rainfall on the geotechnical behavior of volcanic ash soils, rainfall data were obtained from two meteorological stations—Stations 57 and 58—managed by the Aburrá Valley Early Warning System (SIATA, by its Spanish acronym). These stations are in proximity to the sampling site, as shown in Fig. 3 . Three sampling campaigns were conducted at different times of the year, each representing a distinct level of accumulated rainfall. The sampling events were categorized based on total precipitation over the days leading up to the sampling as follows: A1 (intermediate rainfall), A2 (low rainfall), and A3 (high rainfall). These conditions are illustrated in Fig. 4 , and the characteristics of each sampling campaign are detailed below: Sampling Area A1 – April 5th, 2016 (Intermediate Rainfall) : A total of 20 disturbed samples were collected in plastic bags from a test pit approximately 2.0 m × 2.0 m in size at a depth of 0.6 m. The accumulated rainfall during the preceding days was approximately 100 mm. Sampling Area A2 – February 14th, 2017 (Low Rainfall) : Samples were taken from an area of 4.0 m × 2.0 m at depths ranging from 0.5 m to 0.7 m. This campaign included 7 disturbed samples (in plastic bags) and 9 undisturbed samples (6 Shelby tubes and 3 CBR molds). The accumulated rainfall was approximately 10 mm. Sampling Area A3 – May 3rd, 2017 (High Rainfall) : Samples were collected from a 2.0 m × 1.5 m area at similar depths (0.5–0.7 m). The set included 10 disturbed samples and 18 undisturbed samples (12 Shelby tubes and 6 CBR molds). The accumulated rainfall prior to sampling was approximately 192 mm. These three sampling conditions provided a representative dataset of the soil’s behavior under different moisture regimes. This variability allowed for evaluating how short-term accumulated rainfall affects critical geotechnical parameters such as shear strength and California Bearing Ratio (CBR) . 5 GEOTECHNICAL ANALYSIS The index properties and chemical characteristics of the volcanic ash soils under investigation have been previously documented by (Rendón et al., 2020 ). Building upon that foundation, this study evaluates whether short-term variations in rainfall significantly influence key geotechnical properties—particularly water content, shear strength, and CBR—by comparing data collected during three distinct sampling periods (A1, A2, and A3), each associated with different accumulated rainfall conditions. 5.1 Water content To assess the sensitivity of volcanic ash soils to climatic variations, water content was analyzed across the three sampling periods representing intermediate (A1), low (A2), and high (A3) rainfall conditions. A boxplot was constructed to visualize the variability and central tendency of the water content for each sampling event (Fig. 5 ). The results demonstrate a clear correlation between accumulated rainfall and the natural water content of the samples. The lowest water content values correspond to the A2 sampling period, which followed the driest conditions (approximately 10 mm of rainfall). Conversely, the highest water contents were recorded during the A3 period, which followed the most intense rainfall (approximately 192 mm). The A1 campaign, under intermediate rainfall conditions (~ 100 mm), showed water content values between those of A2 and A3. These findings reinforce the hypothesis that volcanic ash soils, due to their high porosity, low dry density, and open microstructure, are highly susceptible to environmental moisture changes. As a superficial deposit, volcanic ash rapidly absorbs or loses moisture depending on ambient humidity and precipitation, resulting in significant short-term variability in geotechnical properties such as water content. 5.2 Grained size distribution A total of 25 samples were classified using the Unified Soil Classification System (USCS) following (ASTM D2487, 2011 ). The particle-size distribution curves for these samples are presented in Fig. 6 . The results show a consistent gradation pattern across all three sampling campaigns (A1, A2, and A3), regardless of the rainfall conditions preceding each collection. The analysis revealed that all samples consisted predominantly of fine-grained soils , with particle sizes smaller than 0.425 mm and more than 70% passing the No. 200 sieve (0.075 mm) . These characteristics are consistent with typical volcanic ash soils, which tend to have high proportions of silt-sized and clay-sized particles due to their pyroclastic origin. Importantly, no significant differences were observed in the grain size distribution among the samples collected during different rainfall conditions. This suggests that seasonal variations in rainfall do not affect the fundamental particle-size composition of the volcanic ash soil. Therefore, the variability in geotechnical behavior observed across the sampling periods is not attributed to changes in soil texture or classification, but rather to water content-dependent properties , such as water content, shear strength, and compaction response. 5.3 Liquid limit (LL) analyses The liquid limit (LL) and plasticity index (PI) values obtained from the samples across the three sampling periods were plotted on a Plasticity Chart , as shown in Fig. 7 . All data points fall below the A-line , consistent with the behavior of volcanic ash soils reported by So ( 1998 ) and later confirmed by Lizcano et al. ( 2006 ), indicating a classification as high-plasticity silts (MH) . Despite their shared classification, the LL and PI values showed notable variation among the three rainfall periods. The samples collected during the high rainfall season (A3) exhibited the highest liquid and plastic limits, with LL values approaching 250%. This increase can be attributed to the high degree of saturation resulting from sustained precipitation and the inherent water retention capacity of the volcanic ash due to its porous microstructure and clay mineral composition. In contrast, during the low rainfall season (A2), the LL values were generally lower, averaging around 190%, indicating reduced saturation. Although this is still a high value by conventional standards, it reflects the persistent ability of volcanic ash soils to retain moisture even during drier conditions. Interestingly, the intermediate rainfall season (A1) showed a wider spread in LL and PI values. This variability may result from non-uniform saturation levels within the soil profile at the time of sampling. Because volcanic ash soils are highly sensitive to moisture changes, even slight differences in water content can influence Atterberg limits significantly, especially when samples are taken from zones with varying micro-drainage or surface exposure. These results highlight the plasticity behavior of volcanic ash soils as strongly dependent on seasonal rainfall, reinforcing the need to consider environmental conditions—particularly soil moisture state at the time of testing—when interpreting or comparing plasticity-based soil classifications for volcanic ash materials. 5.4 Unconfined compressive strength Unconfined compressive strength (UCS) tests were conducted to evaluate the undrained shear strength (qu) of the volcanic ash soils during two of the three sampling periods. Unfortunately, no UCS tests were performed for the intermediate rainfall period (A1). However, reliable measurements were obtained during the low (A2) and high (A3) rainfall seasons, as summarized in Fig. 8 and Tables 3 and 4. Table 1 Summary of unconfined compressive strength results for A2 (low rainfall) and A3 (high rainfall) w (%) ϒ (g/cm3) ϒd (g/cm3) e q u (kPa) S u (kPa) A2 Average 112.35 1.13 0.53 4 31.83 15.91 SD 5.97 0.06 0.04 0.38 10.66 5.33 CV (%) 5 6 7 10 33 33 A3 Average 183.18 1.10 0.39 5 55.86 27.93 SD 8.47 0.03 0.02 0.3 23.81 11.90 CV (%) 5 3 5 5 43 43 The results indicate a marked influence of seasonal moisture conditions on the compressive strength of the soil. During the high rainfall season (A3) , the average undrained compressive strength reached 55.86 kPa , while during the low rainfall season (A2) , it averaged only 31.86 kPa . This represents a reduction of approximately 43% in undrained shear strength during the dry season. This reduction is likely attributed to desiccation processes that occur during drier periods, which cause a breakdown of the soil's microstructure and reduce its apparent cohesion. As a result, the soil's shear strength decreases , which can significantly affect its bearing capacity , especially for shallow foundations. These findings reinforce the critical importance of seasonal timing in geotechnical investigations , particularly in volcanic ash soils where moisture content plays a dominant role in strength development. Designing foundations without accounting for this variability could lead to unconservative assessments of load-bearing performance , especially during the dry season when strength is at its minimum. 5.5 California Bearing Ratio (CBR) CBR tests were performed to assess the bearing capacity of volcanic ash soils under natural moisture conditions. Interestingly, the results show an inverse trend compared to the unconfined compressive strength findings. As illustrated in Fig. 9 , the CBR values were higher during the dry season (A2) than during the rainy season (A3). Specifically, A2 samples (low rainfall) exhibited an average CBR of 2.4% , while A3 samples (high rainfall) showed a significantly lower average of 1.4% . This reflects a 42% reduction in CBR under high rainfall conditions , highlighting the substantial impact of water content on the bearing performance of the soil. Table 2 Summary of CBR results for A2 and A3. w (%) CBR (%) ϒ (g/cm3) ϒd (g/cm3) e A2 Average 116.5 2.4 1.03 0.48 4.4 SD 7.4 0.2 0.11 0.06 0.7 CV (%) 6 7 11 12 15 A3 Average 192.8 1.4 0.98 0.34 6.5 SD 14.3 0.1 0.04 0.03 0.6 CV (%) 7.4 7.6 4.02 8.70 9.8 These results highlight critical behavior of volcanic ash soils: increased water content significantly decreases their CBR , reducing their ability to support pavement and light foundation loads. This partially explains why pavement damage and saturation failures are common in regions with volcanic soils during prolonged rainy periods. 5.5.1 CBR Results Under Immersion To further investigate the effect of saturation on CBR, additional tests were conducted on pre-saturated (fully immersed) samples for both A2 and A3. These tests aimed to simulate worst-case field conditions such as prolonged flooding or capillary rise. The results show that the impact of immersion is more pronounced for A2 (dry season samples) than for A3 (rainy season samples), as shown in Figs. 10 and 11 . This is attributed to soil suction effects : in A2, the drier soil structure retains higher matric suction, and immersion leads to a significant strength reduction. In contrast, A3 samples were already close to saturation, so immersion had a lesser additional effect. These findings emphasize the sensitivity of volcanic ash soils to both natural and induced water content variations , reinforcing the importance of considering moisture state—not just classification—when designing pavements or shallow foundations on such soils. 6 Discussion The index properties determined in this study confirm that soils derived from volcanic ash are generally unsuitable for the design and construction of civil works based on conventional geotechnical standards. Their high porosity, elevated natural water content, low dry unit weight, and mineralogical composition make them particularly challenging for engineering applications , even though these same properties contribute to their agronomic productivity (Shoji et al., 1993 ) However, in the context of pavement design , these materials are often unavoidable, and thus, understanding their behavior under seasonal rainfall variability becomes essential. As shown in Fig. 12 , rainfall regime directly affects soil saturation , which in turn significantly influences CBR performance . An empirical relationship between water content and CBR was derived using the data from this study, resulting in a high correlation ( R² = 0.88 ), as expressed in Eq. 1 . Equation 1. CBR(%) = 5.014 e- 0.007w(%) R 2 = 0.88 This relationship highlights the exponential decay of bearing capacity with increasing water content , reinforcing the importance of timing in sample collection . Soils sampled during dry seasons can lead to overestimation of CBR values , potentially resulting in underdesigned pavement structures if the subgrade later becomes saturated. Furthermore, the findings suggest that the commonly recommended 4-day immersion protocol in laboratory testing may be insufficient to simulate field conditions under prolonged rainfall or extreme saturation scenarios. This finding aligns with broader observations in the pavement engineering literature, where heavy rainfall has been shown to significantly alter subgrade moisture profiles and base layer stiffness , thereby reducing structural performance under load (Jibon et al., 2024 ) To further explore the engineering implications, an evaluation of pavement reliability (Zr) under varying rainfall conditions was performed. In the AASHTO flexible pavement design method, the parameters Zr (reliability index) and So (standard deviation) reflect the confidence required for the design. For instance, major highways demand higher reliability levels (e.g., Zr = 99.9%), whereas local roads can tolerate lower reliability (e.g., Zr = 90%). An illustrative example was developed using initial reliability levels of Zr = 99.9%, 99.0%, and 90.0% . The CBR values were varied to reflect natural, dry, and fully saturated conditions, while keeping the structural pavement section constant. The goal was to determine the equivalent reliability under each scenario. As depicted in Fig. 13 , reliability decreased as CBR decreased due to rainfall-induced saturation . However, this impact was less severe in cases where the initial design reliability was high, demonstrating that high Zr values can buffer against seasonal moisture uncertainty , regardless of road classification. These findings support the incorporation of drainage elements into pavement designs over volcanic ash soils. However, under highly saturated conditions, pavement reliability may fall below acceptable thresholds , indicating the need for conservative design assumptions , moisture control strategies, and, where appropriate, soil stabilization techniques . Lastly, the complex behavior of volcanic ash soils underscores the need for a comprehensive understanding of their formation, depositional environment, and post-depositional weathering . The mechanical properties are not only governed by their mineralogical content but also by ongoing transformation processes that vary significantly across regions and climatic conditions. These nuances must be addressed when interpreting laboratory results and applying them to real-world geotechnical designs. 7 Conclusions This study evaluated the influence of seasonal rainfall on the geotechnical behavior of volcanic ash soils, focusing on critical parameters such as water content , Atterberg limits , unconfined compressive strength , and California Bearing Ratio (CBR) . The results highlight the extreme sensitivity of volcanic ash soils to moisture variation , making them highly problematic yet frequently encountered in infrastructure development, particularly in tropical volcanic regions. The main conclusions are summarized as follows: Volcanic ash soil exhibits high porosity, low dry unit weight, and extreme water retention capacity , which leads to significant seasonal fluctuations in geotechnical properties. Water content and plasticity parameters (LL and PI) increased substantially during high rainfall periods. This variability must be accounted for, as plasticity values near or above the liquid limit can affect soil behavior even before saturation. Unconfined compressive strength showed a 43% reduction from rainy to dry conditions , indicating the mechanical vulnerability of these soils under desiccation or low-moisture states due to microstructural changes. CBR values decreased exponentially with increasing water content , as captured by a high-correlation empirical model (R² = 0.88). Soils in natural saturated conditions (A3) showed CBR values nearly 42% lower than in dry conditions (A2), highlighting the risk of underperformance when rainfall is not properly considered. Standard 4-day immersion procedures may not accurately simulate real field saturation in volcanic soils. The test results suggest that the typical laboratory preparation may underestimate the long-term impacts of wet seasons, especially on pavement performance. Pavement reliability (Zr) is significantly reduced under lower CBR values associated with rainfall , but this effect can be partially mitigated by designing with higher initial reliability levels, regardless of road classification. The use of drainage solutions, moisture control, and conservative design assumptions are strongly recommended when working with volcanic ash soils. In some cases, stabilization or improvement techniques may be necessary to ensure long-term performance. In conclusion, volcanic ash soils present complex behavior governed not only by their mineralogical composition but also by environmental factors such as rainfall and saturation cycles . Their mechanical performance must be evaluated in a site-specific and time-sensitive manner, as seasonal variability has a direct impact on both strength and reliability . These findings contribute to a more resilient approach to geotechnical design in volcanic regions, particularly for pavement and shallow foundation systems Declarations Author Contribution M. Rendón conducted the field and laboratory testing, performed the initial calculations, and contributed to the previous related publication. J.C.V. carried out the statistical analyses, identified the influence of seasonal rainfall on the results, and wrote the manuscript. Both authors reviewed and approved the final version of the manuscript. 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Elsevier. https://doi.org/10.1016/S0166-2481(08)70268-X Picarelli, L., Evangelista, A., Rolandi, G., Paone, A., Nicotera, M. V., Olivares, L., Scotto Di Santolo, A., Lampitiello, S., & Rolandi, M. (2007). Mechanical properties of pyroclastic soils in Campania Region. In K. K. Phoon, D. W. Hight, S. Leroueil, & T. S. Tan (Eds.), Characterisation and Engineering Properties of Natural Soils (Vol. 4, pp. 2331–2383). Taylor & Francis/Balkema. https://doi.org/10.1201/NOE0415426916.ch18 Rao, S. M. (1996). Role of apparent cohesion in the stability of Dominican allophane soil slopes. Engineering Geology , 43 , 265–279. https://doi.org/10.1016/S0013-7952(96)00036-1 Rendón, M. I., Viviescas, J. C., Osorio, J. P., & Hernández, M. S. (2020). Chemical, Mineralogical and Geotechnical Index Properties Characterization of Volcanic Ash Soils. Geotechnical and Geological Engineering . https://doi.org/10.1007/s10706-020-01219-3 Shoji, S., Dahlgren, R., & Nanzyo, M. (1993). Terminology, Concepts and Geographic Distribution of Volcanic Ash Soils. In Volcanic Ash Soils. Genesis, Properties and Utilization. (1st ed., pp. 1–5). Elsevier Science. https://doi.org/10.1016/S0166-2481(08)70262-9 So, E.-K. (1998). Statistical Correlation Between Allophane Content and Index Properties for Volcanic Cohesive Soil. Soils and Foundations , 38 (4), 85–93. Terlien, M. T. J. (1997). Hydrological landslide triggering in ash-covered slopes of Manizales (Colombia). Geomorphology , 20 , 165–175. https://doi.org/10.1016/S0169-555X(97)00022-6 Toro, G., & Hermelín, M. (2012). Tefraestratigrafía Colombiana. Revista Universidad Eafit , 86 , 81–84. Verdugo, R. (2008). Singularities of Geotechnical Properties of Complex Soils in Seismic Regions. Journal of Geotechnical and Geoenvironmental Engineering , 134 (7), 982–992. https://doi.org/10.1061/(ASCE)1090-0241(2008)134:7(982) Wesley, L. D. (1973). Some basic engineering properties of halloysite and allophane clays in Java, Indonesia. Géotechnique , 23 (4), 471–494. https://doi.org/10.1680/geot.1973.23.4.471 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-7511273","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":510409199,"identity":"bdebb18a-8870-4a8d-b740-d116453e068d","order_by":0,"name":"Juan Camilo Viviescas","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABCklEQVRIiWNgGAWjYBACxgYwJcHABuHbMDAcANFsxGtJI6wFHRwmrIV5RvKzB4xtFol97O0PP/z4cz6x73jzA4YPZYcZ+NsbsDtsRpq5AWObRGIbzxljyd6224kzzxwzYJxx7jCDxJkDOLQkmEkwnJEwZpPIYWPgbbiduOFGggEzb9thBgOJBBxa0r9BtMg/f8b458+5xA33n39g/otXSw7QlgoJOTYJBjNmHrYDQFt4DJgZ8WnpeVMmkQDSwpNjLC3blmw880xOwcGec+k8uPxi2J6+TeKDQR2PfPvxhx/f/LGT7Tt+fOODH2XWcrhCzBAkjOEAkPE8WNUDgTwuiVEwCkbBKBgFcAAA+31dQLvkDgAAAAAASUVORK5CYII=","orcid":"","institution":"","correspondingAuthor":true,"prefix":"","firstName":"Juan","middleName":"Camilo","lastName":"Viviescas","suffix":""},{"id":510409201,"identity":"336b0b7b-14a4-47c3-9eb7-23b3745bd9a1","order_by":1,"name":"Maria Isabel Rendon","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Maria","middleName":"Isabel","lastName":"Rendon","suffix":""}],"badges":[],"createdAt":"2025-09-01 19:38:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7511273/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7511273/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":91115965,"identity":"6322a00c-7e3a-4c0c-99f1-7810a021714f","added_by":"auto","created_at":"2025-09-11 17:35:40","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1089658,"visible":true,"origin":"","legend":"\u003cp\u003eLocation of the sampling site in Caldas, Antioquia. \u003cbr\u003e\nRetrieved from Google Earth 2017.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/bae65cd33940ea2d36c0f4ef.png"},{"id":91115182,"identity":"7ee58ca8-774b-4f1c-936c-a883072d94c7","added_by":"auto","created_at":"2025-09-11 17:19:40","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1734143,"visible":true,"origin":"","legend":"\u003cp\u003eVolcanic ash in situ sample photographs.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/054b9288633e5c02a008ca9d.png"},{"id":91116814,"identity":"7bd037d3-ac3e-4a29-8450-41a4c94df941","added_by":"auto","created_at":"2025-09-11 17:51:40","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1160538,"visible":true,"origin":"","legend":"\u003cp\u003eNearest rain gauge stations from the sampling zone.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/ff48b000760ae1423ca66c1c.png"},{"id":91115800,"identity":"000efab4-93fa-45b2-a324-de06b2aaa6e5","added_by":"auto","created_at":"2025-09-11 17:27:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":95898,"visible":true,"origin":"","legend":"\u003cp\u003eRainfall records of stations 57 and 58, since Jan. 2016 to May. 2017 Data processed of SIATA. Retrieved from: \u003ca href=\"http://siata.gov.co:8018/descarga_siata/index.php/index2/\"\u003ehttp://siata.gov.co:8018/descarga_siata/index.php/index2/\u003c/a\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/fbaefff0c4491a86f2fdc852.png"},{"id":91115176,"identity":"335feb3e-54b7-4ff7-a849-15fc1accc7a7","added_by":"auto","created_at":"2025-09-11 17:19:40","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":20542,"visible":true,"origin":"","legend":"\u003cp\u003eNatural water content w (%) box plot for each sample time\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/01175d563710269ae1524a35.png"},{"id":91115181,"identity":"f04da803-47dc-46fb-b0b3-24fc5a59c092","added_by":"auto","created_at":"2025-09-11 17:19:40","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":233850,"visible":true,"origin":"","legend":"\u003cp\u003eParticle-size distribution curves taken from Rendón et al. (2020)\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/ebc57db09729ed203114e30e.png"},{"id":91115187,"identity":"107879a5-3b3d-40b8-ba46-36202d737bba","added_by":"auto","created_at":"2025-09-11 17:19:40","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":108300,"visible":true,"origin":"","legend":"\u003cp\u003ePlasticity chart for volcanic ash samples from different rainfall conditions.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/e172405200a3eeb2b690e261.png"},{"id":91115191,"identity":"1f6abc41-7210-4100-a89d-c36dd6d46172","added_by":"auto","created_at":"2025-09-11 17:19:40","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":184448,"visible":true,"origin":"","legend":"\u003cp\u003eUnconfined compressive strength results for A2 and A3 periods\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/8bbab4f02f958f1b032bd639.png"},{"id":91115803,"identity":"06ffef65-37a7-4653-967a-fecce862c8e9","added_by":"auto","created_at":"2025-09-11 17:27:40","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":119844,"visible":true,"origin":"","legend":"\u003cp\u003eCBR results under natural conditions for sampling periods A2 and A3\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/7fa7fb5b59a74293c0a971eb.png"},{"id":91115194,"identity":"d1c4b3c7-ebbc-4c8d-9a38-d901524321ff","added_by":"auto","created_at":"2025-09-11 17:19:40","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":172417,"visible":true,"origin":"","legend":"\u003cp\u003eCBR comparison between natural and fully saturated conditions (A2).\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/5fa7b8f84fc79a147d0766c5.png"},{"id":91115807,"identity":"3f75d8d3-a84f-40e9-a3d4-f72d1a27e971","added_by":"auto","created_at":"2025-09-11 17:27:40","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":223403,"visible":true,"origin":"","legend":"\u003cp\u003eCBR comparison between natural and fully saturated conditions (A3).\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/7203da3bf08fc46c21237817.png"},{"id":91115189,"identity":"3913bbc1-e00d-402b-9f78-8ff4263b846e","added_by":"auto","created_at":"2025-09-11 17:19:40","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":70577,"visible":true,"origin":"","legend":"\u003cp\u003eCBR versus water content according to rainfall regime and saturation state.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/95e004148646884d5cc023fa.png"},{"id":91115811,"identity":"e9315578-14f5-4572-8dcc-153611246724","added_by":"auto","created_at":"2025-09-11 17:27:40","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":75286,"visible":true,"origin":"","legend":"\u003cp\u003eReliability index (Zr) reduction as a function of rainfall regime and corresponding CBR values.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/402ba80f1c3e5ad74a3575db.png"},{"id":94989244,"identity":"9a9d14b2-e05a-4578-8334-9f14bef0e9e5","added_by":"auto","created_at":"2025-11-03 07:12:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7869221,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7511273/v1/a2999137-2859-4aeb-983b-783ab8a3c963.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Seasonal Rainfall Effects on Shear Strength and CBR in Volcanic Ash Soils","fulltext":[{"header":"1 INTRODUCTION","content":"\u003cp\u003eGeotechnical engineering involves the identification of the physical and geological properties of soil layers for the design of foundations, earth-retaining structures, and the assessment of slope stability. Characterization of soils through laboratory and field testing allows engineers to evaluate mechanical behavior and detect problematic soils, such as those that are collapsible, liquefiable, or expansive. Among these, volcanic soils are often considered particularly challenging due to their high compressibility and susceptibility to moisture-related changes.\u003c/p\u003e\u003cp\u003eVolcanic ash soils are formed from weathered volcanic materials transported by wind and influenced by local geomorphology. These soils cover approximately 0.84% of the Earth's surface, with about 60% located in tropical regions(Betancur et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Nanzyo et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1993\u003c/span\u003e) This distribution often coincides with areas of high population density and significant economic development(Arnalds et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Lizcano et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Picarelli et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). In Colombia, volcanic ash soils occupy roughly 11.6% of the national territory, with notable concentrations in Eastern Antioquia and the southern Aburr\u0026aacute; Valley (Rend\u0026oacute;n et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThese soils typically exhibit high natural water content, elevated liquid limits, low unit weights, and high void ratios. As a result, they are associated with a range of engineering challenges, including erosion, compressibility, collapsibility, slope instability, liquefaction, and inadequate compaction behavior (Lizcano et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Matsumura et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Picarelli et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Terlien, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1997\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe unique behavior of volcanic ash soils is strongly influenced by their geologic origins and the pronounced wet and dry seasonal cycles typical of tropical climates. Since rainfall can substantially alter the moisture content of these soils, understanding how seasonal variations affect their geotechnical properties is critical for reliable infrastructure design. Recent research has also demonstrated how heavy rainfall significantly affects the moisture dynamics and resilient modulus of pavement foundation layers, underlining the importance of accounting for seasonal hydrological cycles in pavement design (Jibon et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2024\u003c/span\u003e)\u003c/p\u003e\u003cp\u003eThis study is a continuation of previously published research titled \u003cem\u003e\"Chemical, Mineralogical and Geotechnical Index Properties Characterization of Volcanic Ash Soils\" by\u003c/em\u003e Rend\u0026oacute;n et al., (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), which established a comprehensive baseline of the soil\u0026rsquo;s physical and compositional characteristics. During post-publication analysis, significant variations were observed in the California Bearing Ratio (CBR) and shear strength values of samples obtained from the same site but at different times of the year. A subsequent review of rainfall data revealed that the sampling campaigns coincided with distinct precipitation conditions: approximately 100 mm in April 2016 (A1, intermediate rainfall), 10 mm in February 2017 (A2, low rainfall), and 192 mm in May 2017 (A3, high rainfall). These differences, initially detected by chance, suggest a strong correlation between seasonal moisture content and soil mechanical behavior.\u003c/p\u003e\u003cp\u003eAccordingly, this paper investigates the influence of accumulated rainfall on the shear strength and CBR values of volcanic ash soils. The findings underscore the importance of considering seasonal climatic variations in geotechnical assessments, particularly when working with water content - sensitive materials such as volcanic ash.\u003c/p\u003e"},{"header":"2 VOLCANIC ASH DESCRIPTION","content":"\u003cp\u003eAndesitic eruptions from stratovolcanoes produce significant quantities of tephra, a term used to describe all solid volcanic materials expelled during an eruption. Tephra includes pyroclastic fragments such as ash, lapilli, and volcanic fragments (Hermel\u0026iacute;n, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1984\u003c/span\u003e; Toro \u0026amp; Hermel\u0026iacute;n, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Among these, volcanic ash is the finest component and is transported primarily by wind, leading to its classification as an aeolian (wind-borne) deposit (Lizcano et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). These deposits form volcanic ash layers with considerable thickness variability\u0026mdash;ranging from a few centimeters to several meters\u0026mdash;and typically exhibit minimal internal structure.\u003c/p\u003e\u003cp\u003eThe morphology and texture of volcanic ash particles undergo significant alteration during their expulsion, transport, and deposition (H\u0026uuml;rlimann et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Finer particles, depending on characteristics such as shape, sphericity, surface roughness, and specific surface area, may be deposited at distances of hundreds of kilometers from their volcanic source (Betancur et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; H\u0026uuml;rlimann et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). This deposition process is geologically rapid, with the transformation from eruption to soil genesis occurring in less than 20,000 years (Toro \u0026amp; Hermel\u0026iacute;n, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe mineralogical composition of volcanic ash is typically dominated by silicates, volcanic glass, crystals, and lithic fragments, followed by feldspars, quartz, hornblende, hypersthene, augite, and magnetite in decreasing order of abundance (H\u0026uuml;rlimann et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Lizcano et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Verdugo, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Once deposited, volcanic ash undergoes weathering due to geophysical and environmental factors, resulting in the dissolution of primary minerals and the formation of secondary clay minerals. This process leads to the formation of volcanic ash soils, characterized by the presence of minerals such as allophane, halloysite, imogolite, and montmorillonite (Garc\u0026iacute;a-Leal \u0026amp; Colmenares, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; H\u0026uuml;rlimann et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Lizcano et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Verdugo, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Wesley, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1973\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThese distinctive clay minerals confer unique chemical and physical properties to volcanic ash soils, differentiating them from other sedimentary soils. In the department of Antioquia (Colombia), volcanic ash soils are believed to originate from the Ruiz\u0026ndash;Tolima volcanic massif, which includes volcanoes such as Cerro Bravo, Nevado del Ruiz, Olleta, Santa Isabel, Tolima, and Mach\u0026iacute;n. While erosional processes have removed these deposits from steep-sloped areas, they remain well-preserved in regions with gentler slopes (Hermel\u0026iacute;n, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1984\u003c/span\u003e).\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Geotechnical Characterization\u003c/h2\u003e\u003cp\u003eThe geotechnical characterization of volcanic ash soils involves determining their index and mechanical properties through field and laboratory testing. These results allow engineers to assess soil behavior, which is often complex due to the unique origin and mineral composition of volcanic materials.\u003c/p\u003e\u003cp\u003eVolcanic ash soils are known for their high liquid limit (LL) values, which increase with water content. However, their plasticity index (PI) is lower than that of sedimentary clays with similar LL values (Gonzalez et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1981\u003c/span\u003e). This is attributed to the dominance of amorphous minerals such as allophane and imogolite, commonly found in the clay fraction of these soils (Lizcano et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Molina et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Rao, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1996\u003c/span\u003e)\u003c/p\u003e\u003cp\u003eIn the previous study by (Rend\u0026oacute;n et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), volcanic ash samples collected in Caldas, Antioquia, were subjected to a series of index tests. The results showed:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eWater content (w): Ranging from 103\u0026ndash;205%, with an average of 156%.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eLiquid limit (LL): Between 158% and 257% (avg. 204%).\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003ePlastic limit (PL): Between 127% and 184% (avg. 150%).\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eVoid ratio (e): Ranging from 3 to 7, with an average of 5.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eDry unit weight (γ\u003csub\u003ed\u003c/sub\u003e): Between 0.36 and 0.62 g/cm\u0026sup3;, indicating extremely loose soils.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eBulk unit weight (γ): Ranging from 1.02 to 1.23 g/cm\u0026sup3;.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eSaturation (S_r): Between 58% and 87%, with an average of 78%.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eThese soils were classified as high plasticity silts (MH) using the Unified Soil Classification System (USCS), with over 70% passing the No. 200 sieve. Their particle size distribution showed low variability and a predominance of fine particles smaller than 0.075 mm.\u003c/p\u003e\u003cp\u003eThe SEM analysis revealed a weakly cemented microstructure with high porosity, which helps explain the relatively low shear strength observed. X-ray diffraction (XRD) and fluorescence (XRF) tests confirmed the presence of quartz, tridymite, and brucite, along with high concentrations of SiO₂ and Al₂O₃, characteristic of aluminum-silicate minerals.\u003c/p\u003e\u003cp\u003eNotably, natural water contents exceeded the liquid limit in many cases, placing the soils in a near-liquid or plastic state under field conditions. This sensitivity to moisture leads to strong drying hysteresis, meaning Atterberg limits can vary dramatically depending on test conditions. Drying (air or oven) was shown to reduce plasticity, confirming findings from previous studies (Lizcano et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Verdugo, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAlthough some samples showed moderate shear strength in their undisturbed state, this was lost upon remolding, as the structure collapsed and plastic behavior became dominant (Betancur et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Gonzalez et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1981\u003c/span\u003e). Furthermore, SPT N-values were consistently low\u0026mdash;between 4 and 8\u0026mdash;reinforcing the classification of these soils as very loose and highly problematic for engineering applications.\u003c/p\u003e\u003cp\u003eIn summary, the previous study confirmed that volcanic ash soils in the Aburr\u0026aacute; Valley present high variability in water-sensitive properties, strong moisture dependency, and significant microstructural fragility. These characteristics warrant a deeper exploration of how seasonal changes in moisture, particularly due to rainfall, affect their mechanical performance\u0026mdash;an issue addressed in the current research.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 SITE DESCRIPTION","content":"\u003cp\u003eThe characterization of the volcanic ash soils was carried out for soil samples taken in the municipality of Caldas, Antioquia, at 22 km south of the city of Medellin, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. The site is located at an approximate altitude of 1.750m above sea level and has an average temperature of 19\u0026deg;C throughout the year.\u003c/p\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Volcanic ash characteristics in situ\u003c/h2\u003e\n \u003cp\u003eAt the study site, volcanic ash soils were found to extend from the surface down to approximately 1.5 meters in depth. These deposits were observed beneath a thin surficial layer (approximately 0.1 meters thick) composed of dark soil with high organic matter content. The volcanic ash layer was underlain by a reddish residual soil mantle, indicative of more weathered and older geological materials as is shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eThe stratigraphy at the site reflects a typical profile of air-fall volcanic ash deposits, where the upper organic-rich layer likely formed due to vegetation accumulation over time, while the underlying residual soil marks the transition to more stable and less transport-influenced horizons. The volcanic ash horizon itself was visually homogeneous, with a light gray to whitish coloration and loose consistency.\u003c/p\u003e\n \u003cp\u003eThis configuration confirms that the studied volcanic ash soils are relatively recent in geological terms, loosely packed, and susceptible to environmental changes\u0026mdash;particularly fluctuations in water content. Their position within the soil profile also supports their classification as surface deposits, prone to direct climatic and hydrological interactions, which are central to the present study.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4 Rainfall records","content":"\u003cp\u003eTo investigate the influence of seasonal rainfall on the geotechnical behavior of volcanic ash soils, rainfall data were obtained from two meteorological stations\u0026mdash;Stations 57 and 58\u0026mdash;managed by the Aburr\u0026aacute; Valley Early Warning System (SIATA, by its Spanish acronym). These stations are in proximity to the sampling site, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThree sampling campaigns were conducted at different times of the year, each representing a distinct level of accumulated rainfall. The sampling events were categorized based on total precipitation over the days leading up to the sampling as follows: A1 (intermediate rainfall), A2 (low rainfall), and A3 (high rainfall). These conditions are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and the characteristics of each sampling campaign are detailed below:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eSampling Area A1 \u0026ndash; April 5th, 2016 (Intermediate Rainfall)\u003c/b\u003e:\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eA total of 20 disturbed samples were collected in plastic bags from a test pit approximately 2.0 m \u0026times; 2.0 m in size at a depth of 0.6 m. The accumulated rainfall during the preceding days was approximately \u003cb\u003e100 mm.\u003c/b\u003e\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eSampling Area A2 \u0026ndash; February 14th, 2017 (Low Rainfall)\u003c/b\u003e:\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eSamples were taken from an area of 4.0 m \u0026times; 2.0 m at depths ranging from 0.5 m to 0.7 m. This campaign included 7 disturbed samples (in plastic bags) and 9 undisturbed samples (6 Shelby tubes and 3 CBR molds). The accumulated rainfall was approximately \u003cb\u003e10 mm.\u003c/b\u003e\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eSampling Area A3 \u0026ndash; May 3rd, 2017 (High Rainfall)\u003c/b\u003e:\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eSamples were collected from a 2.0 m \u0026times; 1.5 m area at similar depths (0.5\u0026ndash;0.7 m). The set included 10 disturbed samples and 18 undisturbed samples (12 Shelby tubes and 6 CBR molds). The accumulated rainfall prior to sampling was approximately \u003cb\u003e192 mm.\u003c/b\u003e\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eThese three sampling conditions provided a representative dataset of the soil\u0026rsquo;s behavior under different moisture regimes. This variability allowed for evaluating how short-term accumulated rainfall affects critical geotechnical parameters such as \u003cb\u003eshear strength\u003c/b\u003e and \u003cb\u003eCalifornia Bearing Ratio (CBR)\u003c/b\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"5 GEOTECHNICAL ANALYSIS","content":"\u003cp\u003eThe index properties and chemical characteristics of the volcanic ash soils under investigation have been previously documented by (Rend\u0026oacute;n et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Building upon that foundation, this study evaluates whether short-term variations in rainfall significantly influence key geotechnical properties\u0026mdash;particularly water content, shear strength, and CBR\u0026mdash;by comparing data collected during three distinct sampling periods (A1, A2, and A3), each associated with different accumulated rainfall conditions.\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e5.1 Water content\u003c/h2\u003e\u003cp\u003eTo assess the sensitivity of volcanic ash soils to climatic variations, water content was analyzed across the three sampling periods representing intermediate (A1), low (A2), and high (A3) rainfall conditions. A \u003cb\u003eboxplot\u003c/b\u003e was constructed to visualize the variability and central tendency of the water content for each sampling event (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe results demonstrate a clear correlation between accumulated rainfall and the natural water content of the samples. The lowest water content values correspond to the A2 sampling period, which followed the driest conditions (approximately 10 mm of rainfall). Conversely, the highest water contents were recorded during the A3 period, which followed the most intense rainfall (approximately 192 mm). The A1 campaign, under intermediate rainfall conditions (~\u0026thinsp;100 mm), showed water content values between those of A2 and A3.\u003c/p\u003e\u003cp\u003eThese findings reinforce the hypothesis that volcanic ash soils, due to their high porosity, low dry density, and open microstructure, are highly susceptible to environmental moisture changes. As a superficial deposit, volcanic ash rapidly absorbs or loses moisture depending on ambient humidity and precipitation, resulting in significant short-term variability in geotechnical properties such as water content.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e5.2 Grained size distribution\u003c/h2\u003e\u003cp\u003eA total of 25 samples were classified using the \u003cb\u003eUnified Soil Classification System (USCS)\u003c/b\u003e following (ASTM D2487, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The particle-size distribution curves for these samples are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The results show a consistent gradation pattern across all three sampling campaigns (A1, A2, and A3), regardless of the rainfall conditions preceding each collection.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe analysis revealed that all samples consisted predominantly of \u003cb\u003efine-grained soils\u003c/b\u003e, with particle sizes smaller than 0.425 mm and \u003cb\u003emore than 70% passing the No. 200 sieve (0.075 mm)\u003c/b\u003e. These characteristics are consistent with typical volcanic ash soils, which tend to have high proportions of silt-sized and clay-sized particles due to their pyroclastic origin.\u003c/p\u003e\u003cp\u003eImportantly, \u003cb\u003eno significant differences were observed in the grain size distribution\u003c/b\u003e among the samples collected during different rainfall conditions. This suggests that \u003cb\u003eseasonal variations in rainfall do not affect the fundamental particle-size composition\u003c/b\u003e of the volcanic ash soil. Therefore, the variability in geotechnical behavior observed across the sampling periods is not attributed to changes in soil texture or classification, but rather to \u003cb\u003ewater content-dependent properties\u003c/b\u003e, such as water content, shear strength, and compaction response.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e5.3 Liquid limit (LL) analyses\u003c/h2\u003e\u003cp\u003eThe \u003cb\u003eliquid limit (LL)\u003c/b\u003e and \u003cb\u003eplasticity index (PI)\u003c/b\u003e values obtained from the samples across the three sampling periods were plotted on a \u003cb\u003ePlasticity Chart\u003c/b\u003e, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. All data points fall below the \u003cb\u003eA-line\u003c/b\u003e, consistent with the behavior of volcanic ash soils reported by So (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) and later confirmed by Lizcano et al. (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), indicating a classification as \u003cb\u003ehigh-plasticity silts (MH)\u003c/b\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDespite their shared classification, the LL and PI values showed notable variation among the three rainfall periods. The samples collected during the high rainfall season (A3) exhibited the highest liquid and plastic limits, with LL values approaching 250%. This increase can be attributed to the high degree of saturation resulting from sustained precipitation and the inherent water retention capacity of the volcanic ash due to its porous microstructure and clay mineral composition.\u003c/p\u003e\u003cp\u003eIn contrast, during the low rainfall season (A2), the LL values were generally lower, averaging around 190%, indicating reduced saturation. Although this is still a high value by conventional standards, it reflects the persistent ability of volcanic ash soils to retain moisture even during drier conditions.\u003c/p\u003e\u003cp\u003eInterestingly, the intermediate rainfall season (A1) showed a wider spread in LL and PI values. This variability may result from non-uniform saturation levels within the soil profile at the time of sampling. Because volcanic ash soils are highly sensitive to moisture changes, even slight differences in water content can influence Atterberg limits significantly, especially when samples are taken from zones with varying micro-drainage or surface exposure.\u003c/p\u003e\u003cp\u003eThese results highlight the plasticity behavior of volcanic ash soils as strongly dependent on seasonal rainfall, reinforcing the need to consider environmental conditions\u0026mdash;particularly soil moisture state at the time of testing\u0026mdash;when interpreting or comparing plasticity-based soil classifications for volcanic ash materials.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e5.4 Unconfined compressive strength\u003c/h2\u003e\u003cp\u003eUnconfined compressive strength (UCS) tests were conducted to evaluate the undrained shear strength (qu) of the volcanic ash soils during two of the three sampling periods. Unfortunately, no UCS tests were performed for the intermediate rainfall period (A1). However, reliable measurements were obtained during the low (A2) and high (A3) rainfall seasons, as summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Tables\u0026nbsp;3 and 4.\u003c/p\u003e\u003cp\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\u003eSummary of unconfined compressive strength results for A2 (low rainfall) and A3 (high rainfall)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\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\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cem\u003ew\u003c/em\u003e (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eϒ (g/cm3)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eϒd (g/cm3)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cem\u003ee\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003eu\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(kPa)\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eu\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e(kPa)\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003eA2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAverage\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003e112.35\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e1.13\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e0.53\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e4\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003e31.83\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e\u003cb\u003e15.91\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5.97\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e10.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e5.33\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCV (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e33\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003eA3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAverage\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003e183.18\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e1.10\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e0.39\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e5\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003e55.86\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e\u003cb\u003e27.93\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e23.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e11.90\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCV (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e43\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 results indicate a \u003cb\u003emarked influence of seasonal moisture conditions\u003c/b\u003e on the compressive strength of the soil. During the \u003cb\u003ehigh rainfall season (A3)\u003c/b\u003e, the average undrained compressive strength reached \u003cb\u003e55.86 kPa\u003c/b\u003e, while during the \u003cb\u003elow rainfall season (A2)\u003c/b\u003e, it averaged only \u003cb\u003e31.86 kPa\u003c/b\u003e. This represents a \u003cb\u003ereduction of approximately 43%\u003c/b\u003e in undrained shear strength during the dry season.\u003c/p\u003e\u003cp\u003eThis reduction is likely attributed to \u003cb\u003edesiccation processes\u003c/b\u003e that occur during drier periods, which cause a breakdown of the soil's microstructure and reduce its apparent cohesion. As a result, the soil's \u003cb\u003eshear strength decreases\u003c/b\u003e, which can significantly affect its \u003cb\u003ebearing capacity\u003c/b\u003e, especially for shallow foundations.\u003c/p\u003e\u003cp\u003eThese findings reinforce the critical importance of \u003cb\u003eseasonal timing in geotechnical investigations\u003c/b\u003e, particularly in volcanic ash soils where moisture content plays a dominant role in strength development. Designing foundations without accounting for this variability could lead to \u003cb\u003eunconservative assessments of load-bearing performance\u003c/b\u003e, especially during the dry season when strength is at its minimum.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e5.5 California Bearing Ratio (CBR)\u003c/h2\u003e\u003cp\u003eCBR tests were performed to assess the bearing capacity of volcanic ash soils under natural moisture conditions. Interestingly, the results show an inverse trend compared to the unconfined compressive strength findings.\u003c/p\u003e\u003cp\u003eAs illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the \u003cb\u003eCBR values were higher during the dry season (A2)\u003c/b\u003e than during the rainy season (A3). Specifically, \u003cb\u003eA2\u003c/b\u003e samples (low rainfall) exhibited an average \u003cb\u003eCBR of 2.4%\u003c/b\u003e, while \u003cb\u003eA3\u003c/b\u003e samples (high rainfall) showed a significantly lower average of \u003cb\u003e1.4%\u003c/b\u003e. This reflects a \u003cb\u003e42% reduction in CBR under high rainfall conditions\u003c/b\u003e, highlighting the substantial impact of water content on the bearing performance of the soil.\u003c/p\u003e\u003cp\u003e\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\u003eSummary of CBR results for A2 and A3.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\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\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cem\u003ew\u003c/em\u003e (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCBR (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eϒ (g/cm3)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eϒd (g/cm3)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cem\u003ee\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003eA2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAverage\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003e116.5\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e2.4\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e1.03\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.48\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003e4.4\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e7.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCV (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003eA3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAverage\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003e192.8\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e1.4\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e0.98\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e0.34\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003e6.5\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e14.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCV (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e7.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e9.8\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\u003eThese results highlight critical behavior of volcanic ash soils: \u003cb\u003eincreased water content significantly decreases their CBR\u003c/b\u003e, reducing their ability to support pavement and light foundation loads. This partially explains why \u003cb\u003epavement damage and saturation failures\u003c/b\u003e are common in regions with volcanic soils during prolonged rainy periods.\u003c/p\u003e\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\u003ch2\u003e5.5.1 CBR Results Under Immersion\u003c/h2\u003e\u003cp\u003eTo further investigate the effect of saturation on CBR, additional tests were conducted on \u003cb\u003epre-saturated (fully immersed)\u003c/b\u003e samples for both A2 and A3. These tests aimed to simulate worst-case field conditions such as prolonged flooding or capillary rise.\u003c/p\u003e\u003cp\u003eThe results show that the \u003cb\u003eimpact of immersion is more pronounced for A2 (dry season samples)\u003c/b\u003e than for A3 (rainy season samples), as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e and \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e. This is attributed to \u003cb\u003esoil suction effects\u003c/b\u003e: in A2, the drier soil structure retains higher matric suction, and immersion leads to a significant strength reduction. In contrast, A3 samples were already close to saturation, so immersion had a lesser additional effect.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThese findings emphasize the \u003cb\u003esensitivity of volcanic ash soils to both natural and induced water content variations\u003c/b\u003e, reinforcing the importance of considering moisture state\u0026mdash;not just classification\u0026mdash;when designing pavements or shallow foundations on such soils.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"6 Discussion","content":"\u003cp\u003eThe index properties determined in this study confirm that soils derived from volcanic ash are generally \u003cb\u003eunsuitable for the design and construction of civil works\u003c/b\u003e based on conventional geotechnical standards. Their high porosity, elevated natural water content, low dry unit weight, and mineralogical composition make them particularly \u003cb\u003echallenging for engineering applications\u003c/b\u003e, even though these same properties contribute to their \u003cb\u003eagronomic productivity\u003c/b\u003e (Shoji et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1993\u003c/span\u003e)\u003c/p\u003e\u003cp\u003eHowever, in the context of \u003cb\u003epavement design\u003c/b\u003e, these materials are often unavoidable, and thus, understanding their behavior under \u003cb\u003eseasonal rainfall variability\u003c/b\u003e becomes essential. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e, rainfall regime directly affects \u003cb\u003esoil saturation\u003c/b\u003e, which in turn significantly influences \u003cb\u003eCBR performance\u003c/b\u003e. An empirical relationship between water content and CBR was derived using the data from this study, resulting in a high correlation (\u003cb\u003eR\u0026sup2; = 0.88\u003c/b\u003e), as expressed in \u003cb\u003eEq.\u0026nbsp;1\u003c/b\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003eEquation 1. CBR(%)\u0026thinsp;=\u0026thinsp;5.014 e-\u003c/em\u003e\u003csup\u003e\u003cem\u003e0.007w(%)\u003c/em\u003e\u003c/sup\u003e \u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;0.88\u003c/em\u003e\u003c/p\u003e\u003cp\u003eThis relationship highlights the \u003cb\u003eexponential decay of bearing capacity with increasing water content\u003c/b\u003e, reinforcing the importance of \u003cb\u003etiming in sample collection\u003c/b\u003e. Soils sampled during \u003cb\u003edry seasons\u003c/b\u003e can lead to \u003cb\u003eoverestimation of CBR values\u003c/b\u003e, potentially resulting in \u003cb\u003eunderdesigned pavement structures\u003c/b\u003e if the subgrade later becomes saturated. Furthermore, the findings suggest that the commonly recommended \u003cb\u003e4-day immersion protocol\u003c/b\u003e in laboratory testing may be \u003cb\u003einsufficient\u003c/b\u003e to simulate field conditions under prolonged rainfall or extreme saturation scenarios. This finding aligns with broader observations in the pavement engineering literature, where heavy rainfall has been shown to significantly alter \u003cb\u003esubgrade moisture profiles and base layer stiffness\u003c/b\u003e, thereby reducing structural performance under load (Jibon et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2024\u003c/span\u003e)\u003c/p\u003e\u003cp\u003eTo further explore the engineering implications, an evaluation of \u003cb\u003epavement reliability (Zr)\u003c/b\u003e under varying rainfall conditions was performed. In the AASHTO flexible pavement design method, the parameters \u003cb\u003eZr\u003c/b\u003e (reliability index) and \u003cb\u003eSo\u003c/b\u003e (standard deviation) reflect the confidence required for the design. For instance, \u003cb\u003emajor highways\u003c/b\u003e demand higher reliability levels (e.g., Zr\u0026thinsp;=\u0026thinsp;99.9%), whereas \u003cb\u003elocal roads\u003c/b\u003e can tolerate lower reliability (e.g., Zr\u0026thinsp;=\u0026thinsp;90%).\u003c/p\u003e\u003cp\u003eAn illustrative example was developed using initial reliability levels of \u003cb\u003eZr\u0026thinsp;=\u0026thinsp;99.9%, 99.0%, and 90.0%\u003c/b\u003e. The CBR values were varied to reflect natural, dry, and fully saturated conditions, while keeping the structural pavement section constant. The goal was to determine the \u003cb\u003eequivalent reliability\u003c/b\u003e under each scenario. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e, reliability \u003cb\u003edecreased as CBR decreased due to rainfall-induced saturation\u003c/b\u003e. However, this impact was \u003cb\u003eless severe\u003c/b\u003e in cases where the \u003cb\u003einitial design reliability\u003c/b\u003e was high, demonstrating that \u003cb\u003ehigh Zr values can buffer against seasonal moisture uncertainty\u003c/b\u003e, regardless of road classification.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThese findings support the \u003cb\u003eincorporation of drainage elements\u003c/b\u003e into pavement designs over volcanic ash soils. However, under highly saturated conditions, \u003cb\u003epavement reliability may fall below acceptable thresholds\u003c/b\u003e, indicating the \u003cb\u003eneed for conservative design assumptions\u003c/b\u003e, moisture control strategies, and, where appropriate, \u003cb\u003esoil stabilization techniques\u003c/b\u003e.\u003c/p\u003e\u003cp\u003eLastly, the complex behavior of volcanic ash soils underscores the need for a \u003cb\u003ecomprehensive understanding of their formation, depositional environment, and post-depositional weathering\u003c/b\u003e. The mechanical properties are not only governed by their mineralogical content but also by \u003cb\u003eongoing transformation processes\u003c/b\u003e that vary significantly across regions and climatic conditions. These nuances must be addressed when interpreting laboratory results and applying them to real-world geotechnical designs.\u003c/p\u003e"},{"header":"7 Conclusions","content":"\u003cp\u003eThis study evaluated the influence of seasonal rainfall on the geotechnical behavior of volcanic ash soils, focusing on critical parameters such as \u003cb\u003ewater content\u003c/b\u003e, \u003cb\u003eAtterberg limits\u003c/b\u003e, \u003cb\u003eunconfined compressive strength\u003c/b\u003e, and \u003cb\u003eCalifornia Bearing Ratio (CBR)\u003c/b\u003e. The results highlight the \u003cb\u003eextreme sensitivity of volcanic ash soils to moisture variation\u003c/b\u003e, making them highly problematic yet frequently encountered in infrastructure development, particularly in tropical volcanic regions.\u003c/p\u003e\u003cp\u003eThe main conclusions are summarized as follows:\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eVolcanic ash soil exhibits high porosity, low dry unit weight, and extreme water retention capacity\u003c/b\u003e, which leads to significant seasonal fluctuations in geotechnical properties.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eWater content and plasticity parameters (LL and PI)\u003c/b\u003e increased substantially during high rainfall periods. This variability must be accounted for, as plasticity values near or above the liquid limit can affect soil behavior even before saturation.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eUnconfined compressive strength showed a 43% reduction from rainy to dry conditions\u003c/b\u003e, indicating the mechanical vulnerability of these soils under desiccation or low-moisture states due to microstructural changes.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eCBR values decreased exponentially with increasing water content\u003c/b\u003e, as captured by a high-correlation empirical model (R\u0026sup2; = 0.88). Soils in natural saturated conditions (A3) showed CBR values nearly 42% lower than in dry conditions (A2), highlighting the risk of underperformance when rainfall is not properly considered.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eStandard 4-day immersion procedures\u003c/b\u003e may not accurately simulate real field saturation in volcanic soils. The test results suggest that the typical laboratory preparation may \u003cb\u003eunderestimate the long-term impacts\u003c/b\u003e of wet seasons, especially on pavement performance.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003ePavement reliability (Zr) is significantly reduced under lower CBR values associated with rainfall\u003c/b\u003e, but this effect can be partially mitigated by designing with higher initial reliability levels, regardless of road classification.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe use of \u003cb\u003edrainage solutions, moisture control, and conservative design assumptions\u003c/b\u003e are strongly recommended when working with volcanic ash soils. In some cases, stabilization or improvement techniques may be necessary to ensure long-term performance.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e\u003cp\u003eIn conclusion, volcanic ash soils present complex behavior governed not only by their mineralogical composition but also by \u003cb\u003eenvironmental factors such as rainfall and saturation cycles\u003c/b\u003e. Their mechanical performance must be evaluated in a site-specific and time-sensitive manner, as seasonal variability has a \u003cb\u003edirect impact on both strength and reliability\u003c/b\u003e. These findings contribute to a more resilient approach to geotechnical design in volcanic regions, particularly for pavement and shallow foundation systems\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM. Rend\u0026oacute;n conducted the field and laboratory testing, performed the initial calculations, and contributed to the previous related publication. J.C.V. carried out the statistical analyses, identified the influence of seasonal rainfall on the results, and wrote the manuscript. Both authors reviewed and approved the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e\u003cp\u003eThe authors would like to acknowledge the financial support to this research project, under the National Doctoral Grant Scheme No. 727 of 2015, provided by the Ministerio de Ciencia Tecnolog\u0026iacute;a e Innovaci\u0026oacute;n of Colombia \u0026ndash; Minciencias (previously Colciencias).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eArnalds, \u0026Oacute;., Bartoli, F., Buurman, P., \u0026Oacute;skarsson, H., Stoops, G., \u0026amp; Garc\u0026iacute;a-Rodeja, E. (2007). \u003cem\u003eSoils of Volcanic Regions in Europe\u003c/em\u003e (\u0026Oacute;. Arnalds, F. Bartoli, P. Buurman, H. \u0026Oacute;skarsson, G. Stoops, \u0026amp; E. Garc\u0026iacute;a-Rodeja, Eds.). Springer.\u003c/li\u003e\n \u003cli\u003eASTM D2487. (2011). \u003cem\u003eStandard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)\u003c/em\u003e (p. 12). ASTM International.\u003c/li\u003e\n \u003cli\u003eBetancur, Y., Builes, M., \u0026amp; Mill\u0026aacute;n, \u0026Aacute;. (2013). Variaci\u0026oacute;n de las Propiedades Mec\u0026aacute;nicas de Arcillas Al\u0026oacute;fanas en Colombia al Variar el Grado de Saturaci\u0026oacute;n. \u003cem\u003eRevista EIA\u003c/em\u003e, \u003cem\u003e10\u003c/em\u003e(20), 173\u0026ndash;181. https://doi.org/10.14508/reia.2013.10.20.173-181\u003c/li\u003e\n \u003cli\u003eGarc\u0026iacute;a-Leal, J. C., \u0026amp; Colmenares, J. E. (2011). 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Suelos derivados de cenizas volc\u0026aacute;nicas en Colombia. \u003cem\u003eRevista Internacional de Desastres Naturales, Accidentes e Infraestructura Civil\u003c/em\u003e, \u003cem\u003e6\u003c/em\u003e(2), 167\u0026ndash;198.\u003c/li\u003e\n \u003cli\u003eMatsumura, S., Miura, S., Yokohama, S., \u0026amp; Kawamura, S. (2015). Cyclic deformation-strength evaluation of compacted volcanic soil subjected to freeze-thaw sequence. \u003cem\u003eSoils and Foundations\u003c/em\u003e, \u003cem\u003e55\u003c/em\u003e(1), 86\u0026ndash;98. https://doi.org/10.1016/j.sandf.2014.12.007\u003c/li\u003e\n \u003cli\u003eMolina, G., Hern\u0026aacute;ndez, E., \u0026amp; Castillo, C. (2012). Determinaci\u0026oacute;n de la correlaci\u0026oacute;n entre el coeficiente de compresi\u0026oacute;n y propiedades \u0026iacute;ndice en suelos de expansi\u0026oacute;n urbana de Pereira. \u003cem\u003eAVANCES Investigaci\u0026oacute;n En Ingenier\u0026iacute;a\u003c/em\u003e, \u003cem\u003e9\u003c/em\u003e(2), 72\u0026ndash;79.\u003c/li\u003e\n \u003cli\u003eNanzyo, M., Shoji, S., \u0026amp; Dahlgren, R. (1993). Chapter 7 Physical Characteristics of Volcanic Ash Soils. In S. Shoji, M. Nanzyo, \u0026amp; R. Dahlgren (Eds.), \u003cem\u003eDevelopments in Soil Science\u003c/em\u003e (Vol. 21, pp. 189\u0026ndash;207). Elsevier. https://doi.org/10.1016/S0166-2481(08)70268-X\u003c/li\u003e\n \u003cli\u003ePicarelli, L., Evangelista, A., Rolandi, G., Paone, A., Nicotera, M. V., Olivares, L., Scotto Di Santolo, A., Lampitiello, S., \u0026amp; Rolandi, M. (2007). Mechanical properties of pyroclastic soils in Campania Region. In K. K. Phoon, D. W. Hight, S. Leroueil, \u0026amp; T. S. Tan (Eds.), \u003cem\u003eCharacterisation and Engineering Properties of Natural Soils\u003c/em\u003e (Vol. 4, pp. 2331\u0026ndash;2383). Taylor \u0026amp; Francis/Balkema. https://doi.org/10.1201/NOE0415426916.ch18\u003c/li\u003e\n \u003cli\u003eRao, S. M. (1996). Role of apparent cohesion in the stability of Dominican allophane soil slopes. \u003cem\u003eEngineering Geology\u003c/em\u003e, \u003cem\u003e43\u003c/em\u003e, 265\u0026ndash;279. https://doi.org/10.1016/S0013-7952(96)00036-1\u003c/li\u003e\n \u003cli\u003eRend\u0026oacute;n, M. I., Viviescas, J. C., Osorio, J. P., \u0026amp; Hern\u0026aacute;ndez, M. S. (2020). Chemical, Mineralogical and Geotechnical Index Properties Characterization of Volcanic Ash Soils. \u003cem\u003eGeotechnical and Geological Engineering\u003c/em\u003e. https://doi.org/10.1007/s10706-020-01219-3\u003c/li\u003e\n \u003cli\u003eShoji, S., Dahlgren, R., \u0026amp; Nanzyo, M. (1993). 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Tefraestratigraf\u0026iacute;a Colombiana. \u003cem\u003eRevista Universidad Eafit\u003c/em\u003e, \u003cem\u003e86\u003c/em\u003e, 81\u0026ndash;84.\u003c/li\u003e\n \u003cli\u003eVerdugo, R. (2008). Singularities of Geotechnical Properties of Complex Soils in Seismic Regions. \u003cem\u003eJournal of Geotechnical and Geoenvironmental Engineering\u003c/em\u003e, \u003cem\u003e134\u003c/em\u003e(7), 982\u0026ndash;992. https://doi.org/10.1061/(ASCE)1090-0241(2008)134:7(982)\u003c/li\u003e\n \u003cli\u003eWesley, L. D. (1973). Some basic engineering properties of halloysite and allophane clays in Java, Indonesia.\u0026nbsp;\u003cem\u003eG\u0026eacute;otechnique\u003c/em\u003e, \u003cem\u003e23\u003c/em\u003e(4), 471\u0026ndash;494. https://doi.org/10.1680/geot.1973.23.4.471\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Seasonal rainfall variability, volcanic ash soils, Shear strength, California Bearing Ratio (CBR), Pavement Reliability","lastPublishedDoi":"10.21203/rs.3.rs-7511273/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7511273/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study evaluates the impact of \u003cb\u003eseasonal rainfall\u003c/b\u003e on the geotechnical behavior of \u003cb\u003evolcanic ash soils\u003c/b\u003e in the Aburr\u0026aacute; Valley, Colombia. Samples were collected during low, intermediate, and high rainfall periods and tested for \u003cb\u003ewater content\u003c/b\u003e, \u003cb\u003eplasticity\u003c/b\u003e, \u003cb\u003eunconfined compressive strength\u003c/b\u003e, and \u003cb\u003eCalifornia Bearing Ratio (CBR)\u003c/b\u003e. Results show that \u003cb\u003emoisture content and Atterberg limits\u003c/b\u003e increase with rainfall, while \u003cb\u003eCBR decreases exponentially\u003c/b\u003e, with values dropping by up to \u003cb\u003e42%\u003c/b\u003e in wet conditions. An empirical correlation (R\u0026sup2; = 0.88) between water content and CBR was developed. \u003cb\u003eUnconfined strength\u003c/b\u003e also decreased by \u003cb\u003e43%\u003c/b\u003e in dry conditions. Pavement reliability (Zr) declined significantly under saturated conditions, emphasizing the importance of accounting for \u003cb\u003eseasonal variability\u003c/b\u003e in design. The findings highlight the need for \u003cb\u003edrainage solutions\u003c/b\u003e, \u003cb\u003econservative designs\u003c/b\u003e, and \u003cb\u003esite-specific assessments\u003c/b\u003e when building on volcanic ash soils.\u003c/p\u003e","manuscriptTitle":"Seasonal Rainfall Effects on Shear Strength and CBR in Volcanic Ash Soils","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-11 17:19:35","doi":"10.21203/rs.3.rs-7511273/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"71084860-1854-413d-a1cf-1de888bb0c19","owner":[],"postedDate":"September 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-02T21:23:23+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-11 17:19:35","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7511273","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7511273","identity":"rs-7511273","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}