Chao Wang, Zhipeng Xiao, Vanessa Di Murro, John Osborne, Miles Friedman, Zili Li
Long-term structural performance of ageing tunnels is influenced by various natural and anthropogenic factors. This study examines the impacts of two rarely-investigated climatic factors: rainfall and temperature. Two dedicated case studies were conducted on the CERN TT10 tunnel and Dublin Port Tunnel (DPT) using distributed fibre optic strain sensing (DFOS) and wireless sensor network (WSN) monitoring respectively. DFOS data showed an increasing deformation in TT10 tunnel, attributed to tunnel deteriorations and ground deformation, with seasonal variation of lining strains linked to rainfall-related seasonal change in pore water pressure. However, inconsistencies in rainfall-strain correlation were also noted due to geological complexities and varying pore water pressure sources. In contrast, WSN measurements showed that DPT deformation correlated with temperature, instead of precipitation. DPT deformation increased in warmer seasons and decreased in colder ones, in the absence of external disturbances, comprising reversible thermal deformation and irreversible deterioration-induced deformation. Over time, cyclic and periodic temperature changes caused elastic deformation to reverse, while plastic deformation accumulated, leading to ongoing tunnel deformation. These findings bring more insights into the resilience of critical underground infrastructure vulnerable to climate change, groundwater variations, and other environmental factors.
{"title":"The impact of thermal-hydraulic variation on tunnel long-term performance: a tale of two tunnels","authors":"Chao Wang, Zhipeng Xiao, Vanessa Di Murro, John Osborne, Miles Friedman, Zili Li","doi":"10.1680/jgeot.23.00074","DOIUrl":"https://doi.org/10.1680/jgeot.23.00074","url":null,"abstract":"Long-term structural performance of ageing tunnels is influenced by various natural and anthropogenic factors. This study examines the impacts of two rarely-investigated climatic factors: rainfall and temperature. Two dedicated case studies were conducted on the CERN TT10 tunnel and Dublin Port Tunnel (DPT) using distributed fibre optic strain sensing (DFOS) and wireless sensor network (WSN) monitoring respectively. DFOS data showed an increasing deformation in TT10 tunnel, attributed to tunnel deteriorations and ground deformation, with seasonal variation of lining strains linked to rainfall-related seasonal change in pore water pressure. However, inconsistencies in rainfall-strain correlation were also noted due to geological complexities and varying pore water pressure sources. In contrast, WSN measurements showed that DPT deformation correlated with temperature, instead of precipitation. DPT deformation increased in warmer seasons and decreased in colder ones, in the absence of external disturbances, comprising reversible thermal deformation and irreversible deterioration-induced deformation. Over time, cyclic and periodic temperature changes caused elastic deformation to reverse, while plastic deformation accumulated, leading to ongoing tunnel deformation. These findings bring more insights into the resilience of critical underground infrastructure vulnerable to climate change, groundwater variations, and other environmental factors.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"40 18","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141924387","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
E. Covilla, D. Mašín, J. Duque, J. Najser, J. Roháč
This article experimentally evaluates the influence of shearing rate on the monotonic and cyclic response of isotropically-consolidated samples of Malaysian kaolin. On the one hand, a series of undrained monotonic triaxial tests were performed with varying shearing displacement rate. On the other hand, undrained cyclic triaxial tests were conducted considering different deviatoric stress amplitudes and loading frequencies. The well-known soil rate-dependency under monotonic loading was confirmed up to a displacement rate threshold. The experimental results under cyclic loading suggest that for the given loading frequency, the variation of the deviatoric stress amplitude remarkably influences the strains and pore water pressure accumulation rates. In addition, the results suggest that depending on the loading frequency different shapes of mobilized effective stress loops are obtained. Larger loading frequencies lead to banana-shaped effective stress loops, while smaller frequencies reproduce eight-shaped effective stress loops. Furthermore, higher loading frequencies result in a larger number of cycles required to reach failure conditions. The reasons for the observed differences in the behavior are thoroughly analyzed and discussed.
{"title":"On the influence of the shearing rate on the monotonic and cyclic response of Malaysian kaolin","authors":"E. Covilla, D. Mašín, J. Duque, J. Najser, J. Roháč","doi":"10.1680/jgeot.23.00310","DOIUrl":"https://doi.org/10.1680/jgeot.23.00310","url":null,"abstract":"This article experimentally evaluates the influence of shearing rate on the monotonic and cyclic response of isotropically-consolidated samples of Malaysian kaolin. On the one hand, a series of undrained monotonic triaxial tests were performed with varying shearing displacement rate. On the other hand, undrained cyclic triaxial tests were conducted considering different deviatoric stress amplitudes and loading frequencies. The well-known soil rate-dependency under monotonic loading was confirmed up to a displacement rate threshold. The experimental results under cyclic loading suggest that for the given loading frequency, the variation of the deviatoric stress amplitude remarkably influences the strains and pore water pressure accumulation rates. In addition, the results suggest that depending on the loading frequency different shapes of mobilized effective stress loops are obtained. Larger loading frequencies lead to banana-shaped effective stress loops, while smaller frequencies reproduce eight-shaped effective stress loops. Furthermore, higher loading frequencies result in a larger number of cycles required to reach failure conditions. The reasons for the observed differences in the behavior are thoroughly analyzed and discussed.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"73 24","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141268467","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
F. Parera Morales, G. A. Siemens, M. McKellar, N. Pinyol, E. E. Alonso, W. Andy Take
The dry fringe that appears beneath a footprint on a beach is a visual manifestation of dilation. Dilation causes the sand to attempt to increase in volume on shearing, resulting in negative pore pressure, and enhanced bearing capacity. Conventional theory cannot explain this increase in strength, suggesting instead that bearing capacity with the phreatic surface at the sand surface should be approximately half that of the dry case. To explore this apparent contradiction, ten load-controlled bearing capacity experiments were performed on transparent soil with phreatic surfaces ranging from 100 mm below to 30 mm above the footing. For phreatic surfaces deeper than the footing width, failure occurred within the dry material. In shallower phreatic surface conditions, bearing capacity increased as the phreatic surface approached the surface. Air entry was observed to initiate along the top surface of the saturated layer and extend to the depth of the bearing capacity mechanism. For the case of the phreatic and soil surfaces being coincident, the bearing capacity was observed to be more than double that of the dry case. In submerged cases bearing capacity was less than at-surface, as there was no air-fluid interface to enhance the development of negative pore pressure.
{"title":"Footprints on the beach: visualizing dilation-induced air entry","authors":"F. Parera Morales, G. A. Siemens, M. McKellar, N. Pinyol, E. E. Alonso, W. Andy Take","doi":"10.1680/jgeot.23.00243","DOIUrl":"https://doi.org/10.1680/jgeot.23.00243","url":null,"abstract":"The dry fringe that appears beneath a footprint on a beach is a visual manifestation of dilation. Dilation causes the sand to attempt to increase in volume on shearing, resulting in negative pore pressure, and enhanced bearing capacity. Conventional theory cannot explain this increase in strength, suggesting instead that bearing capacity with the phreatic surface at the sand surface should be approximately half that of the dry case. To explore this apparent contradiction, ten load-controlled bearing capacity experiments were performed on transparent soil with phreatic surfaces ranging from 100 mm below to 30 mm above the footing. For phreatic surfaces deeper than the footing width, failure occurred within the dry material. In shallower phreatic surface conditions, bearing capacity increased as the phreatic surface approached the surface. Air entry was observed to initiate along the top surface of the saturated layer and extend to the depth of the bearing capacity mechanism. For the case of the phreatic and soil surfaces being coincident, the bearing capacity was observed to be more than double that of the dry case. In submerged cases bearing capacity was less than at-surface, as there was no air-fluid interface to enhance the development of negative pore pressure.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"74 8","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141268461","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Guo-qing Cai, Xian-feng Diao, Xu-zhen He, Shuai Gao, Tao Liu
Different layers of soil often have distinct particle sizes. When exposed to the natural environment, soil is easily affected by natural rainfall, rising groundwater levels, and human activities, leading to particle contact erosion, which reduces the safety and service performance of the soil structure. In this paper, a coupled computational fluid dynamics–discrete element method (CFD–DEM) model was employed to investigate the particle migration phenomena, mechanical response of contact interfaces, variations in flow fields, and macroscopic deformation during the contact erosion process under cyclic loads at different frequencies and amplitudes. The conclusions are presented as follows: (1) Within one cycle of cyclic loading, both compression during loading and stress relaxation during unloading are the main factors triggering the migration of fine particles. (2) The migration and loss of fine particles mainly occur in the early stages of cyclic loading, where strong contact force chains are formed within the fine particle layer, leading to significant plastic deformation of the soil at the macroscopic level. (3) Under cyclic loading, changes in the soil pore structure cause an upwards hydraulic gradient in the initial quiescent water flow field. This hydraulic gradient can rupture weak contact force chains and cause particle pumping. (4) Increasing the frequency and amplitude of cyclic loading intensifies the erosion of fine particles, causing greater axial deformation of the soil. Compared to cyclic loading frequency, the amplitude of cyclic loading has a greater impact on contact erosion.
{"title":"Micromechanical analysis of contact erosion under cyclic loads using the coupled CFD‒DEM method","authors":"Guo-qing Cai, Xian-feng Diao, Xu-zhen He, Shuai Gao, Tao Liu","doi":"10.1680/jgeot.23.00458","DOIUrl":"https://doi.org/10.1680/jgeot.23.00458","url":null,"abstract":"Different layers of soil often have distinct particle sizes. When exposed to the natural environment, soil is easily affected by natural rainfall, rising groundwater levels, and human activities, leading to particle contact erosion, which reduces the safety and service performance of the soil structure. In this paper, a coupled computational fluid dynamics–discrete element method (CFD–DEM) model was employed to investigate the particle migration phenomena, mechanical response of contact interfaces, variations in flow fields, and macroscopic deformation during the contact erosion process under cyclic loads at different frequencies and amplitudes. The conclusions are presented as follows: (1) Within one cycle of cyclic loading, both compression during loading and stress relaxation during unloading are the main factors triggering the migration of fine particles. (2) The migration and loss of fine particles mainly occur in the early stages of cyclic loading, where strong contact force chains are formed within the fine particle layer, leading to significant plastic deformation of the soil at the macroscopic level. (3) Under cyclic loading, changes in the soil pore structure cause an upwards hydraulic gradient in the initial quiescent water flow field. This hydraulic gradient can rupture weak contact force chains and cause particle pumping. (4) Increasing the frequency and amplitude of cyclic loading intensifies the erosion of fine particles, causing greater axial deformation of the soil. Compared to cyclic loading frequency, the amplitude of cyclic loading has a greater impact on contact erosion.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"6 5","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141271064","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Prithvendra Singh, D. N. Singh, Chuangxin Lyu, Gustav Grimstad, Satoshi Nishimura
{"title":"Discussion on “Pore pressure coefficient in frozen soils”","authors":"Prithvendra Singh, D. N. Singh, Chuangxin Lyu, Gustav Grimstad, Satoshi Nishimura","doi":"10.1680/jgeot.24.00100","DOIUrl":"https://doi.org/10.1680/jgeot.24.00100","url":null,"abstract":"","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"10 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-06-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141273669","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Wenli Lin, Ang Liu, Erkang Zhang, Shuyu Tian, Deqi He
This study investigates the impact of cementation on the mechanical behavior of sands with various cement content (CSR) in drained triaxial compression, employing both Acoustic Emission (AE) and Environmental Scanning Electron Microscopy (ESEM) measurements. The experimental findings, encompassing quantitative statistics of stress-strain relations, microstructure variations, and AE characteristics, demonstrate that: the addition of CSR from 1% to 20% leads to an exponential rise in peak strength and stiffness, marking a transition from ductile to brittle mechanical failure, which is pinpointed between CSR levels of 5% to 10%. AE characteristics unveil an upward-opening parabola of normalized AE hits with CSR, a clear transition zone identification, and three distinct types of AE rate evolutions corresponding to failure patterns of ductile bulging, shear banding, and brittle fracturing, respectively. It suggests an intimate correlation with the intrinsic differences in micro-mechanical behaviors and AE propagation properties of cemented sands with varying CSRs. Notably, the bulging and shear banding processes are divided by AE into three stages, whereas fracturing is characterized into five stages. Two precursory AE anomalies associated with incipient failure and complex failure modes are observed, emphasizing the advantage of using AE to reflect the internal micro-mechanical behavior of cemented sands over conventional stress-strain manifestations.
{"title":"Effect of cementation on the mechanical response of sands using acoustic emission technique","authors":"Wenli Lin, Ang Liu, Erkang Zhang, Shuyu Tian, Deqi He","doi":"10.1680/jgeot.23.00053","DOIUrl":"https://doi.org/10.1680/jgeot.23.00053","url":null,"abstract":"This study investigates the impact of cementation on the mechanical behavior of sands with various cement content (<i>CSR</i>) in drained triaxial compression, employing both Acoustic Emission (AE) and Environmental Scanning Electron Microscopy (ESEM) measurements. The experimental findings, encompassing quantitative statistics of stress-strain relations, microstructure variations, and AE characteristics, demonstrate that: the addition of <i>CSR</i> from 1% to 20% leads to an exponential rise in peak strength and stiffness, marking a transition from ductile to brittle mechanical failure, which is pinpointed between <i>CSR</i> levels of 5% to 10%. AE characteristics unveil an upward-opening parabola of normalized AE hits with <i>CSR</i>, a clear transition zone identification, and three distinct types of AE rate evolutions corresponding to failure patterns of ductile bulging, shear banding, and brittle fracturing, respectively. It suggests an intimate correlation with the intrinsic differences in micro-mechanical behaviors and AE propagation properties of cemented sands with varying <i>CSR</i>s. Notably, the bulging and shear banding processes are divided by AE into three stages, whereas fracturing is characterized into five stages. Two precursory AE anomalies associated with incipient failure and complex failure modes are observed, emphasizing the advantage of using AE to reflect the internal micro-mechanical behavior of cemented sands over conventional stress-strain manifestations.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"83 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140830216","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
H. Jin, Zhen Huang, Kang Yao, Li Shi, Yuanqiang Cai
The undrained cyclic behaviour of clay under three-dimensional (3D) stress states is vital in marine geotechnical applications, especially concerning deformation and stability issues related to pile foundations under oceanic environmental loadings. However, the 3D cyclic behaviour of clay is rarely undertaken in the laboratory. This paper presents an experimental study on the undrained 3D responses of clay performed with a true triaxial apparatus, where horizontal one-way and vertical two-way cyclic stresses are applied to the specimen simultaneously. Emphasis is put on the effects of different combinations of horizontal and vertical stresses on the behaviour of strain, pore water pressure, mobilised friction, and modulus. Test results indicated that cyclic behaviour strongly depended on the 3D stress conditions or stress paths. By introducing a new cyclic stress ratio (CSRA), it was demonstrated that the cyclic shakedown behaviours of clay could be made independent of stress paths. An allowable CSRA, acting as a cyclic stress boundary ratio, was established at 0·08. Interestingly, an increase in the modulus of clay was observed during cyclic loading. This phenomenon contradicted the typically observed modulus degradation of clay under undrained cyclic loadings, adding a new perspective to our understanding of the material's behaviour.
{"title":"Undrained three-dimensional behaviour of soft clay under horizontal one-way and vertical two-way cyclic loading","authors":"H. Jin, Zhen Huang, Kang Yao, Li Shi, Yuanqiang Cai","doi":"10.1680/jgeot.23.00181","DOIUrl":"https://doi.org/10.1680/jgeot.23.00181","url":null,"abstract":"The undrained cyclic behaviour of clay under three-dimensional (3D) stress states is vital in marine geotechnical applications, especially concerning deformation and stability issues related to pile foundations under oceanic environmental loadings. However, the 3D cyclic behaviour of clay is rarely undertaken in the laboratory. This paper presents an experimental study on the undrained 3D responses of clay performed with a true triaxial apparatus, where horizontal one-way and vertical two-way cyclic stresses are applied to the specimen simultaneously. Emphasis is put on the effects of different combinations of horizontal and vertical stresses on the behaviour of strain, pore water pressure, mobilised friction, and modulus. Test results indicated that cyclic behaviour strongly depended on the 3D stress conditions or stress paths. By introducing a new cyclic stress ratio (CSRA), it was demonstrated that the cyclic shakedown behaviours of clay could be made independent of stress paths. An allowable CSRA, acting as a cyclic stress boundary ratio, was established at 0·08. Interestingly, an increase in the modulus of clay was observed during cyclic loading. This phenomenon contradicted the typically observed modulus degradation of clay under undrained cyclic loadings, adding a new perspective to our understanding of the material's behaviour.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":" 12","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140687059","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Davood Dadras-Ajirlou, Gustav Grimstad, Seyed Ali Ghoreishian Amiri, Samson Abate Degago, Guy Tinmouth Houlsby
Clays exhibit complex mechanical behaviour with significant viscous, nonlinear, and hysteric characteristics, beyond the prediction capacity of the well-known modified cam clay (MCC) model. This paper extends the MCC model to address these important limitations. The proposed family of models is constructed entirely within the hyperplasticity framework deduced from thermodynamic extremal principles. More specifically, the previously developed MCC hyper-viscoplastic model based on the isotache concept is extended to incorporate multiple internal variables and to capture recent loading history, hysteresis, and smooth response of the material. This is achieved by defining an inelastic free energy and an element that implements a bounding surface within hyperplasticity, resulting in pressure dependency in both reversible and irreversible processes with a unique critical state envelope, and only eight material parameters with a readily measurable viscous parameter. A kinematic hardening in the logistic differential form in stress space is derived that enables the proposed model to function effectively across a wide range of stresses. Based on this kinematic hardening rule, the current stress state acts as an asymptotic attractor for the back/shift stresses whose evolution rates are proportional to their current state.
{"title":"Modified cam clay bounding surface hyper-viscoplastic model","authors":"Davood Dadras-Ajirlou, Gustav Grimstad, Seyed Ali Ghoreishian Amiri, Samson Abate Degago, Guy Tinmouth Houlsby","doi":"10.1680/jgeot.23.00308","DOIUrl":"https://doi.org/10.1680/jgeot.23.00308","url":null,"abstract":"Clays exhibit complex mechanical behaviour with significant viscous, nonlinear, and hysteric characteristics, beyond the prediction capacity of the well-known modified cam clay (MCC) model. This paper extends the MCC model to address these important limitations. The proposed family of models is constructed entirely within the hyperplasticity framework deduced from thermodynamic extremal principles. More specifically, the previously developed MCC hyper-viscoplastic model based on the isotache concept is extended to incorporate multiple internal variables and to capture recent loading history, hysteresis, and smooth response of the material. This is achieved by defining an inelastic free energy and an element that implements a bounding surface within hyperplasticity, resulting in pressure dependency in both reversible and irreversible processes with a unique critical state envelope, and only eight material parameters with a readily measurable viscous parameter. A kinematic hardening in the logistic differential form in stress space is derived that enables the proposed model to function effectively across a wide range of stresses. Based on this kinematic hardening rule, the current stress state acts as an asymptotic attractor for the back/shift stresses whose evolution rates are proportional to their current state.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"41 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140625825","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Matteo Pedrotti, Alessandro Tarantino, Antonio Annese, Federica Cotecchia, Claudia Vitone
Scaly clays are intensely fissured clays with lens shaped elements of millimetre size and show a complex compression behaviour that poses challenges to the design and construction of geostructures (excavations, retaining diaphragms, and tunnels). Scaly clays show a Normal Compression Line (NCL) where plastic deformation accumulates as typically observed in non-scaly clays. Yet the response observed upon unloading and subsequent reloading is very peculiar, i) the unloading-reloading cycle is typically a close-loop with relatively large hysteresis; ii) the compressibility recorded at high OCR ratio of the unloading or reloading branches is close to the NCL compressibility. This paper presents a microstructural study on an Italian scaly clay where SEM observations are integrated with Mercury Intrusion Porosimetry (MIP) analyses and X-ray Computed Tomography (XCT) images. The mechanism associated with the closing of inter-scale porosity and the generation of new intra-scale porosity was identified as the process responsible for the plastic deformation. Experimental observation of reconstituted clay showed a “quasi-reversible” behaviour upon loading and unloading and a pore size distribution characterized only by interparticle porosity. The observation that unloading and reloading curves are parallel in natural and reconstituted clays, led to postulate that the interparticle porosity is controlling the elastic response.
鳞片状粘土是具有毫米级透镜状元素的强烈裂隙粘土，具有复杂的压缩行为，给土工结构（挖掘、挡土墙和隧道）的设计和施工带来了挑战。鳞片状粘土显示出正常压缩线 (NCL)，塑性变形在此累积，这是在非鳞片状粘土中观察到的典型现象。然而，在卸载和后续重新加载时观察到的反应却非常奇特：i）卸载-重新加载循环通常是一个具有相对较大滞后的闭环；ii）在卸载或重新加载分支的高 OCR 比下记录到的压缩性接近 NCL 压缩性。本文对意大利的一种鳞片状粘土进行了微观结构研究，将扫描电镜观察结果与水银渗入孔隙度（MIP）分析和 X 射线计算机断层扫描（XCT）图像相结合。与尺度间孔隙度闭合和尺度内新孔隙度生成相关的机制被确定为塑性变形的过程。对重组粘土的实验观察表明，在加载和卸载时，粘土具有 "准可逆 "行为，孔径分布仅以颗粒间孔隙率为特征。由于观察到天然粘土和重组粘土的卸载和重载曲线是平行的，因此推测颗粒间孔隙率控制着弹性响应。
{"title":"A microstructural insight into the compression behaviour of scaly clays","authors":"Matteo Pedrotti, Alessandro Tarantino, Antonio Annese, Federica Cotecchia, Claudia Vitone","doi":"10.1680/jgeot.23.00269","DOIUrl":"https://doi.org/10.1680/jgeot.23.00269","url":null,"abstract":"Scaly clays are intensely fissured clays with lens shaped elements of millimetre size and show a complex compression behaviour that poses challenges to the design and construction of geostructures (excavations, retaining diaphragms, and tunnels). Scaly clays show a Normal Compression Line (NCL) where plastic deformation accumulates as typically observed in non-scaly clays. Yet the response observed upon unloading and subsequent reloading is very peculiar, i) the unloading-reloading cycle is typically a close-loop with relatively large hysteresis; ii) the compressibility recorded at high OCR ratio of the unloading or reloading branches is close to the NCL compressibility. This paper presents a microstructural study on an Italian scaly clay where SEM observations are integrated with Mercury Intrusion Porosimetry (MIP) analyses and X-ray Computed Tomography (XCT) images. The mechanism associated with the closing of inter-<i>scale</i> porosity and the generation of new intra-<i>scale</i> porosity was identified as the process responsible for the plastic deformation. Experimental observation of reconstituted clay showed a “quasi-reversible” behaviour upon loading and unloading and a pore size distribution characterized only by interparticle porosity. The observation that unloading and reloading curves are parallel in natural and reconstituted clays, led to postulate that the interparticle porosity is controlling the elastic response.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"87 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140625694","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Chuanjin Tang, Andrea Franza, Jingmin Xu, Alec M. Marshall
This paper presents results from geotechnical centrifuge tests of tunnelling in dry dense sand beneath loaded non-displacement piles, focusing on the effect of a pile cap (representative also of a raft or grade beam) in contact with the soil surface on load transfer mechanisms. Experiments included loading tests to ascertain the foundation capacity and load-displacement response in the presence/absence of an underlying model tunnel. Individual ‘reference’ pile response is compared for cases with and without a pile cap, including pile displacements and load distributions between the head, shaft, and base; the case of ‘friction’ piles with a compressible base are also considered. Results show that uncapped piles with relatively large service loads experience ‘geotechnical failure’ (i.e. large settlements or a significant increase in settlement rate with tunnel volume loss) in order to mobilise base or shaft resistance. Pile caps are shown to reduce settlements and prevent geotechnical failure of both reference and friction piles; they also improve the post-tunnelling foundation performance under loading. The paper provides evidence to enable engineers to consider the beneficial role of shallow elements in contact with the surface on the performance of pile foundations both during and subsequent to tunnelling.
{"title":"Load transfer mechanisms for capped and uncapped non-displacement piles above tunnels: a centrifuge modelling study","authors":"Chuanjin Tang, Andrea Franza, Jingmin Xu, Alec M. Marshall","doi":"10.1680/jgeot.23.00201","DOIUrl":"https://doi.org/10.1680/jgeot.23.00201","url":null,"abstract":"This paper presents results from geotechnical centrifuge tests of tunnelling in dry dense sand beneath loaded non-displacement piles, focusing on the effect of a pile cap (representative also of a raft or grade beam) in contact with the soil surface on load transfer mechanisms. Experiments included loading tests to ascertain the foundation capacity and load-displacement response in the presence/absence of an underlying model tunnel. Individual ‘reference’ pile response is compared for cases with and without a pile cap, including pile displacements and load distributions between the head, shaft, and base; the case of ‘friction’ piles with a compressible base are also considered. Results show that uncapped piles with relatively large service loads experience ‘geotechnical failure’ (i.e. large settlements or a significant increase in settlement rate with tunnel volume loss) in order to mobilise base or shaft resistance. Pile caps are shown to reduce settlements and prevent geotechnical failure of both reference and friction piles; they also improve the post-tunnelling foundation performance under loading. The paper provides evidence to enable engineers to consider the beneficial role of shallow elements in contact with the surface on the performance of pile foundations both during and subsequent to tunnelling.","PeriodicalId":501472,"journal":{"name":"Géotechnique","volume":"49 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140611457","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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