Showing papers in "Acta Geotechnica in 2016"
TL;DR: In this article, the authors reviewed the terminologies commonly used to quantify the amount of water stored in rocks and reviewed the water-weakening effects on rock strengths, particularly focusing on uniaxial compressive strength and modulus, as well as tensile strength, under quasi-static loading and dynamic loading.
Abstract: Reduction in strength and stiffness in rocks attributed to an increase in water content has been extensively researched on a large variety of rock types over the past decades. Due to the considerable variations of texture and lithology, the extent of water-weakening effect is highly varied among different rock types, spanning from nearly negligible in quartzite to 90 % of uniaxial compressive strength reduction in shale. Readers, however, often face difficulties in comparing the data published in different sources due to the discrepancy of experimental procedures of obtaining the water saturation state and how the raw laboratory data is interpreted. In view of this, the present paper first reviews the terminologies commonly used to quantify the amount of water stored in rocks. The second part of the paper reviews the water-weakening effects on rock strengths, particularly focusing on uniaxial compressive strength and modulus, as well as tensile strength, under quasi-static loading and dynamic loading. The correlation relationships established among various parameters, including porosity, density and fabric of rocks, and external factors such as strain rate, surface tension and dielectric constant of the saturating liquid, absorption percentage and suction pressure, are reviewed and presented toward the end of the paper.
208 citations
TL;DR: In this paper, a constitutive model for the soil under cyclic loading under constant volume (undrained) conditions is presented, which describes the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors).
Abstract: For numerical studies of geotechnical structures under earthquake loading, aiming to examine a possible failure due to liquefaction, using a sophisticated constitutive model for the soil is indispensable. Such a model must adequately describe the material response to a cyclic loading under constant volume (undrained) conditions, amongst others the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors). The soil behaviour under undrained cyclic loading is manifold, depending on the initial conditions (e.g. density, fabric, effective mean pressure, stress ratio) and the load characteristics (e.g. amplitude of the cycles, application of stress or strain cycles). In order to develop, calibrate and verify a constitutive model with focus to undrained cyclic loading, the data from high-quality laboratory tests comprising a variety of initial conditions and load characteristics are necessary. The purpose of these two companion papers was to provide such database collected for a fine sand. The database consists of numerous undrained cyclic triaxial tests with stress or strain cycles applied to samples consolidated isotropically or anisotropically. Monotonic triaxial tests with drained or undrained conditions have also been performed. Furthermore, drained triaxial, oedometric or isotropic compression tests with several un- and reloading cycles are presented. Part I concentrates on the triaxial tests with monotonic loading or stress cycles. All test data presented herein will be available from the homepage of the first author. As an example of the examination of an existing constitutive model, the experimental data are compared to element test simulations using hypoplasticity with intergranular strain.
102 citations
TL;DR: In this article, a method that enables chemo-mechanical characterization of this highly heterogeneous source rock at the micron and submicron length scale through a statistical analysis of a large array of energy-dispersive X-ray spectroscopy (EDX) data coupled with nanoindentation data was proposed.
Abstract: The organic–inorganic nature of organic-rich source rocks poses several challenges for the development of functional relations that link mechanical properties with geochemical composition. With this focus in mind, we herein propose a method that enables chemo-mechanical characterization of this highly heterogeneous source rock at the micron and submicron length scale through a statistical analysis of a large array of energy-dispersive X-ray spectroscopy (EDX) data coupled with nanoindentation data. The ability to include elemental composition to the indentation probe via EDX is shown to provide a means to identify pure material phases, mixture phases, and interfaces between different phases. Employed over a large array, the statistical clustering of this set of chemo-mechanical data provides access to the properties of the fundamental building blocks of clay-dominated organic-rich source rocks. The versatility of the approach is illustrated through the application to a large number of source rocks of different origin, chemical composition, and organic content. We find that the identified properties exhibit a unique scaling relation between stiffness and hardness. This suggests that organic-rich shale properties can be reduced to their elementary constituents, with several implications for the development of predictive functional relations between chemical composition and mechanical properties of organic-rich source rocks such as the intimate interplay between clay-packing, organic maturity, and mechanical properties of porous clay/organic phase.
87 citations
TL;DR: In this paper, the authors used the discrete element method (DEM) to conduct undrained cyclic biaxial compression simulations on granular assemblies consisting of 2D circular particles.
Abstract: In an effort to study undrained post-liquefaction shear deformation of sand, the discrete element method (DEM) is adopted to conduct undrained cyclic biaxial compression simulations on granular assemblies consisting of 2D circular particles. The simulations are able to successfully reproduce the generation and eventual saturation of shear strain through the series of liquefaction states that the material experiences during cyclic loading after the initial liquefaction. DEM simulations with different deviatoric stress amplitudes and initial mean effective stresses on samples with different void ratios and loading histories are carried out to investigate the relationship between various mechanics- or fabric-related variables and post-liquefaction shear strain development. It is found that well-known metrics such as deviatoric stress amplitude, initial mean effective stress, void ratio, contact normal fabric anisotropy intensity, and coordination number, are not adequately correlated to the observed shear strain development and, therefore, could not possibly be used for its prediction. A new fabric entity, namely the Mean Neighboring Particle Distance (MNPD), is introduced to reflect the space arrangement of particles. It is found that the MNPD has an extremely strong and definitive relationship with the post-liquefaction shear strain development, showing MNPD’s potential role as a parameter governing post-liquefaction behavior of sand.
87 citations
TL;DR: In this paper, three constitutive models were used to simulate the collapse of two granular columns with different geometries and for two densities, and the results of the simulations showed that the constitutive model had a twofold influence on the collapse behaviour.
Abstract: The granular column collapse is a well-established experiment which consists of having a vertical column of granular material on a flat surface and letting it collapse by gravity. Despite its simplicity in execution, the numerical modelling of a column collapse remains challenging. So far, much attention has been dedicated in assessing the ability of various numerical methods in modelling the large deformation and little to the role of the constitutive model on both the triggering mechanism and the flow behaviour. Furthermore, the influence of the initial density, and its associated dilatancy and strength characteristics, have never been included in the analyses. Most past numerical investigations had relied on simple constitutive relations which do not consider the softening behaviours. The aim of this study is to illustrate the influence of the constitutive model on the on-set of failure, the flow behaviour and the deposition profile using the material point method. Three constitutive models were used to simulate the collapse of two granular columns with different geometries and for two densities. The results of the simulations showed that the constitutive model had a twofold influence on the collapse behaviour. It defined the volume of the mobilised mass which spread along the flat surface and controlled the dissipation of its energy. The initial density was found to enhance the failure angle and flow behaviours and was more significant for small columns than for larger ones. The analysis of the potential energy of the mobilised mass explained the existence of two collapse regimes.
79 citations
TL;DR: In this article, the authors focused on the quantitative description of the evolution of the creep coefficient with both soil density and soil structure under 1D compression, and proposed a nonlinear creep formulation accounting for both soil densities and soil structures.
Abstract: This study focuses on the quantitative description of the evolution of creep coefficient (C
αe) with both soil density and soil structure under 1D compression. Firstly, conventional consolidation test results on various reconstituted clays are selected in order to investigate the evolution of C
αe with void ratio of soils, which can be described by a simple nonlinear creep formulation. Secondly, the contributions of the inter-particle bonding and debonding for soft structured clays to C
αe are analyzed based on test results on intact and reconstituted samples of the same clay. A material constant ρ, function of the bonding ratio χ, is introduced in order to quantify the contribution of the soil structure to C
αe, and a nonlinear creep formulation accounting for both soil density and soil structure is finally proposed. Furthermore, the parameters used in the formulation are correlated with Atterberg limits, allowing us to suggest a relationship between C
αe, Atterberg limits and inter-particle bonding for a given soil. Finally, the validity of the proposed formulation is examined by comparing experimental and predicted C
αe values for both reconstituted and intact samples of natural soft clays. The proposed formulation is also validated by comparing the computed and measured void ratio with time on two intact clays.
75 citations
TL;DR: In this article, a unified numerical framework for granular modelling is presented, which is capable of describing both quasi-static and dynamic behaviours of granular material, and is validated by simulating element tests under both solid and fluid regimes.
Abstract: In this paper, we present a unified numerical framework for granular modelling. A constitutive model capable of describing both quasi-static and dynamic behaviours of granular material is developed. Two types of particle interactions controlling the mechanical responses, frictional contact and collision, are considered by a hypoplastic model and a Bagnold-type rheology relation, respectively. The model makes no use of concepts like yield stress or flow initiation criterion. A smooth transition between the solid-like and fluid-like behaviour is achieved. The Smoothed Particle Hydrodynamics method is employed as the unified numerical tool for both solid and fluid regimes. The numerical model is validated by simulating element tests under both quasi-static and flowing conditions. We further proceed to study three boundary value problems, i.e. collapse of a granular pile on a flat plane, and granular flows on an inclined plane and in a rotating drum.
74 citations
TL;DR: In this paper, a critical-state-based sand model has been developed to simulate three triaxial compression tests performed on loose and dense Hostun sand, and two widely used genetic algorithms with two initialisation methods are examined.
Abstract: Adding refinement and accuracy to constitutive models of soil results in the introduction of complexities along with more model parameters. These parameters (such as hardening-/softening-, dilatancy-/contractancy-related parameters and critical state parameters) are usually not easily obtained in a straightforward way. How to identify these key parameters and estimate their correlations of advanced soil models is a particular issue for geotechnical engineering. This paper was aimed to investigate multi-objective genetic algorithms for identifying parameters of advanced sand models based on standard laboratory tests, followed by the correlation analysis of parameters. A critical-state-based sand model has been developed to simulate three triaxial compression tests performed on loose and dense Hostun sand. Two widely used genetic algorithms with two initialisation methods are examined. The performance of the two genetic algorithms is assessed by comparing their simulation performance using optimal parameters, the convergence speed and the distribution of solutions on the Pareto front. The optimal parameters can then be classified into two factors by their correlation relationship.
72 citations
TL;DR: A new DEM scheme (LS-DEM) is introduced that takes advantage of level sets to enable the inclusion of real grain shapes into a classical discrete element method and trends for the CS parameters (CSP) dependency on sphericity, roundness, and regularity are obtained.
Abstract: We introduce a new DEM scheme (LS-DEM) that takes advantage of level sets to enable the inclusion of real grain shapes into a classical discrete element method. Then, LS-DEM is validated and calibrated with respect to real experimental results. Finally, we exploit part of LS-DEM potentiality by using it to study the dependency of critical state (CS) parameters such as critical state line (CSL) slope $$\lambda $$
, CSL intercept $$\varGamma $$
, and CS friction angle $$\varPhi _{\mathrm{CS}}$$
on the grain’s morphology, i.e., sphericity, roundness, and regularity. This study is carried out in three steps. First, LS-DEM is used to capture and simulate the shape of five different two-dimensional cross sections of real grains, which have been previously classified according to the aforementioned morphological features. Second, the same LS-DEM simulations are carried out for idealized/simplified grains, which are morphologically equivalent to their real counterparts. Third, the results of real and idealized grains are compared, so the effect of “imperfections” on real particles is isolated. Finally, trends for the CS parameters (CSP) dependency on sphericity, roundness, and regularity are obtained as well as analyzed. The main observations and remarks connecting particle’s morphology, particle’s idealization, and CSP are summarized in a table that is attempted to help in keeping a general picture of the analysis, results, and corresponding implications.
69 citations
TL;DR: In this article, an experimental investigation revisiting the anisotropic stress-strain-strength behavior of geomaterials in drained monotonic shear using hollow cylinder apparatus is presented.
Abstract: This paper presents an experimental investigation revisiting the anisotropic stress–strain–strength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of Leighton Buzzard sand. Experiments have also been performed on glass beads to understand the effect of particle shape. This paper explains phenomenological observations based on recently acquired understanding in micromechanics, with attention focused on strength anisotropy and deformation non-coaxiality, i.e. non-coincidence between the principal stress direction and the principal strain rate direction. The test results demonstrate that the effects of initial anisotropy produced during sample preparation are significant. The stress–strain–strength behaviour of the specimen shows strong dependence on the principal stress direction. Preloading history, material density and particle shape are also found to be influential. In particular, it was found that non-coaxiality is more significant in presheared specimens. The observations on the strength anisotropy and deformation non-coaxiality were explained based on the stress–force–fabric relationship. It was observed that intermediate principal stress parameter b(b = (σ2 − σ3)/(σ1 − σ3)) has a significant effect on the non-coaxiality of sand. The lower the b-value, the higher the degree of non-coaxiality is induced. Visual inspection of shear band formed at the end of HCA testing has also been presented. The inclinations of the shear bands at different loading directions can be predicted well by taking account of the relative direction of the mobilized planes to the bedding plane.
68 citations
TL;DR: In this article, a new approach and new devices were proposed to study clay conditioning with laboratory tests, and the results of many tests carried out with the proposed device are presented and discussed.
Abstract: Tunnelling in difficult and challenging conditions such as soft soils in urban areas is increasing. In this condition, it is important to minimise the possible negative effect of the tunnel excavation, such as settlement or, in the worst case, collapses. To achieve this result, earth pressure balance machines are commonly used. One of the key parameters that must be considered for an optimal management of the EPB-TBM excavation is soil conditioning since the excavated muck must properly transmit the pressure to the tunnel face. Soil conditioning is also necessary to reduce the effect of the problems, such as clogging in clay layers, that can occur during the excavation and that can affect the performance of the tools and of the entire tunnelling process. For this reason, in the last decade, much research has been carried out to understand how to deal with and reduce the effects of clogging and stickiness, using different conditioning additives. These studies have proposed several different test procedures to evaluate the effect of the conditioning on the adhesion of the soil on the metallic parts of the machines. The present research has been carried out with the aim of proposing a new approach and new devices to study clay conditioning with laboratory tests, and the results of many tests carried out with the proposed device are presented and discussed.
TL;DR: In this article, the effect of different factors such as grain size and grain shape, confining pressure, level of shear, stress path, and formation of one or several shear bands on the permeability of dense unconsolidated sands under triaxial loading was investigated.
Abstract: Various mechanisms can affect the permeability of dense unconsolidated sands: Volumetric dilation can lead to permeability increase, whereas strain localization in shear bands may increase or decrease the permeability depending on the state of compaction and on the level of grains breakage inside the band. To investigate these various mechanisms, an experimental study has been performed to explore the effect of different factors such as grain size and grain shape, confining pressure, level of shear, stress path, and formation of one or several shear bands on the permeability of dense sands under triaxial loading. The experimental results show a reduction of permeability during the consolidation phase and during the volumetric contraction phase of shear loading, which can be related to the decrease of porosity. The experimental results also show that, depending on the confining pressure, the permeability remains stable or decreases during the volumetric dilation phase despite the increase of total porosity. This permeability reduction is attributed to the presence of fine particles, which result from grains attrition during pre-localization and grains breakage inside the shear band during the post-localization phase.
TL;DR: In this article, the effects of water content and fine content on the resilient modulus of interlayer soil were analyzed and it was shown that under unsaturated conditions, the soil containing high fine content has higher resilient properties due to the contribution of suction.
Abstract: This paper deals with the resilient behavior of the interlayer soil which is created mainly by the interpenetration of ballast and subgrade soils. The interlayer soil studied was taken from a site in the southeast of France. Large-scale cyclic triaxial tests were carried out at three water contents (w = 4, 6 and 12 %) and three fines contents corresponding to 5, 10 % subgrade added to the natural interlayer soil and 10 % fine particles (<80 μm) removed from the natural interlayer soil. Soil specimens underwent various deviator stresses, and for each deviator stress, a large number of cycles was applied. The effects of deviator stress, number of cycles, water content and fines content on the resilient modulus (M r) were analyzed. It appears that the effects of water content and fines content must be analyzed together because the two effects are closely linked. Under unsaturated conditions, the soil containing high fines content has higher resilient modulus due to the contribution of suction. When the soil approaches the saturated state, it loses its mechanical enhancement with a sharp decrease in resilient modulus.
TL;DR: In this article, an unconfined compressive strength (UCS) test was performed as an index of soil improvement on mix designs treated with calcium-based powder stabilizer (SH-85).
Abstract: Non-traditional soil stabilizers are widely used for treating weak materials. These additives are cost- and time-effective alternatives to more traditional materials such as lime and cement. It has been well established that the treatment of natural soil with chemical additives will gradually affect the size, shape, and arrangement of soil particles. Furthermore, the degree of improvement is dependent on the quantity and the pattern of new products formed on and around the soil particles. In this paper, unconfined compressive strength (UCS) test was performed as an index of soil improvement on mix designs treated with calcium-based powder stabilizer (SH-85). The time-dependent changes in shear strength parameter and compressibility behavior of treated soil were also studied using standard direct shear and one-dimensional consolidation tests. In order to better understand the shape and surface area of treated particles, FESEM, N2-BET, and particle size distribution analysis were performed on soil-stabilizer matrix. From engineering standpoint, the UCS results showed that the degree of improvement for SH-85-stabilized laterite soil was roughly five times stronger than the untreated soil at the early stages of curing (7-day period). Also, a significant increase in the compressibility resistance of treated samples with curing time was observed. Based on the results, less porous and denser soil fabric was seen on the surface of clay particles. FESEM images of the treated mix designs showed the formation of white lumps in the soil fabric with the cementitious gel filling the pores in the soil structure.
TL;DR: In this paper, a suite of experimental and theoretical microporomechanics methods are developed to associate the mechanical properties of organic-rich shales both to their maturity level and to the organic content at micrometer and sub-micrometer length scales.
Abstract: Despite the importance of organic-rich shales, microstructural characterization and theoretical modeling of these rocks are limited due to their highly heterogeneous microstructure, complex chemistry, and multiscale mechanical properties. One of the sources of complexity in organic-rich shales is the intricate interplay between microtextural evolution and kerogen maturity. In this study, a suite of experimental and theoretical microporomechanics methods are developed to associate the mechanical properties of organic-rich shales both to their maturity level and to the organic content at micrometer and sub-micrometer length scales. Recent results from chemomechanical characterization experiments involving grid nanoindentation and energy-dispersive X-ray spectroscopy (EDX) are used in new micromechanical models to isolate the effects of maturity levels and organic content from the inorganic solids. These models enable attribution of the role of organic maturity to the texture of the indented material, with immature systems exhibiting a matrix-inclusion morphology, while mature systems exhibit a polycrystal morphology. Application of these models to the interpretation of nanoindentation results on organic-rich shales allows us to identify unique clay mechanical properties that are consistent with molecular simulation results for illite and independent of the maturity of shale formation and total organic content. The results of this investigation contribute to the design of a multiscale model of the fundamental building blocks of organic-rich shales, which can be used for the design and validation of multiscale predictive poromechanics models.
TL;DR: In this paper, preserved kerogen-rich Woodford shale samples manufactured in micro-beam and micro-pillar geometries were mechanically tested and brought to failure in tension and compression, respectively.
Abstract: In the past decade, mechanical, physical, and chemical characterization of reservoir shale rocks, such as the Woodford shale, which is kerogen-rich shale (KRS), has moved toward micro- and nanoscale testing and analyses. Nanoindentation equipment is now widely used in many industrial and university laboratories to measure shale anisotropic Young’s moduli, kerogen stiffness, plastic yield parameters, and other isotropic and anisotropic poromechanical and viscoelastic properties. However, to date, failure analyses of KRS and the effects of organic components on the tensile strength have not been observed or measured at the micro- or nanoscales. In this study, preserved kerogen-rich Woodford shale samples manufactured in micro-beam and micro-pillar geometries were mechanically tested and brought to failure in tension and compression, respectively. These tests were conducted in situ using a nanoindenter inside a scanning electron microscope (SEM). The load versus displacement curves of prismatic micro-cantilever beams were analyzed in light of high-resolution images collected during tensile fracture initiation, propagation, and ultimately sample failure. The micro-pillar geometries were subjected to a uniaxial compressive load and were also brought to failure while capturing measurements of stress and strain. It was found that, within just a few hundred microns of the KRS micro-cantilever beams, both brittle and ductile failure modes were observed. In the ductile plastic domain, strain-softening and strain-hardening behaviors were identified and characterized. These were not due to confining stress variations, but due to the volume of the organic matter and the way it is interlaced with the shale minerals in and around the failure planes. The tensile strength characteristics and the large modulus of toughness of kerogen, which is a cross-linked polymer, definitely weigh heavily in our engineering field applications, such as hydraulic fracking, which is a Mode I tensile fracture opening and propagation phenomenon. This practice demands that, due to the complex composite nature of KRS, mechanical characterization be not only for unconfined compressive strength but also for unconfined tensile strength and moduli of ruptures. At the end of this study, the need for nanometer scale mechanical characterization of KRS will become apparent. These nano- and micro-scale shale failure tests reinforce our previous understanding of the heterogeneous composite nature of Woodford KRS and its complex behavior, as well as other source shale reservoir formations.
TL;DR: In this paper, a novel macroelement for single vertical piles in sand developed within the hypoplasticity theory is presented, where the incremental nonlinear constitutive equations are defined in terms of generalized forces, displacements and rotations.
Abstract: This paper presents a novel macroelement for single vertical piles in sand developed within the hypo-plasticity theory, where the incremental nonlinear constitutive equations are defined in terms of generalized forces, displacements and rotations. Inspired from the macroelement for shallow foundations of Salciarini and Tamagnini (Acta Geotech, 4(3):163–176, 2009), the new element adopts the “intergranular displacement” mutuated from Niemunis and Herle (Mech Cohes Frict Mater, 2:279–299, 1997) to reproduce the behavior under cyclic loading. Analytical and numerical strategies are provided to calibrate the macroelement’s parameters. Comparisons with experimental results show the performance of the macroelement that while being simple and computational fast is suitable for finite element calculations and engineering design.
TL;DR: In this article, the physical and analytical fragmentation characteristics of pre-flawed Portland cement samples are analyzed through the sieve test and fractal theory, respectively, and three different patterns of tensile cracks and shear cracks are observed.
Abstract: Experiments on Portland cement samples containing mixed flaws are conducted to investigate the strength, fragmentation and fractal properties. Flaw geometry is a new combination of two edge-notched flaws and an imbedded flaw, which is different from those in the previous studies, where parallel or coplanar flaws are used. The physical implications of the shear-box test applied to result to rock slopes are studied. The physical and analytical fragmentation characteristics of preflawed samples are analyzed through the sieve test and fractal theory, respectively. Three different patterns of tensile cracks and shear cracks are observed. A sliding crack model is presented to elucidate the brittle failure flaws. In all of the cases of the shear-box tests, the coalescence is produced by the linkage of shear cracks, and two types of coalescence (Type C1 and Type C2) have been classified, which tend to confirm the observations from the numerical model and field of jointed rock slopes. The shear strength is a function of the flaw geometry and the shear–normal stress ratio. The result of sieve tests indicates that the fragment size distribution of fragments has the fractal property, providing a physical understanding of the fragmentation mechanism. The fragments under the shear-box test have fractal dimensions between 2.2 and 2.6, which are larger than those under the compression test but similar to those in the fault cores. The fragmentation in the case of Type C2 has a smaller fractal dimension, corresponding to a larger shear strength.
TL;DR: In this article, an enhanced hypoplastic interface model that incorporates the in-plane stresses at the interface is proposed, which can be used in the existing continuum model with few modifications.
Abstract: Modelling of interfaces in geotechnical engineering is an important issue. Interfaces between structural elements (e.g., anchors, piles, tunnel linings) and soils are widely used in geotechnical engineering. The objective of this article is to propose an enhanced hypoplastic interface model that incorporates the in-plane stresses at the interface. To this aim, we develop a general approach to convert the existing hypoplastic model with a predefined limit state surface for sands into an interface model. This is achieved by adopting reduced stress and stretching vectors and redefining tensorial operations which can be used in the existing continuum model with few modifications. The enhanced interface model and the previous model are compared under constant-load, stiffness and volume conditions. The comparison is followed by a verification of two the approaches for modelling the different surface roughness. Subsequently, a validation between available experimental data from the literature versus simulations is presented. The new enhanced model gives improved predictions by the incorporation of in-plane stresses into the model formulation.
TL;DR: In this paper, a dynamic analysis model comprising track, embankment and layered ground was presented based on the two-and-half-dimensional (2.5D) finite elements combining with thin-layer elements to predict vibrations generated by train moving loads.
Abstract: High-speed train induced vibrations of track structure and underlying soils differ from that induced by low-speed train. Determining the critical speed of train operation remains difficult due to the complex properties of the track, embankment and ground. A dynamic analysis model comprising track, embankment and layered ground was presented based on the two-and-half-dimensional (2.5D) finite elements combining with thin-layer elements to predict vibrations generated by train moving loads. The track structure is modeled as an Euler–Bernoulli beam resting on embankment. The train is treated as a series of moving axle loads; the embankment and ground are modeled by the 2.5D finite elements. The dynamic responses of the track structure and the ground under constant and vibrating moving loads at various speeds are presented. The results show that the critical speed of a train moving on an embankment is higher than the Rayleigh wave velocity of the underlying soil, attributed to the presence of the track structure and the embankment. It is found that the dynamic response of ground induced by moving constant loads is mostly dominated by train speed. While for the moving load with vibration frequency, the ground response is mostly affected by the vibration frequency instead of train speed. Mach effect appears when the train speed exceeds the critical speed of the track–embankment–ground system.
TL;DR: In this article, anisotropic critical state theory (ACST) is proposed to enhance the classical critical states theory by adding an additional condition that fabric must satisfy at critical states.
Abstract: Critical state (CS) is a physically observed state of granular materials at failure, based on which the critical state theory (CST) was founded. CST attempts to describe analytically the material response when CS failure occurs and constitutes the framework within which constitutive modeling techniques have been developed in the last half-century. The conditions of CST defined as necessary and sufficient to characterize CS do not include fabric orientation of material samples. In the present work, this absence of fabric is discussed in light of a newly developed anisotropic critical state theory (ACST) that enhances the classical CST by introducing an additional condition that fabric must satisfy at CS. The use of ACST framework in constitutive modeling is presented in a generic way, usable in conjunction with various constitutive models that comply with CST. Answers to some fundamental questions are attempted or suggested, e.g., as to whether the conditions of classical CST are necessary and sufficient for CS to occur, whether the ACST is simply a convenient supplement to CST or a necessary enhancement, and if the latter is true, the merits and open questions that arise. The presentation focuses on the justification and thoughts behind the proposed specific features of ACST, provides clarifications not made before, while several suggestions and disclaimers are made, and alternate approaches are proposed for further investigations on the subject matter.
TL;DR: In this paper, a constitutive model for the soil under cyclic loading under constant volume (undrained) conditions is presented, which describes the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors).
Abstract: For numerical studies of geotechnical structures under earthquake loading, aiming to examine a possible failure due to liquefaction, using a sophisticated constitutive model for the soil is indispensable. Such model must adequately describe the material response to a cyclic loading under constant volume (undrained) conditions, amongst others the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors). The soil behaviour under undrained cyclic loading is manifold, depending on the initial conditions (e.g. density, fabric, effective mean pressure, stress ratio) and the load characteristics (e.g. amplitude of the cycles, application of stress or strain cycles). In order to develop, calibrate and verify a constitutive model with focus to undrained cyclic loading, the data from high-quality laboratory tests comprising a variety of
initial conditions and load characteristics are necessary. It is the purpose of these two companion papers to provide such database collected for a fine sand. Part II concentrates on the undrained triaxial tests with strain cycles, where a large range of strain amplitudes has been studied. Furthermore, oedometric and isotropic compression tests as well as drained triaxial tests with un- and reloading cycles are discussed. A combined monotonic and cyclic loading has been also studied in undrained triaxial tests. All test data presented herein will be available from the homepage of the first author. As an example of the examination of an existing constitutive model, the experimental data are compared to element test simulations using hypoplasticity with intergranular strain.
TL;DR: In this paper, the dilatancy boundary of the unloading confining pressure test began with unloading, and the volume deformation of rock salt under unloading was more than under triaxial loading, but less than under uniaxial loading (with the same deviatoric stress).
Abstract: Rock mechanical behaviors and deformation characteristics are associated with stress history and loading path. Unloading conditions occur during the formation of a salt cavity as a result of washing techniques. Such conditions require an improved understanding of the mechanical and deformation behaviors of rock salt. In our study, rock salt dilatancy behaviors under triaxial unloading confining pressure tests were analyzed and compared with those from conventional uniaxial and triaxial compression tests. The volume deformation of rock salt under unloading was more than under triaxial loading, but less than under uniaxial loading (with the same deviatoric stress). Generally, under the same axial compression, the corresponding dilatancy rate decreased as the confining compression increased, and under the same confining compression, the corresponding dilatancy rate increased as the axial compression increased. The dilatancy boundary of the unloading confining pressure test began with unloading. This is different from the dilatancy of the uniaxial and triaxial compression tests. The accelerated dilatancy point boundary stress value was affected by confining and axial compressions. The specimens entered into a creep state after unloading. The associated creep rate depends on the deviatoric stress and confining compression values at the end of the unloading process. Based on unloading theory and the experimental data, we propose a constitutive model of rock salt damage. Our model reflects the dilatancy progression at constant axial stress and reduced lateral confinement.
TL;DR: In this article, a full-scale accelerated railway testing platform for ballastless high-speed railways was proposed and the feasibility of the sequential loading method in simulating train moving loads, and the boundary effect of the proposed physical model of Ballastless railways, was investigated using three-dimensional finite element models.
Abstract: Ballastless high-speed railways have dynamic performances that are quite different from those of conventional ballasted railways. The essential dynamic characteristics of high-speed railways due to passing train wheels, such as the cyclic effect, moving effect, and speed effect, were put forward and discussed. A full-scale accelerated railway testing platform for ballastless high-speed railways was proposed in this study. The feasibility of the sequential loading method in simulating train moving loads, and the boundary effect of the proposed physical model of ballastless railways, was investigated using three-dimensional finite element models. A full-scale physical model, 5 m long, 15 m wide, and 6 m high, was then established according to practical engineering design methods. Using a sequential loading system composed of eight high-performance hydraulic actuators, loads of a moving train with highest speed of 360 km/h were simulated. Preliminary experimental results of vibration velocities were presented and compared with field measurements of the Wuguang high-speed railway in China. Results showed that the experimental results coincided with the field measurements, demonstrating that the full-scale accelerated railway testing platform can simulate the process of a moving train and realistically reproduce the dynamic behaviors of ballastless high-speed railways.
TL;DR: In this article, the results of suction-controlled triaxial tests performed on compacted samples of two well-graded granular materials in the range of coarse sand-medium gravel particle sizes: a quartzitic slate and a hard limestone were discussed.
Abstract: The paper reports the results of suction-controlled triaxial tests performed on compacted samples of two well-graded granular materials in the range of coarse sand–medium gravel particle sizes: a quartzitic slate and a hard limestone. The evolution of grain size distributions is discussed. Dilatancy rules were investigated. Dilatancy could be described in terms of stress ratio, plastic work input and average confining stress. The shape of the yield locus in a triaxial plane was established by different experimental techniques. Yielding loci in both types of lithology is well represented by approximate elliptic shapes whose major axis follows approximately the K0 line. Relative humidity was found to affect in a significant way the evolution of grain size distribution, the deviatoric stress–strain response and the dilatancy rules.
TL;DR: The salt concentration gradually increases at the soil free surface when the evaporation rate exceeds the diffusive counter transport, and crystals form a porous sodium chloride crust with a porosity of 0.43 ± 0.14.
Abstract: The salt concentration gradually increases at the soil free surface when the evaporation rate exceeds the diffusive counter transport. Eventually, salt precipitates and crystals form a porous sodium chloride crust with a porosity of 0.43 ± 0.14. After detaching from soils, the salt crust still experiences water condensation and salt deliquescence at the bottom, brine transport across the crust driven by the humidity gradient, and continued air-side precipitation. This transport mechanism allows salt crust migration away from the soil surface at a rate of 5 μm/h forming salt domes above soil surfaces. The surface characteristics of mineral substrates and the evaporation rate affect the morphology and the crystal size of precipitated salt. In particular, substrate hydrophobicity and low evaporation rate suppress salt spreading.
TL;DR: In this article, a unified constitutive model for clean and silty sands is presented, where the concept of equivalent void ratio is used in the model formulation instead of the global void ratio.
Abstract: Adding a small amount of non-plastic silt to clean sands may lead to dramatic loss of shear strength and a noteworthy tendency toward contraction when the mechanical behavior of the mixture is compared with that of the clean host sand. Thus, simulation of the behavior of silty sands with varying fines content is still a challenging subject in geomechanics. A unified constitutive model for clean and silty sands is presented in this paper. To eliminate the factitious decrease of void ratio associated with inactive silt particles in various silty sand mixtures, the concept of equivalent void ratio is used in the model formulation instead of the global void ratio. In addition, the instantaneous soil state is expressed in terms of intergranular state parameter taking into account the combined influence of intergranular void ratio, mean principal effective stress and fines content. Then, dilatancy and plastic hardening modulus are directly linked to the intergranular state parameter. To improve the model capacity in simulation of cyclic tests, new features are added to the plastic hardening modulus. It is shown that the proposed model can reasonably reproduce the mechanical behavior as well as the onset of flow liquefaction instability of clean and silty sands using a unique set of parameters.
TL;DR: In this article, a random virtual crack DEM model is proposed for creep behavior of rockfill in PFC2D according to the theory of subcritical crack propagation induced by stress corrosion mechanisms.
Abstract: The post-construction settlement of rockfill dams and high filled ground of airport, which is a phenomenon of much significance, is mainly caused by the time-dependent breakage of the rockfill material. In this paper, a random virtual crack DEM model is proposed for creep behavior of rockfill in PFC2D according to the theory of subcritical crack propagation induced by stress corrosion mechanisms. The bonded clusters are adopted to represent the rockfill particles so as to simulate their irregular shapes. Virtual cracks are set at the bonds of the clusters, and the length of the crack is considered as a random value, which leads the crushing strength of a single particle to follow the Weibull’s statistical model and the corresponding size rules. Oedometric creep tests for rockfill are simulated by using this proposed model. The results show that the model, validated preliminarily by some test data, can reflect qualitatively the creep mechanism as well as the size effects reasonably. Particles can develop various breakage patterns during creep, including global breakage, local breakage and even complex mixed breakage. The increase in stress levels and particle size will lead to an obvious growth of the creep strain and creep rate of the rockfill. The scale effects on the creep behavior of rockfill are analyzed through 35 specimens, and formulas including the effects of scales and stress levels are tentatively proposed.
TL;DR: In this paper, three types of Hong Kong turf grass including Cynodon Dactylon, Paspalum Notatum, and Zoysia Matrella were planted on three soil grounds with degrees of compaction of 80, 90, and 100% respectively.
Abstract: Grass cover is considered as a sustainable means of controlling soil erosion and enhancing durability of soil slopes. A number of grass species are commonly available for soil bioengineering in Hong Kong, but their capacities to control soil erosion have not been characterized quantitatively. The main objectives of this paper are to study the influence of soil density on characteristics of grass roots, to measure the erodibility parameters of the root-permeated soils at two growth stages, and to select the proper Hong Kong grass species that effectively control soil erosion. Three types of Hong Kong turf grass including Cynodon Dactylon, Paspalum Notatum, and Zoysia Matrella were planted on three soil grounds with degrees of compaction of 80, 90, and 100 %, respectively. The featural parameters of grass roots on each compacted ground, including root mass density, root volume density, and root depth, were measured in two growth stages. A jet index apparatus was applied to measure two erodibility properties (i.e., coefficient of erodibility and critical shear stress) of these vegetated soils in the two test stages. Cynodon Dactylon and Zoysia Matrella have higher root mass density values than Paspalum Notatum does, and reduce the susceptibility of soil erosion more effectively. Therefore, the two grass species are suggested for soil bioengineering in Hong Kong.
TL;DR: In this article, the authors studied the shear behavior and acoustic emission (AE) characteristics of joints under different experimental conditions (asperity height, shear rate, and normal load), irregular artificial saw-tooth joints with different asperity heights were sheared in the laboratory, and the AE signals were detected and analysed.
Abstract: Shear failure of rockmass along a weak plane occurs frequently in rock slopes and in underground tunnels. To study the shear behaviour and acoustic emission (AE) characteristics of joints under different experimental conditions (asperity height, shear rate, and normal load), irregular artificial saw-tooth joints with different asperity heights were sheared in the laboratory, and the AE signals were detected and analysed. The results demonstrated that the strength of the joints increased with increases in normal load and asperity height, while the strength of the joints first increased and then decreased when the shear rate was elevated. The ideal curve of the cumulative hits could be divided into a quiet period, a slow rise period, and a sharp growth period, which could be used to monitor and predict the potential shear failure of the joints. The higher the asperity was, the higher the peak energy rate and the lower the peak hit rate, and cumulative hits at failure were because of differences in asperity size and number. The peak hit rate and cumulative energy tended to increase gradually with the decrease in shear rate, and the minimum peak hit rate and energy rate at failure were both attained at the maximum shear rate. In addition, curves of the energy rate and hit rate showed large fluctuations at higher shear rates. The peak energy rate and cumulative energy under low normal stress could be greater than the peak energy rate and cumulative energy under high normal stress, and the peak hit rate and cumulative hit number under high normal stress could be larger than that of under low normal stress.