Geomechanical Back Assessment of a Major Ground Collapse of a Twin Tunnel located in the Eastern Section of the Algerian Highway

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These tunnels are often excavated with large cross-sections and often in double-tubes line, leaving a clear spacing between each other, in order to ensure traffic fluidity and security services. High section tunnels usually result in large stress perturbations and deformations leading to a ground collapse phenomenon in misunderstood behaviour conditions of the groundmass, more particularly freeway twin tunnels with clear spacing. This paper takes into consideration a twin 3-lanes tunnel, part of a national freeway project of around 1200 km, with a non-circular section of a 190 m² each and a clear spacing of 17 m. Given the surrounding rock conditions, mentioned above, at a certain project stage, substantial displacements reached a peak of 41 mm/day, moreover, shotcrete cracks were observed prior to a major collapse extending up to 130 m in the tunnel direction. Back assessment is proposed in this work of post-collapse and ante-collapse main disturbance factors, feedback outcomes will be valuable for the ascertaining and understanding of underground failures phenomenon. Civil Engineering Back assessment Twin-tunnel Major collapse Support performance 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 The continuous expansion of the underground excavations leads to considerable issues in some cases, especially with large sections to ensure the most fluid transportation condition, issuing with challenging engineering tasks to carry out. Consequent excavation sections result of a larger deformation range to consider taking into account a freeway tunnel. Large deformations of the surrounding rocks are often encountered during tunnel coving within soft rock mass with squeezing characteristics [ 1 ], generally lasting from several weeks to several months [ 2 – 5 ], up to even more in soft rock tunnel [ 6 , 7 ], often accompanied by the lining cracking and even collapsing [ 1 ]. These phenomena are widely observed at relatively shallow depths in soft rocks such as phyllite, mudstone, shale, and siltstone [ 2 ]. The laboratory testing explored the failure mechanisms of twin-tunnels [ 8 ]. Jiang et al. (2021) [ 9 ] has shown the overall failure process of twin-tunnels through 3D printing-based physical models, and they identified the vital part of the structure failure by observation and automated measurements [ 10 ]. Jia et al. (2021) [ 11 ] developed a test system for use with the twin-tunnel excavation model to find out the deformation pattern of the surrounding rock and supporting structure. Their research indicated that the failure of twin-tunnels is obviously caused by the concentration of stress that occurring mainly in the middle rock pillar between the twin-tunnels. However, the stress concentration phenomenon will gradually decrease with the clear spacing increasing, when the clear spacing between the twin-tunnels exceeded twice the span of the single tunnel, its influence on the stability of the surrounding rock and supporting structure of the twin-tunnels is ignorable then. With continuous development of the transport infrastructures, several tunnels are dug in complicated surrounding rocks structure associated with poor mechanical properties conditions. In low buried tunnels, the surrounding rocks deformations mechanisms at actual construction process have been studied[ 12 – 16 ]. In shallow or ultra-shallow tunnel excavations, with associated poor mechanical properties, surrounding groundmass instability and unsymmetrical loading, the tunnel’s bearing arch is not properly formed, and construction causes a large additional loading resulting to excessive deformations or partial collapse of the tunnel face during construction process [ 17 , 18 ]. In this research, we will discuss the prior failure conditions leading to major collapse of a large section freeway twin tunnel, taking into account the degraded nature of the surrounding ground, moreover the discontinuities parallel disposition. The above mentioned mechanism are part of the failure process encountered during excavation, also we should underline the support system performance limit, where lining cracks were observed and lining cracks became obvious on; the first, the final lining starts cracking; the second, alarming displacements were recorded and steel arch have been observed in an offset position. 2. Project, ground conditions and methodology overview The tunnel of “Djebel Ouahch”, Constantine province, Algeria, is taken as reference for this research work. This tunnel is a part of the Maghreb unity motorway (AUM, French acronym). It is approximately 7000km long, crossing Algeria with 1200km long. The tunnel belongs to Sect. 4.1 of this highway. It crosses Djebel Ouahch Mountain in the northeast of Algeria at Constantine province, where it is included between the two mileages, starting from pk205 + 393 and ending at 207 + 384.5, with a total length of 1909 m. It is a1909 m long twin-tunnel, with a radius of 9.57 m, separated by 17 m distance from each other. Average depth is around 120, and could reach a maximum of 140 m locally with the weakest coverage reaching 12 m, and a section approaching 190m 2 shaped as a lowered vault as shown in Fig. 2 . According to the reconnaissance campaign carried out, both tunnels have always crossed the marl-limestone area corresponding to the Tellian layer, particularly in the argillite unit of the cretaceous age. At the surface, quaternary colluviums flush locally. Rough colluviums are represented by heterometric sandstone elements, pebbles and cornered blocs with red silty mounting, as shown in the next figure. The geological formations encountered are mainly greyish clays, with calcite in places. There are also some compact passages in places. Their dispositions are varying from horizontal to sub-horizontal with calcite joints at 30°. The next table describes the lithology disposition. Table 1 Lithology disposition of the surrounding ground Depth (m) Encountered terrain with joints angle in degrees (°) 30–32.5 Brittle grey argillite, with compact passages of gravel and pebbles 32.5–34 Compact grey argillite with passage of dark argillite with calcite joints and very smooth 34–41 Compact grey argillite with passage of dark argillite with calcite joints at 30° and whitish patches 41–47 Greyish, smooth, bluish argillite, sometimes firm, with calcite joints, 30 to 70° 47–59.5 Greyish, smooth, bluish, plastic argillite with 30° joints 59.5–110 Greyish, smooth, sometimes bluish, firm argillite, fractured rock, with calcite joints at 30° Table 2 Main mechanical properties of the surrounding ground mass Unit weight ɣ(KN/M 3 ) Young modulus E (MPa) Cohesion C (KPa) Frictional angle ɸ (°) Poisson’s ratio ʋ 24 110 100 24 0.3 Table 3 Summarized in-situ tests Depth (m) Vertical in situ stress P 0 (MPa) Uniaxial compressive strength of the intact material σ ci (MPa) RMR Coefficient of lateral pressure Kp 100 2.5–3.0 5–8 14–18 0.71 120 3.0–3.5 2–7 14–20 0.71 In the construction record, the right tube has entirely been dug and in use, then the construction process of the second tube took place (left tube). The NATM (New Austrian tunnelling method) was adopted in the construction phase of the left tube; the bench-cut excavation section. A double-layered support system was carried out in the construction process of the left tunnel, the temporary support scheme materials was constituted of: HEB 200 steel-arch with 0.75 m pitch spacing, 6 m hollow micro-fissured grouting fiberglass bolts, and 30 cm thick shotcrete layer. For the secondary lining, a 60 cm thick layer of reinforced concrete was used. On January 1st 2014 at between 16h00 and 16h30, a major ground collapse occurred in the left tube, expanding from the mileage pk206 + 150 to pk206 + 280, The collapse has severely damaged the right tube (already achieved and in use back then) causing serious damages on the lining of the right tube, and the total collapse length has reached 140 m long. Neither wounded nor casualties have been registered after the accident’s occurrence. 3. Surveying, pre-collapse situation and countermeasures adopted The monitoring activity provides quantitative data to measure for analyses at a real situation in the tunneling process. Accurately determining the surrounding rocks is crucial for ensuring safety in underground works [ 19 – 23 ]. Many researches underscored the monitoring importance to study the deformation of surrounding rocks and have proposed measures to ensure safe construction during tunnel excavation [ 24 – 26 ]. Indeed, monitoring process is relevant to study/identify the surrounding rock mass deformation in order to take the appropriate measures [ 16 ] and empower the decision making process. The displacements were obtained by the use of topographic station, working with optical targets reflectors, installed around tunnel profile (Five targets), with a stable reference point outside the zone of convergence to measure displacements. Topographic stations are more or less 0.2 mm over 100 m accurate for tunneling process [ 27 , 28 ]. The previously mentioned support scheme has been adopted during the construction of the left tunnel. At a certain construction advance, important displacements were recorded exceeding the allowed project’s safety deformations limits, established as written in Table 4 . Table 4 Maximum deformations allowed during excavation process Vigilance limit (mm) Alert limit (mm) Side wall deformations 80 110 Crown settlements 100 130 When the excavation progression reached certain mileages, the adopted support system started to show obvious weakness (observable with the naked eye), where serious shotcrete cracks appeared and the steel-archs started offsetting from the initially installed position. It has been reported that the ground displacements were continuously increasing with the advance of the excavation, confirming both the vulnerability of the adopted support system and the groundmass softening due to excessive displacements monitored over the previously mentioned limits in Table 3 . These displacements were lasting during a considerable period, extending up to a month along all the previously underlined mileages i.e. pk206 + 150 to pk206 + 280; we will take the following four monitoring points PK206 + 221, PK206 + 226, PK206 + 233, and PK206 + 236, these monitoring points are part of the area where the support countermeasures were not applied, the values are the average daily measurements. Table 5 Recorded displacements (layout + pk projection) Monitoring points Recorded crown settlements, min/max (mm) Recorded side wall deformations, min/max (mm) PK206 + 221 20.6 / 96.9 38.7 / 53.7 PK206 + 226 59.6 / 87.3 22.4 / 43.5 PK206 + 233 99.2 / 145.6 41.9 / 75.6 PK206 + 236 97.9 / 114 89.3 / 38.4 Given the recorded displacements shown by the table above, an increasing displacement situation of the tunnel deformation has taken place, sometimes registered over the limits set by the project’s limit safety deformations. After the previously deformations have been recorded, urgent measures were proposed to mitigate the danger observed during excavation, knowing the right tube was already in use. In the scope of risk management and collapse prevention, the selected construction technique is a key role for the timely project’s achievements [ 16 , 29 , 30 ]. In section 2, paragraph 5, the support scheme is described; the fastest measure taken into application was to double the HEB-200 steel-arch due to the situation’s emergency, thus apply another shotcrete layer 25 cm thick and apply the first layer with 35 instead of 30 cm as previously cited. Indeed, the upgraded reinforcement system shown effectiveness of its reliability by the ground movements decreasing in the applied area of this improved support scheme, also a clearly broad difference settlement rate is observed in the section where the initial support system is used as shown in the next Fig. 10 . As observed in the next figure, settlements rate is divided in two obvious sections, i.e. first where the urgent support measure is applied prior to mileage PK206 + 220, settlements rate is more or less reasonable given the recorded values, second section is represented beyond the mileage PK206 + 220, in this section single HEB-200 steel-arch support are deployed, displacements are alarming reaching a peak of 41.7 mm/day at mileage PK206 + 230. The adopted fitted the required support resistance taking into account the ground reaction to the excavation as shown in the next figure. The countermeasure support is obviously fitting the required resistance despite the drastic displacements previously observed. The finite elements analysis confirms both support scheme competence, where the Mohr-coulomb constitutive model is adopted. The decompressed zone elevates up to 40 m over the tunnel crown, making an excessive stress for the left tunnel bearing capacity relevant to the simple arch support, which is widely reduced putting to evidence the treatments competency after exceeding the maximum shear strength capacity as summed up in Fig. 11 . The next table sums up the Mohr-Coulomb in-put for the modelling. 4. Collapse mechanism feedback In soft rock tunnels, geological conditions and mechanical properties are the essential causes to large deformations issues during tunneling [ 1 , 6 , 31 – 33 , 5 , 34 ]. Strength/stress (SSR) ratio, defined as the ratio between rock mass uniaxial compressive strength σcm and the in situ stress P0, is recognized as rock squeezing extent factor [1.35.36]. Mechanical properties of the rock mass and geo-stress levels are important factors inducing large-deformation phenomenon in tunnels. Previous research [ 37 , 5 ] concluded that excavation induced plastic deformations is considered to be one of the most important factors leading to squeezing in soft rock tunnels. Plastic deformations at different extent, moderate to severe, depending on the geo-stress magnitude in vertical direction and rock strength e.g. σcm/P0. Over the past several decades, based on the rock mass classification system, many empirical equations for calculating the strength of rock mass (σcm) have been proposed. The most widely used equations and the results of σcm at the geostress test areas are listed in Table 6 . In these equations, the uniaxial compressive strength of intact rock is taken as σci = 8 MPa and the RMR = 14. Based on the equations in Table 6 , the average value the σcm is 171 KPa, and P0 as 3.0 MPa Table 6 Empirical equations and results of strength of rock mass (σ cm ) at the geostress test areas Researcher Equation (MPa) σ CM (kPa) Kalmaris and Bieniawski (1995) [ 37 ] \(\:\sigma\:cm=\sigma\:ci\:\text{e}\text{x}\text{p}\left(\frac{RMR-100}{24}\right)\) 222 Sheorey (1997) [ 38 ] \(\:\sigma\:cm=\sigma\:ci\:exp\left(\frac{RMR-100}{20}\right)\) 108 Truman (1998) [ 39 ] \(\:\sigma\:cm=0.06\text{e}\text{x}\text{p}\left(0.06RMR\right)\) 139 Aydan and Dalgic (1998) [ 40 ] \(\:\sigma\:cm=\sigma\:ci\frac{RMR}{RMR+6(100-RMR)}\) 211 The SSR, being σ cm /P 0 [ 41 – 43 ], is 0.07 lower than 0.1, which is less than the cut-off value of the SSR (severely squeezing rock) [ 35 ], and severely squeezing with support problem [ 4 ]. Consequently, the surrounding rock are severely squeezing of the considered tunnel with an issue of an unfit support. Table 7 Mohr-Coulomb yield criterion in-put Young modulus (E, MPa) Poisson’s ration Cohesion (KPa) Frictional angle (°) 110 0.3 100 18 According to the Mohr-Coulomb criterion, the yielding of rock mass has close relationships with both the deviator stress and the rock mass strength. The adjusted deviator stress after excavation in a high stress case is obviously higher than that in a low stress case, which leads to the rock fracture propagation and further accelerates the deformation and failure of rock masses. Compared with the case of high rock strength, rock masses with low strength yield more easily, resulting in a larger deformation of plastic flow. At a certain range, interaction between two tunnels cannot be ignored. Due to a certain clear spacing of twin tunnels, the right tunnel status is affected by the excavation of the left. When the right tunnel is excavated, it can be regarded as one tunnel excavation. When the right tunnel is excavated, the middle rock pillar has a tendency of further crushing due to the disturbance by the excavation of the left tunnel. This can be confirmed by the monitoring observation of the right tunnel, the displacements in the crown and the sidewalls on the middle rock pillar side were very close, this can be considered as a major sign taking into account the cracks spread along the right tunnel final lining as shown in the next figure. The plastic stress region is caused by the plasticity of the weak surrounding rock, and plastic deformation occurs when the stress exceeds a certain value. In the range of the plastic zone, even if the stress does not increase, the deformation will continue to increase, so the distribution of plastic zone is one of the important indexes that are used to measure the stability of tunnels in soft rock [ 10 ]. A critically stressed fault, with shear stresses exceeding the shear strength, can slip when the degree of freedom is changed as it is intersected by a mine opening, or low depth tunnel excavation. Alternatively, it may slip when the shear strength is reduced due to a drop in clamping stress or water infiltration into the fault, it may also slip when the mining-induced shear stress is increased and exceeds the strength of the fault, which is a function of the normal stress, the coefficient of friction of the fault surface, its waviness or dilation characteristics, and, in the case of fracture propagation, the strength of the rock mass [ 44 ]. The discontinuities are disposed in parallel with tunnel axis [ 45 ]. Discontinuities forming a 20° − 45° are unfavorable to underground excavation making thus the task one of the most challenging in such situation, with a lateral extension in case of a rock mass failure pushing along the discontinuities, as recorded in the next figure showing important side expansion of the ground collapse. 5. Conclusion In this study, a back assessment has been conducted in pre and post-collapse situation where the surrounding ground has experienced large deformations that lasted considerably with the time, and simultaneously with the excavation process, until reaching then exceeding the threshold, that consequently lead to a major collapse. According to the post-collapse investigation campaign, the collapsed area extends up to 130 m in the left tube, with an estimated decompressed zone reaching up the 40 m above the crown of the tunnel, and a lateral expansion damaging the right tunnel, achieved and in service back then, the back analysis outcomes are summed up in the following points: At a certain stage, the support system showed severe malfunction according to recorded ground deformations, knowing that it is a 190 m2 tunnel and in non-circular shape, nevertheless; an upgraded support system was adopted which has reduced the surrounding ground mass displacements, consisting in the doubling the HEB-200 steel-archs with an additional 25 cm shotcrete layer, given the fact that the previously adopted support scheme was not able to bear the ground pressure, moreover the extension of the excavated portion was too important to prevent failure occurrence, taking into account the tunnel section, the applied upgraded support have been efficient in the portion of its application, the previous support obviously could not bear the ground pressure in the direct vicinity of the excavation, notably above the crown of the tunnel. The complex geological conditions have also been proven to be problematic, indeed, the smooth ground conditions and parallel disposition of the discontinuities i.e. 30° were subjected to squeezing, leading to lateral expansion reaching the achieved right tunnel, which these factors shown unfavorable conditions to tunnel coving making the excavation task of the most complicated to take into consideration. In this back analyses, it has been concluded that, given the fact that of the start of increasing displacements and crown settlements, the support system has shown limits of its performance taking into account the measured deformations along the construction of the tunnel. In fact, the adopted support could no longer increase the bearing capacity, resulting to shotcrete cracking and the steel-arch offsetting from the initially installed position. The countermeasure support scheme has proven efficiency given the recorded displacements in their applied portion. 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Q J Eng Geol 5:119–120 Nakano R (1979) Geotechnical properties of mudstone of neogene tertiary in Japan. Int Syrup Soil Mech Oaxaca 1:75–92 Hoek E, Marinos P (2000) Predicting tunnel squeezing problems in weak heterogeneous rock masses. Tunnels Tunn Int 32(11):45–51 Kaiser PK, Cai M (2012) Design of rock support system under rockburst condition. J Rock Mech Geotech Eng 4(3):215–227 Bieniawski ZT (1973) Engineering classification of jointed rock masses. Trans S Afr Inst Civ Engrs 15:335–344 Additional Declarations The authors declare no competing interests. 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5331051","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":370316075,"identity":"60274fd1-975b-4671-a626-f2beb678ab15","order_by":0,"name":"ALLOUACHE Abdelaziz N","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABO0lEQVRIie2QQUvDMBTHXyh0l0KvGcL2FVIGU5m6r5JSaC91CgMpCFoQusvAa0E/RP0GkcB2mfNacYfs5EWku0gPIqZTdkhFdvSQHyQhj/z4vxcAjeZ/gpioDgxIbGp0vVtymb86jH4rBvl5iuJtFRNvFPhDsWNDMBrxtn1zNYlWJQd7Z/QiBBwNdhsP9wWcXagKZiZhdMaddDHxn1LKoXk7c2Rj3nB/PPAwzOsxDAhzE44yHHafLamQPESx+2m4GQsJRomlGm3WKCqln+GT9+GHVPp5sJQpl272+NopUYJVhTBrneLKFNOAKgXTqjFZycOuTCGq4nDrVM4SeGnud5pjP7BwHjophalU3vw9Oqeq0pqO7kQR9Q6vU29ZlAe9lp0GYlXCuWzsmOdF/cfAUO7KtLUMjUaj0WzDFxpodyJhvFJcAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0009-0007-7638-6770","institution":"National higher school of technology and engineering","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"ALLOUACHE","middleName":"Abdelaziz","lastName":"N","suffix":""}],"badges":[],"createdAt":"2024-10-25 09:06:36","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false,"coiExplicitlySet":false},"doi":"10.21203/rs.3.rs-5331051/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5331051/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":67610027,"identity":"efa268f6-f228-4b25-b030-b1627a66bd20","added_by":"auto","created_at":"2024-10-28 05:30:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":371500,"visible":true,"origin":"","legend":"\u003cp\u003eFigure 1. Tunnel’s geographic location (Project’s provided document)\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/26d4d9fa9ce3b63794946f42.png"},{"id":67610391,"identity":"a9d238be-34a4-483b-8ccc-6f0c86b1f391","added_by":"auto","created_at":"2024-10-28 05:38:48","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":53615,"visible":true,"origin":"","legend":"\u003cp\u003eTunnel’s cross sections in metre (Project provided document).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/af2558333bc8830c4894e560.png"},{"id":67610024,"identity":"5375790c-e50b-4cd9-9323-d16c3f2b2c0e","added_by":"auto","created_at":"2024-10-28 05:30:48","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":437764,"visible":true,"origin":"","legend":"\u003cp\u003eArgillite geological formation crossed along by the tunnel construction (Project provided document)\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/975e81c13d414edeb0f75be6.png"},{"id":67610033,"identity":"f02a79cf-aa80-4a45-8e22-259a62b80b70","added_by":"auto","created_at":"2024-10-28 05:30:49","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":144576,"visible":true,"origin":"","legend":"\u003cp\u003eSimplified log strata\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/0f49e0b2eb6e4ebce84b712d.png"},{"id":67610031,"identity":"b7f43eea-5f6f-4e0e-9feb-a9a7d2724e94","added_by":"auto","created_at":"2024-10-28 05:30:48","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":291298,"visible":true,"origin":"","legend":"\u003cp\u003eCollapse phenomenon occasioned damages (Project provided document)\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/34dac1faa0c9fc85a07ab668.png"},{"id":67612232,"identity":"bd450887-aa7c-4b19-b307-322bc1db1b69","added_by":"auto","created_at":"2024-10-28 06:02:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":251834,"visible":true,"origin":"","legend":"\u003cp\u003eRecorded support weakness (steel-arch offsetting with observed cracks on the shotcrete)\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/c21fffbb9d7e3b0153ac108a.png"},{"id":67610021,"identity":"dbdbb1a0-cc25-4f02-9fde-4652901bab04","added_by":"auto","created_at":"2024-10-28 05:30:48","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":120364,"visible":true,"origin":"","legend":"\u003cp\u003eMonitoring points\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/4fdbde838ddc76d190381aa4.png"},{"id":67610020,"identity":"afacbaa3-e764-4e8b-9d69-a337020288b5","added_by":"auto","created_at":"2024-10-28 05:30:48","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":48749,"visible":true,"origin":"","legend":"\u003cp\u003ePlane view of the monitoring points\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/4dc29a332199b3511a567989.png"},{"id":67610394,"identity":"08f5abee-a148-425b-9979-8fd5275d93b7","added_by":"auto","created_at":"2024-10-28 05:38:48","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":76887,"visible":true,"origin":"","legend":"\u003cp\u003eRecorded settlements rate in both section with doubled HEB-200 steel-arch and single HEB-200 steel-archs\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/b3748c0d7c3a0c4322177f9a.png"},{"id":67610023,"identity":"a5c5a523-e5ba-436e-a8c5-35bfac4085ba","added_by":"auto","created_at":"2024-10-28 05:30:48","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":91251,"visible":true,"origin":"","legend":"\u003cp\u003eSingle and double steel-arch support pattern\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/fac07eb67c466f64e9c7593a.png"},{"id":67610030,"identity":"b32ab492-8227-4b20-b3c6-5591df9cb030","added_by":"auto","created_at":"2024-10-28 05:30:48","extension":"jpeg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1245502,"visible":true,"origin":"","legend":"\u003cp\u003eFinite elements analysis results applied\u003c/p\u003e","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/ec8bdb8a498f3f07082f794a.jpeg"},{"id":67610393,"identity":"86a60c1f-a856-44ca-a861-7fcb4b4f0d2e","added_by":"auto","created_at":"2024-10-28 05:38:48","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":112287,"visible":true,"origin":"","legend":"\u003cp\u003eRegistered displacement in the right tunnel, 1; is the crown, 2; middle rock pillar side, 3; free tunnel sidewall\u003c/p\u003e","description":"","filename":"floatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/daaef8eb96a2474e75280fcb.png"},{"id":67610395,"identity":"4035ee39-ac56-42aa-9344-84b59ab3988c","added_by":"auto","created_at":"2024-10-28 05:38:49","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":75410,"visible":true,"origin":"","legend":"\u003cp\u003eCollapse extension view\u003c/p\u003e","description":"","filename":"floatimage13.png","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/3ef89a5b5a7b6981b206fc45.png"},{"id":67612235,"identity":"1f7308bf-27c4-4f4b-997f-5d0dc313ce72","added_by":"auto","created_at":"2024-10-28 06:03:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3611417,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5331051/v1/82c5caa6-4858-4ac8-ab16-85ba88fc3077.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eGeomechanical Back Assessment of a Major Ground Collapse of a Twin Tunnel located in the Eastern Section of the Algerian Highway\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe continuous expansion of the underground excavations leads to considerable issues in some cases, especially with large sections to ensure the most fluid transportation condition, issuing with challenging engineering tasks to carry out. Consequent excavation sections result of a larger deformation range to consider taking into account a freeway tunnel.\u003c/p\u003e \u003cp\u003eLarge deformations of the surrounding rocks are often encountered during tunnel coving within soft rock mass with squeezing characteristics [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], generally lasting from several weeks to several months [\u003cspan additionalcitationids=\"CR3 CR4\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], up to even more in soft rock tunnel [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], often accompanied by the lining cracking and even collapsing [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. These phenomena are widely observed at relatively shallow depths in soft rocks such as phyllite, mudstone, shale, and siltstone [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe laboratory testing explored the failure mechanisms of twin-tunnels [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Jiang et al. (2021) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] has shown the overall failure process of twin-tunnels through 3D printing-based physical models, and they identified the vital part of the structure failure by observation and automated measurements [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Jia et al. (2021) [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] developed a test system for use with the twin-tunnel excavation model to find out the deformation pattern of the surrounding rock and supporting structure. Their research indicated that the failure of twin-tunnels is obviously caused by the concentration of stress that occurring mainly in the middle rock pillar between the twin-tunnels. However, the stress concentration phenomenon will gradually decrease with the clear spacing increasing, when the clear spacing between the twin-tunnels exceeded twice the span of the single tunnel, its influence on the stability of the surrounding rock and supporting structure of the twin-tunnels is ignorable then.\u003c/p\u003e \u003cp\u003eWith continuous development of the transport infrastructures, several tunnels are dug in complicated surrounding rocks structure associated with poor mechanical properties conditions. In low buried tunnels, the surrounding rocks deformations mechanisms at actual construction process have been studied[\u003cspan additionalcitationids=\"CR13 CR14 CR15\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. In shallow or ultra-shallow tunnel excavations, with associated poor mechanical properties, surrounding groundmass instability and unsymmetrical loading, the tunnel\u0026rsquo;s bearing arch is not properly formed, and construction causes a large additional loading resulting to excessive deformations or partial collapse of the tunnel face during construction process [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this research, we will discuss the prior failure conditions leading to major collapse of a large section freeway twin tunnel, taking into account the degraded nature of the surrounding ground, moreover the discontinuities parallel disposition. The above mentioned mechanism are part of the failure process encountered during excavation, also we should underline the support system performance limit, where lining cracks were observed and lining cracks became obvious on; the first, the final lining starts cracking; the second, alarming displacements were recorded and steel arch have been observed in an offset position.\u003c/p\u003e"},{"header":"2. Project, ground conditions and methodology overview","content":"\u003cp\u003eThe tunnel of \u0026ldquo;Djebel Ouahch\u0026rdquo;, Constantine province, Algeria, is taken as reference for this research work. This tunnel is a part of the Maghreb unity motorway (AUM, French acronym). It is approximately 7000km long, crossing Algeria with 1200km long. The tunnel belongs to Sect. 4.1 of this highway. It crosses Djebel Ouahch Mountain in the northeast of Algeria at Constantine province, where it is included between the two mileages, starting from pk205\u0026thinsp;+\u0026thinsp;393 and ending at 207\u0026thinsp;+\u0026thinsp;384.5, with a total length of 1909 m.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt is a1909 m long twin-tunnel, with a radius of 9.57 m, separated by 17 m distance from each other. Average depth is around 120, and could reach a maximum of 140 m locally with the weakest coverage reaching 12 m, and a section approaching 190m\u003csup\u003e2\u003c/sup\u003e shaped as a lowered vault as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAccording to the reconnaissance campaign carried out, both tunnels have always crossed the marl-limestone area corresponding to the Tellian layer, particularly in the argillite unit of the cretaceous age. At the surface, quaternary colluviums flush locally. Rough colluviums are represented by heterometric sandstone elements, pebbles and cornered blocs with red silty mounting, as shown in the next figure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe geological formations encountered are mainly greyish clays, with calcite in places. There are also some compact passages in places. Their dispositions are varying from horizontal to sub-horizontal with calcite joints at 30\u0026deg;. The next table describes the lithology disposition.\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\u003eLithology disposition of the surrounding ground\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDepth (m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEncountered terrain with joints angle in degrees (\u0026deg;)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e30\u0026ndash;32.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBrittle grey argillite, with compact passages of gravel and pebbles\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e32.5\u0026ndash;34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCompact grey argillite with passage of dark argillite with calcite joints and very smooth\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e34\u0026ndash;41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCompact grey argillite with passage of dark argillite with calcite joints at 30\u0026deg; and whitish patches\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e41\u0026ndash;47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGreyish, smooth, bluish argillite, sometimes firm, with calcite joints, 30 to 70\u0026deg;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e47\u0026ndash;59.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGreyish, smooth, bluish, plastic argillite with 30\u0026deg; joints\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e59.5\u0026ndash;110\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGreyish, smooth, sometimes bluish, firm argillite, fractured rock, with calcite joints at 30\u0026deg;\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\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\u003eMain mechanical properties of the surrounding ground mass\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUnit weight ɣ(KN/M\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYoung modulus E (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCohesion C (KPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFrictional angle ɸ (\u0026deg;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePoisson\u0026rsquo;s ratio ʋ\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e110\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.3\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\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummarized in-situ tests\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" 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=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDepth (m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eVertical in situ stress P\u003csub\u003e0\u003c/sub\u003e (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eUniaxial compressive strength of the intact material σ\u003csub\u003eci\u003c/sub\u003e (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRMR\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCoefficient of lateral pressure Kp\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.5\u0026ndash;3.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5\u0026ndash;8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14\u0026ndash;18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.71\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.0\u0026ndash;3.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2\u0026ndash;7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14\u0026ndash;20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.71\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\u003eIn the construction record, the right tube has entirely been dug and in use, then the construction process of the second tube took place (left tube). The NATM (New Austrian tunnelling method) was adopted in the construction phase of the left tube; the bench-cut excavation section. A double-layered support system was carried out in the construction process of the left tunnel, the temporary support scheme materials was constituted of: HEB 200 steel-arch with 0.75 m pitch spacing, 6 m hollow micro-fissured grouting fiberglass bolts, and 30 cm thick shotcrete layer. For the secondary lining, a 60 cm thick layer of reinforced concrete was used. On January 1st 2014 at between 16h00 and 16h30, a major ground collapse occurred in the left tube, expanding from the mileage pk206\u0026thinsp;+\u0026thinsp;150 to pk206\u0026thinsp;+\u0026thinsp;280, The collapse has severely damaged the right tube (already achieved and in use back then) causing serious damages on the lining of the right tube, and the total collapse length has reached 140 m long. Neither wounded nor casualties have been registered after the accident\u0026rsquo;s occurrence.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"3. Surveying, pre-collapse situation and countermeasures adopted","content":"\u003cp\u003eThe monitoring activity provides quantitative data to measure for analyses at a real situation in the tunneling process. Accurately determining the surrounding rocks is crucial for ensuring safety in underground works [\u003cspan additionalcitationids=\"CR20 CR21 CR22\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Many researches underscored the monitoring importance to study the deformation of surrounding rocks and have proposed measures to ensure safe construction during tunnel excavation [\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Indeed, monitoring process is relevant to study/identify the surrounding rock mass deformation in order to take the appropriate measures [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] and empower the decision making process. The displacements were obtained by the use of topographic station, working with optical targets reflectors, installed around tunnel profile (Five targets), with a stable reference point outside the zone of convergence to measure displacements. Topographic stations are more or less 0.2 mm over 100 m accurate for tunneling process [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe previously mentioned support scheme has been adopted during the construction of the left tunnel. At a certain construction advance, important displacements were recorded exceeding the allowed project\u0026rsquo;s safety deformations limits, established as written in Table \u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMaximum deformations allowed during excavation process\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eVigilance limit (mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAlert limit (mm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSide wall deformations\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e110\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCrown settlements\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e130\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\u003eWhen the excavation progression reached certain mileages, the adopted support system started to show obvious weakness (observable with the naked eye), where serious shotcrete cracks appeared and the steel-archs started offsetting from the initially installed position.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt has been reported that the ground displacements were continuously increasing with the advance of the excavation, confirming both the vulnerability of the adopted support system and the groundmass softening due to excessive displacements monitored over the previously mentioned limits in Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. These displacements were lasting during a considerable period, extending up to a month along all the previously underlined mileages i.e. pk206\u0026thinsp;+\u0026thinsp;150 to pk206\u0026thinsp;+\u0026thinsp;280; we will take the following four monitoring points PK206\u0026thinsp;+\u0026thinsp;221, PK206\u0026thinsp;+\u0026thinsp;226, PK206\u0026thinsp;+\u0026thinsp;233, and PK206\u0026thinsp;+\u0026thinsp;236, these monitoring points are part of the area where the support countermeasures were not applied, the values are the average daily measurements.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eRecorded displacements (layout\u0026thinsp;+\u0026thinsp;pk projection)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMonitoring points\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRecorded crown settlements, min/max (mm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRecorded side wall deformations, min/max (mm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePK206\u0026thinsp;+\u0026thinsp;221\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20.6 / 96.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e38.7 / 53.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePK206\u0026thinsp;+\u0026thinsp;226\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e59.6 / 87.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e22.4 / 43.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePK206\u0026thinsp;+\u0026thinsp;233\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e99.2 / 145.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e41.9 / 75.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePK206\u0026thinsp;+\u0026thinsp;236\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e97.9 / 114\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e89.3 / 38.4\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\u003e \u003c/p\u003e \u003cp\u003eGiven the recorded displacements shown by the table above, an increasing displacement situation of the tunnel deformation has taken place, sometimes registered over the limits set by the project\u0026rsquo;s limit safety deformations.\u003c/p\u003e \u003cp\u003eAfter the previously deformations have been recorded, urgent measures were proposed to mitigate the danger observed during excavation, knowing the right tube was already in use. In the scope of risk management and collapse prevention, the selected construction technique is a key role for the timely project\u0026rsquo;s achievements [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn section 2, paragraph 5, the support scheme is described; the fastest measure taken into application was to double the HEB-200 steel-arch due to the situation\u0026rsquo;s emergency, thus apply another shotcrete layer 25 cm thick and apply the first layer with 35 instead of 30 cm as previously cited. Indeed, the upgraded reinforcement system shown effectiveness of its reliability by the ground movements decreasing in the applied area of this improved support scheme, also a clearly broad difference settlement rate is observed in the section where the initial support system is used as shown in the next Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eAs observed in the next figure, settlements rate is divided in two obvious sections, i.e. first where the urgent support measure is applied prior to mileage PK206\u0026thinsp;+\u0026thinsp;220, settlements rate is more or less reasonable given the recorded values, second section is represented beyond the mileage PK206\u0026thinsp;+\u0026thinsp;220, in this section single HEB-200 steel-arch support are deployed, displacements are alarming reaching a peak of 41.7 mm/day at mileage PK206\u0026thinsp;+\u0026thinsp;230.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe adopted fitted the required support resistance taking into account the ground reaction to the excavation as shown in the next figure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe countermeasure support is obviously fitting the required resistance despite the drastic displacements previously observed.\u003c/p\u003e \u003cp\u003eThe finite elements analysis confirms both support scheme competence, where the Mohr-coulomb constitutive model is adopted. The decompressed zone elevates up to 40 m over the tunnel crown, making an excessive stress for the left tunnel bearing capacity relevant to the simple arch support, which is widely reduced putting to evidence the treatments competency after exceeding the maximum shear strength capacity as summed up in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e. The next table sums up the Mohr-Coulomb in-put for the modelling.\u003c/p\u003e "},{"header":"4. Collapse mechanism feedback","content":"\u003cp\u003eIn soft rock tunnels, geological conditions and mechanical properties are the essential causes to large deformations issues during tunneling [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Strength/stress (SSR) ratio, defined as the ratio between rock mass uniaxial compressive strength σcm and the in situ stress P0, is recognized as rock squeezing extent factor [1.35.36]. Mechanical properties of the rock mass and geo-stress levels are important factors inducing large-deformation phenomenon in tunnels.\u003c/p\u003e \u003cp\u003ePrevious research [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] concluded that excavation induced plastic deformations is considered to be one of the most important factors leading to squeezing in soft rock tunnels. Plastic deformations at different extent, moderate to severe, depending on the geo-stress magnitude in vertical direction and rock strength e.g. σcm/P0. Over the past several decades, based on the rock mass classification system, many empirical equations for calculating the strength of rock mass (σcm) have been proposed. The most widely used equations and the results of σcm at the geostress test areas are listed in Table\u0026nbsp;\u003cspan refid=\"Tab7\" class=\"InternalRef\"\u003e6\u003c/span\u003e. In these equations, the uniaxial compressive strength of intact rock is taken as σci\u0026thinsp;=\u0026thinsp;8 MPa and the RMR\u0026thinsp;=\u0026thinsp;14.\u003c/p\u003e \u003cp\u003eBased on the equations in Table \u003cspan refid=\"Tab7\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the average value the σcm is 171 KPa, and P0 as 3.0 MPa\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab7\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEmpirical equations and results of strength of rock mass (σ\u003csub\u003ecm\u003c/sub\u003e) at the geostress test areas\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\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=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eResearcher\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEquation (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eσ\u003csub\u003eCM\u003c/sub\u003e (kPa)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKalmaris and Bieniawski (1995) [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:cm=\\sigma\\:ci\\:\\text{e}\\text{x}\\text{p}\\left(\\frac{RMR-100}{24}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e222\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSheorey (1997) [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:cm=\\sigma\\:ci\\:exp\\left(\\frac{RMR-100}{20}\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e108\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTruman (1998) [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:cm=0.06\\text{e}\\text{x}\\text{p}\\left(0.06RMR\\right)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e139\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAydan and Dalgic (1998) [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:cm=\\sigma\\:ci\\frac{RMR}{RMR+6(100-RMR)}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e211\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 SSR, being σ\u003csub\u003ecm\u003c/sub\u003e/P\u003csub\u003e0\u003c/sub\u003e [\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e], is 0.07 lower than 0.1, which is less than the cut-off value of the SSR (severely squeezing rock) [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], and severely squeezing with support problem [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Consequently, the surrounding rock are severely squeezing of the considered tunnel with an issue of an unfit support.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 7\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMohr-Coulomb yield criterion in-put\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eYoung modulus (E, MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePoisson\u0026rsquo;s ration\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCohesion (KPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFrictional angle (\u0026deg;)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e110\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e18\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\u003e \u003c/p\u003e\u003cp\u003eAccording to the Mohr-Coulomb criterion, the yielding of rock mass has close relationships with both the deviator stress and the rock mass strength. The adjusted deviator stress after excavation in a high stress case is obviously higher than that in a low stress case, which leads to the rock fracture propagation and further accelerates the deformation and failure of rock masses. Compared with the case of high rock strength, rock masses with low strength yield more easily, resulting in a larger deformation of plastic flow.\u003c/p\u003e \u003cp\u003eAt a certain range, interaction between two tunnels cannot be ignored. Due to a certain clear spacing of twin tunnels, the right tunnel status is affected by the excavation of the left. When the right tunnel is excavated, it can be regarded as one tunnel excavation. When the right tunnel is excavated, the middle rock pillar has a tendency of further crushing due to the disturbance by the excavation of the left tunnel. This can be confirmed by the monitoring observation of the right tunnel, the displacements in the crown and the sidewalls on the middle rock pillar side were very close, this can be considered as a major sign taking into account the cracks spread along the right tunnel final lining as shown in the next figure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe plastic stress region is caused by the plasticity of the weak surrounding rock, and plastic deformation occurs when the stress exceeds a certain value. In the range of the plastic zone, even if the stress does not increase, the deformation will continue to increase, so the distribution of plastic zone is one of the important indexes that are used to measure the stability of tunnels in soft rock [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA critically stressed fault, with shear stresses exceeding the shear strength, can slip when the degree of freedom is changed as it is intersected by a mine opening, or low depth tunnel excavation. Alternatively, it may slip when the shear strength is reduced due to a drop in clamping stress or water infiltration into the fault, it may also slip when the mining-induced shear stress is increased and exceeds the strength of the fault, which is a function of the normal stress, the coefficient of friction of the fault surface, its waviness or dilation characteristics, and, in the case of fracture propagation, the strength of the rock mass [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The discontinuities are disposed in parallel with tunnel axis [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Discontinuities forming a 20\u0026deg; \u0026minus;\u0026thinsp;45\u0026deg; are unfavorable to underground excavation making thus the task one of the most challenging in such situation, with a lateral extension in case of a rock mass failure pushing along the discontinuities, as recorded in the next figure showing important side expansion of the ground collapse.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eIn this study, a back assessment has been conducted in pre and post-collapse situation where the surrounding ground has experienced large deformations that lasted considerably with the time, and simultaneously with the excavation process, until reaching then exceeding the threshold, that consequently lead to a major collapse. According to the post-collapse investigation campaign, the collapsed area extends up to 130 m in the left tube, with an estimated decompressed zone reaching up the 40 m above the crown of the tunnel, and a lateral expansion damaging the right tunnel, achieved and in service back then, the back analysis outcomes are summed up in the following points:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eAt a certain stage, the support system showed severe malfunction according to recorded ground deformations, knowing that it is a 190 m2 tunnel and in non-circular shape, nevertheless; an upgraded support system was adopted which has reduced the surrounding ground mass displacements, consisting in the doubling the HEB-200 steel-archs with an additional 25 cm shotcrete layer, given the fact that the previously adopted support scheme was not able to bear the ground pressure, moreover the extension of the excavated portion was too important to prevent failure occurrence, taking into account the tunnel section, the applied upgraded support have been efficient in the portion of its application, the previous support obviously could not bear the ground pressure in the direct vicinity of the excavation, notably above the crown of the tunnel.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe complex geological conditions have also been proven to be problematic, indeed, the smooth ground conditions and parallel disposition of the discontinuities i.e. 30\u0026deg; were subjected to squeezing, leading to lateral expansion reaching the achieved right tunnel, which these factors shown unfavorable conditions to tunnel coving making the excavation task of the most complicated to take into consideration.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIn this back analyses, it has been concluded that, given the fact that of the start of increasing displacements and crown settlements, the support system has shown limits of its performance taking into account the measured deformations along the construction of the tunnel. In fact, the adopted support could no longer increase the bearing capacity, resulting to shotcrete cracking and the steel-arch offsetting from the initially installed position. The countermeasure support scheme has proven efficiency given the recorded displacements in their applied portion. Despite support efficiency, a major ground collapse has occurred, due to the important excavated section added to this the included volume extension. In the feedback contribution, the timely and appropriate support scheme are crucial assets to consider and reconsider if necessary given the ground complexity, it would be better to timely apply an upgraded support system in order to prevent or mitigate ground deformations, especially in large sections and rock mass with poor mechanical properties.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBian K, Liu J, Liu Z, Liu S, Ai F, Zheng X, Ni S, Zhang W (2019) Mechanisms of large deformation in soft rock tunnels: a case study of Huangjiazhai Tunnel. Bull Eng Geol Environ 78:431\u0026ndash;444\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAydan O, Dalgic S (1998) Prediction of deformation behavior of 3 lanes Bolu tunnels through squeezing rocks of North Anotolian Fault Zone (NAFZ). 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Trans S Afr Inst Civ Engrs 15:335\u0026ndash;344\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"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":"Back assessment, Twin-tunnel, Major collapse, Support performance","lastPublishedDoi":"10.21203/rs.3.rs-5331051/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5331051/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHighway tunnels are sometimes driven in a challenging ground conditions and technical constrains, within surrounding groundmass with poor mechanical properties, namely the cretaceous soil, which is a highly weathered and degraded rock with scaly structure. These tunnels are often excavated with large cross-sections and often in double-tubes line, leaving a clear spacing between each other, in order to ensure traffic fluidity and security services. High section tunnels usually result in large stress perturbations and deformations leading to a ground collapse phenomenon in misunderstood behaviour conditions of the groundmass, more particularly freeway twin tunnels with clear spacing.\u003c/p\u003e\n\u003cp\u003eThis paper takes into consideration a twin 3-lanes tunnel, part of a national freeway project of around 1200 km, with a non-circular section of a 190 m² each and a clear spacing of 17 m. Given the surrounding rock conditions, mentioned above, at a certain project stage, substantial displacements reached a peak of 41 mm/day, moreover, shotcrete cracks were observed prior to a major collapse extending up to 130 m in the tunnel direction. Back assessment is proposed in this work of post-collapse and ante-collapse main disturbance factors, feedback outcomes will be valuable for the ascertaining and understanding of underground failures phenomenon.\u003c/p\u003e","manuscriptTitle":"Geomechanical Back Assessment of a Major Ground Collapse of a Twin Tunnel located in the Eastern Section of the Algerian Highway","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-10-28 05:30:43","doi":"10.21203/rs.3.rs-5331051/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":"bdee882b-1bee-4f66-9078-1a38b158c40c","owner":[],"postedDate":"October 28th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":39413349,"name":"Civil Engineering"}],"tags":[],"updatedAt":"2024-10-28T05:30:43+00:00","versionOfRecord":[],"versionCreatedAt":"2024-10-28 05:30:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5331051","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5331051","identity":"rs-5331051","version":["v1"]},"buildId":"-HB7Z8yhvgn0wM9Nzuekk","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}