Showing papers in "Acta Geotechnica in 2018"

Effect of particle size distribution on the bio-cementation of coarse aggregates

Abstract: The effect of grain size distribution on the unconfined compressive strength (UCS) of bio-cemented granular columns is examined. Fine and coarse aggregates were mixed in various percentages to obtain five different grain size distributions. A four-phase percolation strategy was adopted where a bacterial suspension and a cementation solution (urea and calcium chloride) were percolated sequentially. The results show that a gap-graded particle size distribution can improve the UCS of bio-cemented coarser granular materials. A maximum UCS of approximately 575 kPa was achieved with a particle size distribution containing 75% coarse aggregate and 25% fine aggregate. Furthermore, the minimum UCS obtained has applications where mitigation of excessive bulging of stone/sand columns, and possible slumping that might occur during their installation, is needed. The finding also implies that the amount of biochemical treatments can be reduced by adding fine aggregate to coarse aggregate resulting in effective bio-cementation within the pore matrix of the coarse aggregate column as it could substantially reduce the cost associated with bio-cementation process. Scanning electron microscopy results confirm that adding fine aggregate to coarse aggregate provides more bridging contacts (connected by calcium carbonate precipitation) between coarse aggregate particles, and hence, the maximum UCS achieved was not necessarily associated with the maximum calcium carbonate precipitation.

The critical state friction angle of granular materials: does it depend on grading?

Abstract: Whether the critical state friction angle of granular materials depends on grading is a fundamental question of both academic and practical interest. The present study attempts to address this question through a specifically designed experimental program where the influence of particle grading was carefully isolated from other influencing factors. The laboratory experiments show that under otherwise similar conditions, the angle of friction at critical state is a constant independent of grading, but, for a given grading, the angle of friction at critical state is highly dependent on particle shape. This finding suggests that the commonly adopted practice of separately allowing for the effect of particle shape and the effect of grading on critical state friction angle is conceptually inappropriate and, hence, should be taken with caution in geotechnical design to avoid the risk of underestimating safety requirements. The study also reveals that varying particle gradation can impose a marked impact on liquefaction susceptibility of granular soils: Under the same post-consolidation state in terms of void ratio and confining pressure, a well-graded soil tends to be more susceptible to liquefaction than a uniformly graded soil. This variation of liquefaction susceptibility is shown to be consistent with the variation of location of the critical state locus in the compression space and is explainable by the critical state theory.

Monotonic and cyclic tests on kaolin: a database for the development, calibration and verification of constitutive models for cohesive soils with focus to cyclic loading

Abstract: A database with about 60 undrained monotonic and cyclic triaxial tests on kaolin is presented. In the monotonic tests, the influences of consolidation pressure, overconsolidation ratio, displacement rate and sample cutting direction have been studied. In the cyclic tests, the stress amplitude, the initial stress ratio and the control (stress vs. strain cycles) have been additionally varied. Isotropic consolidation leads to a failure due to large strain amplitudes with eight-shaped effective stress paths in the final phase of the cyclic tests, while a failure due to an excessive accumulation of axial strain and lens-shaped effective stress paths was observed in the case of anisotropic consolidation with $$q^{\text{ ampl }}< |q^{\text{ av }}|$$ . The rate of pore pressure accumulation grew with increasing amplitude and void ratio (i.e. decreasing consolidation pressure and overconsolidation ratio). The “cyclic flow rule” well known for sand has been confirmed also for kaolin: With increasing value of the average stress ratio $$|\eta ^{\text{ av }}| = |q^{\text{ av }}|/p^{\text{ av }}, $$ the accumulation of deviatoric strain becomes predominant over the accumulation of pore water pressure. The tests on the samples cut out either horizontally or vertically revealed a significant effect of anisotropy. In the cyclic tests, the two kinds of samples exhibited an opposite inclination of the effective stress path. Furthermore, the horizontal samples showed a higher stiffness and could sustain a much larger number of cycles to failure. All data of the present study are available from the homepage of the first author. They may serve for the examination, calibration or improvement in constitutive models dedicated to cohesive soils under cyclic loading, or for the development of new models.

Multiscale modeling and analysis of compaction bands in high-porosity sandstones

Abstract: We present a multiscale investigation on the initiation and development of compaction bands in high-porosity sandstones based on an innovative hierarchical multiscale approach. This approach couples the finite element method and the discrete element method (DEM) to offer direct, rigorous linking of the microscopic origins and mechanisms with complex macroscopic phenomena observed in granular rocks such as strain localization and failure. To simulate compaction band in granular cementitious sandstone, we adopt a bonded contact model with normal and tangential interparticle cohesions in the DEM and propose a dual-porosity structure consisting of macro-pores and interstitial voids for the representative volume element to mimic the typical meso-structure of high-porosity sandstones. In the absence of particle crushing, our multiscale analyses identify debonding and pore collapses as two major contributors to the formation of compaction bands. The critical pressures predicted by our simulations, corresponding to surges of debonding and pore collapse events, agree well with the estimations from field data. The occurrence patterns of compaction band are found closely related to specimen heterogeneity, porosity and confining pressure. Other deformation band patterns, including shear-enhanced compaction bands and compactive shear bands, were also observed under relatively low confining pressure conditions with a rough threshold at $$0.55P^{*}$$ ( $$P^{*}$$ is the critical pressure) on the failure envelop. Key microscopic characteristics attributable to the occurrence of these various deformation patterns, including fabric anisotropy, particle rotation, debonding and pore collapse, are examined. Shear-enhanced compaction bands and pure compaction bands bear many similarities in terms of these microscopic characteristics, whereas both differ substantially from compactive shear bands.

Face stability analysis of shield-driven tunnels shallowly buried in dry sand using 1-g large-scale model tests

Abstract: This paper investigates the face stability of shield-driven tunnels shallowly buried in dry sand using 1-g large-scale model tests. A half-circular tunnel model with a rigid front face was designed and tested. The ground movement was mobilized by pulling the tunnel face backwards at different speeds. The support pressure at tunnel face, settlement at ground surface, and internal movement of soil body were measured by load cells, linear variable differential transducers, and a camera, respectively, and the progress of face failure was observed through a transparent lateral wall of model tank. The tests show that, as the tunnel face moves backwards, the support pressure at the tunnel face drops sharply initially, then rebounds slightly, and tends to be stabilized at the end. Similarly, the ground surface settlement shows a three-stage variation pattern. Using the particle image velocimetry technique, the particle movement, shear strain, and vortex location of soil are analyzed. The variation of support pressure and ground surface settlement related to the internal movement of soil particles is discussed. The impact of the tunnel face moving speed on the face stability is discussed. As the tunnel face moves relatively fast, soil failure originates from a height above tunnel invert and an analytical model is developed to analyze such failure.

Cyclic degradation and non-coaxiality of soft clay subjected to pure rotation of principal stress directions

Abstract: Foundation soils are often under non-proportional cyclic loadings. The deformation behaviour and the mechanism of non-coaxiality under continuous pure principal stress rotation for clays are not clearly investigated up to now. In order to study the effect of pure principal stress rotation, a series of cyclic undrained tests on Shanghai soft clay subjected to cyclic rotation of principal stress directions keeping the deviatoric stress constant under the pure rotation condition were conducted using hollow cylinder apparatus. Based on this, the evolutions of excess pore pressure and strains during cyclic loading were investigated, together with the effects of the intermediate principal stress parameter and the deviatoric stress level on stress–strain stiffness and non-coaxiality. The result can provide an experimental basis for constitutive modelling of clays describing the behaviour under non-proportional loadings.

Study on the freezing temperature of saline soil

Abstract: Freezing temperature is an important parameter in studying the freezing mechanism of saline soil. An equation for calculating the freezing temperature is proposed based on the phase transition theory in porous medium, including two main influencing factors, the water activity and pore size. In this equation, the effect of the water activity on the freezing temperature of soil is calculated by Pitzer model, while the impact of pore size is replaced by water content. Through comparing the calculated results with the published experimental data, the equation is proved to be competent in predicting the freezing temperature for the saline soil with sodium chloride or calcium chloride. For the saline soil with sodium carbonate, the effect of salt hydrate crystallization should be taken into consideration. With respect to the saline soil with sodium sulfate, it is difficult to determine the freezing temperature, since there is uncertainty of the resultant when freezing (that is, heptahydrate or decahydrate). In addition, the effects of pore size and multi-component solutes on freezing temperature are also discussed. The study would be helpful for revealing the freezing mechanism and also providing a useful theoretical method for engineering design of saline soil in cold regions.

Continuum hydrodynamics of dry granular flows employing multiplicative elastoplasticity

Abstract: We present a Lagrangian formulation for simulating the continuum hydrodynamics of dry granular flows based on multiplicative elastoplasticity theory for finite deformation calculations. The formulation is implemented within the smoothed particle hydrodynamics (SPH) method along with a variant of the usual dynamic boundary condition. Three benchmark simulations on dry sands are presented to validate the model: (a) a set of plane strain collapse tests, (b) a set of 3D collapse tests, and (c) a plane strain simulation of the impact force generated by granular flow on a rigid wall. Comparison with experimental results suggests that the formulation is sufficiently robust and accurate to model the continuum hydrodynamics of dry granular flows in a laboratory setting. Results of the simulations suggest the potential of the formulation for modeling more complex, field-scale scenarios characterized by more elaborate geometry and multi-physical processes. To the authors’ knowledge, this is the first time the multiplicative plasticity approach has been applied to granular flows in the context of the SPH method.

Undrained expansion of a cylindrical cavity in clays with fabric anisotropy: theoretical solution

Abstract: This paper presents a novel, exact, semi-analytical solution for the quasi-static undrained expansion of a cylindrical cavity in soft soils with fabric anisotropy. This is the first theoretical solution of the undrained expansion of a cylindrical cavity under plane strain conditions for soft soils with anisotropic behaviour of plastic nature. The solution is rigorously developed in detail, introducing a new stress invariant to deal with the soil fabric. The semi-analytical solution requires numerical evaluation of a system of six first-order ordinary differential equations. The results agree with finite element analyses and show the influence of anisotropic plastic behaviour. The effective stresses at critical state are constant, and they may be analytically related to the undrained shear strength. The initial vertical cross-anisotropy caused by soil deposition changes towards a radial cross-anisotropy after cavity expansion. The analysis of the stress paths shows that proper modelling of anisotropic plastic behaviour involves modelling not only the initial fabric anisotropy but also its evolution with plastic straining.

Deformation mechanism of strain localization in 2D numerical interface tests

Abstract: Heterogeneity arises in soil subjected to interface shearing, with the strain gradually localizing into a band area How the strain localization accumulates and develops to form the structure is crucial in explaining some significant constitutive behaviors of the soil–structural interface during shearing, for example, stress hardening, softening, and shear-dilatancy Using DEM simulation, interface shear tests with a periodic boundary condition are performed to investigate the strain localization process in densely and loosely packed granular soils Based on the velocity field given by grains’ translational and rotational velocities, several kinematic quantities are analyzed during the loading history to demonstrate the evolution of strain localization Results suggest that tiny concentrations in the shear deformation have already been observed in the very early stage of the shear test The degree of the strain localization, quantified by a proposed new indicator, α, steadily ascends during the stress-hardening regime, dramatically jumps prior to the stress peak, and stabilizes at the stress steady state Loose specimen does not develop a steady pattern at the large strain, as the deformation pattern transforms between localized and diffused failure modes During the stress steady state of both specimens, remarkable correlations are observed between α and the shear stress, as well as between α and the volumetric strain rate

DEM modeling of hydraulic fracturing in permeable rock: influence of viscosity, injection rate and in situ states

Abstract: Hydraulic fracturing in permeable rock is a complicated process which might be influenced by various factors including the operational parameters (e.g., fluid viscosity, injection rate and borehole diameter) and the in situ conditions (e.g., in situ stress states and initial pore pressure level). To elucidate the effects of these variables, simulations are performed on hollow-squared samples at laboratory scale using fully coupled discrete element method. The model is first validated by comparing the stress around the borehole wall measured numerically with that calculated theoretically. Systematic parametric studies are then conducted. Modeling results reveal that the breakdown pressure and time to fracture stay constant when the viscosity is lower than 0.002 Pa s or higher than 0.2 Pa s but increases significantly when it is between 0.002 and 0.2 Pa s. Raising the injection rate can shorten the time to fracture but dramatically increase the breakdown pressure. Larger borehole diameter leads to the increase in the time to fracture and the reduction in the breakdown pressure. Higher in situ stress requires a longer injection time and higher breakdown pressure. The initial pore pressure, on the other hand, reduces the breakdown pressure as well as the time to fracture. The increase in breakdown pressure with viscosity or injection rate can be attributed to the size effect of greater tensile strength of samples with smaller infiltrated regions.

Centrifuge shaking table tests on 4 × 4 pile groups in liquefiable ground

Abstract: A series of centrifuge shaking table model tests are conducted on 4 × 4 pile groups in liquefiable ground in this study, achieving horizontal–vertical bidirectional shaking in centrifuge tests on piles for the first time. The dynamic distribution of forces on piles within the pile groups is analysed, showing the internal piles to be subjected to greater bending moment compared with external piles, the mechanism of which is discussed. The roles of superstructure–pile inertial interaction and soil–pile kinematic interaction in the seismic response of the piles within the pile groups are investigated through cross-correlation analysis between pile bending moment, soil displacement, and structure acceleration time histories and by comparing the test results on pile groups with and without superstructures. Soil–pile kinematic interaction is shown to have a dominant effect on the seismic response of pile groups in liquefiable ground. Comparison of the pile response in two tests with and without vertical input ground motion shows that the vertical ground motion does not significantly influence the pile bending moment in liquefiable ground, as the dynamic vertical total stress increment is mainly carried by the excess pore water pressure. The influence of previous liquefaction history during a sequence of seismic events is also analysed, suggesting that liquefaction history could in certain cases lead to an increase in liquefaction susceptibility of sand and also an increase in dynamic forces on the piles.

Numerical investigation of tunneling in saturated soil: the role of construction and operation periods

Abstract: This paper numerically investigates the slurry shield tunneling in fully saturated soils with different hydraulic conductivities in short- and long-term scales. A fully coupled hydromechanical three-dimensional model that accounts for the main aspects of tunnel construction and the hydromechanical interactions due to tunneling process is developed. An elasto-plastic constitutive model obeying a double hardening rule, namely hardening soil model, is employed in the numerical simulations. The research mainly focuses on assessing the influence of soil hydraulic conductivity and the method to simulate backfill grouting in the tail void on the evolution of ground subsidence, excess pore water pressure and lining forces. Two different consolidation schemes have been taken into account to computationally address the tunnel construction in soil with low and high hydraulic conductivities. In addition, different methods are employed to simulate the tail void grouting as a hydromechanical boundary condition and to study its effects on the model responses. Finally, the influences of infiltration of the fluidized particles of grouting suspension into the surrounding soil and its corresponding time–space hydraulic conductivity evolution on the displacements and lining forces are studied.

Localized failure in unsaturated soils under non-isothermal conditions

Abstract: Fluctuations of temperature and degree of saturation have considerable influence on the mechanical, hydraulic and retention properties of unsaturated soils. Localized failure is a ubiquitous feature of geomaterials. Major research on localized failure of geomaterials has been focused on geomaterials under the isothermal condition. In this article, we study the localized failure of unsaturated soils under non-isothermal conditions. In particular, we derive the isothermal and adiabatic bifurcation conditions from a homogeneous deformation at the constitutive level under a locally drained condition. A recently proposed meso-scale constitutive model for thermal unsaturated soils is used to derive the isothermal and adiabatic acoustic tensors. We present the spectral form of the consistent tangential elasto-plastic operator from a local material integration algorithm. The numerical simulations at the material level are conducted to study the impact of temperature on localized failure of unsaturated soils under the plane strain condition. The numerical results show that the timing and the critical angle of bifurcation are dependent on temperature.

Fabric evolution of granular materials along imposed stress paths

Abstract: The stress–strain behavior of a granular material is dominated by its internal structure, which is related to the spatial connectivity of particles, and the force chain network. In this study, a series of discrete element simulations were carried out to investigate the evolution of internal structure and force chain networks in initially isotropic granular materials along various imposed stress paths. The fabric tensor of the strong sub-network, which is the bearing network toward loading, can be related to the applied stresses uniquely. The principal directions of fabric tensor of the strong sub-network coincide with those of stress tensor during the loading process in the Lode coordinate system. The fabric of the whole contact network in the pre- and post-peak deformation stages can be related to the applied stresses as $$q_{\phi } = B\left( {q/p} \right)^{z}$$ (B and z are constants depending on loading condition, such as the stress paths and mean stress level) and $$\phi_{1} :\phi_{2} :\phi_{3} \approx \left( {\sigma_{1} } \right)^{0.4} :\left( {\sigma_{2} } \right)^{0.4} :\left( {\sigma_{3} } \right)^{0.4}$$ , respectively. At the critical stress state, the deviator of fabric tensor of the strong sub-network is much larger than that of the whole contact network. When plotted on the π-plane, the fabric state of the strong sub-network can be expressed as a Lade’s surface, while the fabric state of the whole network corresponds to an inverted Lade’s surface.

Soil wettability in ground engineering: fundamentals, methods, and applications

Abstract: Wettability is a fundamental property controlling the extent of wetting in flat and granular solids. In natural soils, wettability affects a wide variety of processes including infiltration, preferential flow and surface runoff. In mineral processing, wettability is paramount in enhancing the efficiency of separation of minerals from gangue. The manipulation of surface wettability is equally crucial in many industrial applications. For instance, superhydrophobic surfaces are those on which water drops roll off easily and as such are used for self-cleaning applications. Therefore, while wettability is strongly cross-disciplinary, its evolution has been discipline-specific with a direct extrapolation or transfer of concepts, approaches, and methods to ground engineering unlikely to remain valid. This paper synthesizes relevant aspects from surface chemistry, materials science, mining engineering, and soil science, and discusses their implications within the context of new granular materials that resist wetting, for use in barriers or ground improvement and, in unsaturated soils, where the effects of wettability have been documented.

Role of particle crushing on particle kinematics and shear banding in granular materials

Abstract: The paper provides an in-depth exploration of the role of particle crushing on particle kinematics and shear banding in sheared granular materials. As a two-dimensional approximation, a crushable granular material may be represented by an assembly of irregularly shaped polygons to include shape diversity of realistic granular materials. Particle assemblies are subjected to biaxial shearing under flexible boundary conditions. With increasing percentage of crushed particles, mesoscale deformation becomes increasingly unstable. Fragmented deformation patterns within the granular assemblies are unable to form stable and distinct shear bands. This is confirmed by the sparsity of large fluctuating velocities in highly crushable assemblies. Without generating distinct shear bands, deformation patterns and failure modes of a highly crushable assembly are similar to those of loose particle assemblies, which are regarded as diffuse deformation. High degrees of spatial association amongst the kinematical quantities confirm the key role that non-affine deformation and particle rotation play in the generation of shear bands. Therefore, particle kinematical quantities can be used to predict the onset and subsequent development of shear zones, which are generally marked by increased particle kinematic activity, such as intense particle rotation and high granular temperature. Our results indicate that shear band thickness increases, and its speed of development slows down, with increasing percentage of crushed particles. As particles crush, spatial force correlation becomes weaker, indicating a more diffuse nature of force transmission across particle contacts.

Effects of the micro-structure and micro-parameters on the mechanical behaviour of transversely isotropic rock in Brazilian tests

Abstract: Based on the Brazilian test results of 23 kinds of transversely isotropic rocks, five trends are obtained for the variation of normalized failure strength (NFS) as a function of the weak plane-loading angles. For each angle, three kinds of fracture patterns are obtained. Furthermore, a new numerical approach based on the particle discrete element method is put forward to systematically investigate the influence of the micro-structure of rock matrix and strength of weak plane on NFS and fracture patterns. The results reveal that the trend of NFS and fracture patterns are slightly influenced by coordination number of rock particles and tensile strength of weak plane, but greatly influenced by percentage of pre-existing cracks and shear strength of weak plane. Micro-parameters of the numerical approach are calibrated to reproduce behaviours of transversely isotropic rocks with different trends, and the simulation results are well matched with experimental results in terms of NFS and fracture patterns. Finally, the numerical approach is applied to study the failure process of layered surrounding rock after tunnel excavation. The simulation results also agree well with observation results of engineering projects.

Constitutive modeling of principal stress rotation by considering inherent and induced anisotropy of soils

Abstract: In order to simulate the soil response during principal stress rotation, anisotropic unified hardening (UH) model is developed within the framework of elastoplastic theory. Without introducing any additional mechanism to display the role of stress rotation specifically, this model achieves the simulation by considering the material anisotropy. The effect of inherent anisotropy is reflected using the anisotropic transformed stress method, but a new formula for the stress mapping is adopted to keep the mean stress unchanged. Analysis indicates that from the view of the transformed stress tensor, the anisotropic soil is subjected to loading during pure rotation of principal stress axes, so that plastic strains can be calculated. To represent the induced anisotropy, a fabric evolution law is proposed based on laboratory and numerical test results. At the critical state, the fabric tensor reaches a stable value determined by the stress state, while the critical state line is unique in the plane of void ratio versus mean stress. The anisotropic UH model has concise formulation and explicit elastoplastic flexibility matrix and can provide reasonable predictions for the deformation of anisotropic soils when principal stresses rotate.

On the soil water characteristic curves of poorly graded granular materials in aqueous polymer solutions

Abstract: Tempe cell with a self-developed horizontal hanging column attachment was used to measure the soil water characteristic curves (SWCCs) of granular materials initially saturated with either water or polymer solutions. For SWCCs of six poorly graded granular materials with d 50 ranging from 0.04 to 0.7 mm in water, it was found that (1) as grain size decreases, air entry value increases, and matric suction (ψ) for the funicular and pendular regimes increases and that (2) steep desaturation curves over narrow ψ range in the funicular regime were observed. Air entry values obtained from the fitting parameter 1/α in van Genuchten SWCC equation fall in the boundaries calculated from the pore throat sizes in both simple cubic and face-centered cubic packings. A toroidal meniscus water model, which incorporates the measured surface tension and contact angle values between aqueous solutions and solid surfaces, was proposed for the SWCC in pendular regime and was compared to well-received numerical methods. This toroid model successfully depicts SWCC of poorly graded granular materials in water. However, SWCCs predicted by this toroid model underestimate the degree of saturation in the pendular regime for Ottawa 20–30 sands in polymer solutions. Herschel–Bulkley fluid, which is a type of non-Newtonian fluids, is postulated to increase the ψ needed to drain the polymer solution due to the nonzero shear stress intercept. In addition, it is also postulated by scanning electron microscopy and optical confocal imaging results that the rough surfaces of Ottawa 20–30 sand, which have many micron-sized “kinks”, together with the possible chemical attractions, help retain the polymer solutions on the solid surfaces, or water film.

Tensile strength of large-scale incipient rock joints: a laboratory investigation

Abstract: In this paper, a testing methodology was developed in the laboratory to measure the tensile strength of large-scale incipient rock joints. In the test, an expansive grout was used to develop the tensile force. Each test comprises two phases: Phase i test and Phase ii test. The Phase i test identified sample failure time, while the Phase ii test measured the corresponding tensile force arising from the expansive grout. Ostensibly homogeneous rock samples without incipient joints were firstly tested to establish the methodology. Tensile strength of block samples containing incipient rock joints was then measured using the established testing scheme. The test results have been compared with those obtained from conventional Brazilian and uniaxial tension tests as suggested by ISRM. The proposed approach is capable of giving a measure of tensile strength of large-scale incipient rock joints, although somewhat smaller strength than that from the standard approaches was occasionally measured in the preliminary tests on ostensibly homogeneous samples. Effects of stress concentration, sample scale, loading rate and expansive tensile force on the testing results were discussed. Furthermore, this simple and practical testing scheme is proposed for the measurement of the in situ tensile strength of rock and incipient discontinuities in the field, which if successful will provide a more scientific guidance on the rock mass classification and engineering design.

DEM analysis of the onset of flow deformation of sands: linking monotonic and cyclic undrained behaviours

Abstract: This study explores the link between the monotonic and cyclic undrained behaviour of sands using the discrete element method (DEM). It is shown that DEM can effectively capture the flow deformation of sands sheared under both monotonic and cyclic undrained loading conditions. When subjected to cyclic shearing, flow-type failure is observed for a loose sample, while cyclic mobility is observed for a dense sample. A strong correlation between the monotonic and cyclic loading behaviour that has been revealed experimentally is also confirmed in DEM simulations: (a) flow deformation occurs in the compressive loading direction when the cyclic stress path intersects the monotonic compression stress path prior to the monotonic extension stress path, and vice versa; (b) the onset of flow deformation in q– $$p^{\prime }$$ space is located in the zone bounded by the critical state line and the instability line determined from monotonic simulations. Hill’s condition of instability is shown to be effective to describe the onset of flow failure. Micro-mechanical analyses reveal that flow deformation is initiated when the index of redundancy excluding floating particles drops to below 1.0 under both monotonic and cyclic loading conditions. Flow deformation induced by either monotonic or cyclic loading is characterized by an abrupt change of structural fabric which is highly anisotropic. The reason why the dense sample dilated during monotonic loading but showed cyclic mobility (temporary liquefaction) during cyclic loading is attributed to the repeating reversal of loading direction, which leads to the periodic change of microstructure.

A micro–macro investigation of the capillary strengthening effect in wet granular materials

Abstract: Wet granular materials are three-dimensionally simulated by the discrete element method with water bridges incorporated between particles. The water bridges are simplified as toroidal shapes, and the matric suction is constantly maintained in the material. A comparison with experimental tests in the literature indicates that the toroidal shape approximation may be one of the best choices with high practicability and decent accuracy. Mechanical behaviours of wet granular materials are studied by triaxial tests. Effects of particle size distributions and void ratios are investigated systematically in this study. The hydraulic limit of the pendular state is also discussed. It gives the capillary cohesion function which is not only determined by the degree of saturation but also positively correlated to relative density and particle size polydispersity and inversely proportional to mean particle size. Furthermore, the capillary strengthening effect is also analysed microscopically in aid of the Stress–Force–Fabric relationship, mainly in fabric anisotropy, coordination number and stress transmission pattern, which revealed the micro-mechanisms of the additional effective stress induced by capillary effect.

Bearing capacity of strip footings on c – φ soils with square voids

Abstract: The presence of underground voids has an adverse influence on the performance of shallow foundations. In this study, the bearing capacity and failure mechanism of footings placed on cohesive-frictional soils with voids are evaluated using discontinuity layout optimization. By introducing a reduction coefficient, a set of design charts that can be directly applied to the classical bearing capacity formulation is presented. The results indicate that the undrained bearing capacity with voids is sensitive to soil weight and cohesion, as both the bearing capacity and stability issues exist in the problem. The failure mechanism is directly related to a variety of soil properties, the locations of single voids, and the horizontal distance between two voids. The presence of voids has a more dominant effect on c–φ soils compared to that on undrained soil. An interpretation of the critical and adverse locations for single-void and dual-void cases with various soil strengths is presented.

Separate-ice frost heave model for one-dimensional soil freezing process

Abstract: For one-dimensional soil freezing process, a separate-ice frost heave model is established, and the coupled process of heat transfer, fluid flow and stress development is considered in the model. First, a coupled heat–fluid–stress model describing the growth of a single ice lens is developed by extending the coupled heat–fluid model presented by Zhou and Zhou (Can Geotech J 49(6):686–693, 2012). Second, the mechanism for the formation of a new ice lens in the frozen fringe is studied, and we indicate that if the total vertical disjoining pressure at certain place exceeds the sum of the external pressure and the critical pressure, a new ice lens will emerge. By combining the growth model of a single ice lens and the criterion for the formation of a new ice lens, the separate-ice frost heave model is then established. The difference between the separate-ice model and the rigid-ice model is explained, and the relations for different mathematical models which describe the soil freezing process are also discussed. Numerical analysis of the separate-ice model is conducted using the finite volume method. The freezing tests for Devon silt under no external pressure and Xuzhou silty clay under a constant external pressure are applied to verify the computational results. The consistence between the calculation and the observation validates the separate-ice frost heave model.

Theoretical analysis of desiccation crack spacing of a thin, long soil layer

Abstract: Soil desiccation cracking is important for a range of engineering applications, but the theoretical advancement of this process is less than satisfactory. In particular, it is not well understood how the crack spacing-to-depth ratio depends on soil material behaviour. In the past, two approaches, namely stress relief and energy balance, have been used to predict the crack spacing-to-depth ratio. The current paper utilises these two approaches to predict the approximate spacing-to-depth ratio of parallel cracks that form in long desiccating soil layers subjected to uniform tensile stress (or suction profile) while resting on a hard base. The theoretical developments have examined the formation of simultaneous and sequential crack patterns and have identified an important relationship between the stress relief and energy approaches. In agreement with experimental observations, it was shown that the spacing-to-depth ratio decreases with layer depth, and crack spacing generally increases with layer depth. The influence of the stiffness at the base interface indicated that decreasing the basal interface stiffness makes the crack spacing to increase in sequential crack formation. The experimental observations also show a decrease in cracking water content with the decrease in layer thickness, and this behaviour was explained on the basis of a critical depth concept.

Numerical integration and FE implementation of a hypoplastic constitutive model

Abstract: Hypoplastic constitutive equation based on nonlinear tensor functions possesses a failure surface but no yield surface. In this paper, we consider the numerical integration and FE implementation of a simple hypoplastic constitutive equation. The accuracy of several integration methods, including implicit and explicit methods, is examined by performing a set of triaxial compression tests. Adaptive explicit schemes show the best performance. In addition, the stress drift away from the failure surface is corrected with a predictor-corrector scheme, which is verified by two boundary value problems, i.e. rigid footing tests and slope stability.

Liquefaction analysis and damage evaluation of embankment-type structures

Abstract: The increasing importance of performance-based earthquake engineering analysis points out the necessity to assess quantitatively the risk of liquefaction of embankment-type structures. In this extreme scenario of soil liquefaction, devastating consequences are observed, e.g., excessive settlements, lateral spreading and slope instability. The present work discusses the global dynamic response and interaction of an earth structure-foundation system, so as to determine quantitatively the collapse mechanism due to foundation’s soil liquefaction. A levee-foundation system is simulated, and the influence of characteristics of input ground motion, as well as of the position of liquefied layer on the liquefaction-induced failure, is evaluated. For the current levee model, its induced damage level (i.e., induced crest settlements) is strongly related to both liquefaction apparition and dissipation of excess pore water pressure on the foundation. The respective role of input ground motion characteristics is a key component for soil liquefaction apparition, as long duration of mainshock can lead to important nonlinearity and extended soil liquefaction. A circular collapse surface is generated inside the liquefied region and extends toward the crest in both sides of the levee. Even so, when the liquefied layer is situated in depth, no significant effect on the levee response is found. This research work provides a reference case study for seismic assessment of embankment-type structures subjected to earthquake and proposes a high-performance computational framework accessible to engineers.

A micromechanical experimental study of highly/completely decomposed tuff granules

Abstract: In this paper, an experimental micromechanical study is presented investigating the contact mechanics and tribological behaviour of highly/completely decomposed tuff granules. The parent material was taken from two locations—named the top and bottom—from a recent landslide in Hong Kong, and in this study the tested granules were obtained from the parent material after drying and sieving processes. Basic material characterisation was conducted quantifying the particle shape, the surface roughness and the strength of a set of grains. A set of twenty-nine monotonic inter-particle shearing tests were conducted on pairs of granules taken from the top and bottom of the landslide. It was found that the granules had very high friction angles at their contacts, in general greater in comparison with other materials reported in the literature. The slightly greater inter-particle friction for the granules taken from the top of the landslide might be because of their higher roughness in comparison with the ones from the bottom. Additional experiments were conducted to investigate the normal and tangential load–displacement response of the granules subjected to cyclic loading. A good curve fitting for the normal load–displacement response could be obtained by using very low apparent Young’s moduli in the Hertzian model. In general, the decomposed tuff granules showed significant plastic response during the first normal load cycle, and this plastic behaviour continued for the subsequent third and fourth cycles. In the cyclic inter-particle shearing tests, the nonlinearity and hysteresis increased for larger cyclic displacements, but the effect of the number of shearing cycles on the energy loss was generally small. Finally, a limited discussion is presented on the applicability of a theoretical model on the tangential load–displacement behaviour of the granules.

An improved numerical approach in surrounding rock incorporating rockbolt effectiveness and seepage force

Abstract: The solutions of stress and displacement of a circular opening excavated in brittle and strain-softening rock mass incorporating rockbolts effectiveness and seepage force are presented in this study. The evolution equation is reconstructed for the strength parameters that incorporate these factors. Based on the evolution equation, an improved numerical method and stepwise procedure are presented which are compatible with the Mohr–Coulomb (M–C) and the generalized Hoek–Brown (H–B) failure criteria, respectively. Then given three interaction mechanisms between rockbolts and surrounding rock, solutions for stress and displacement are proposed in line with the improved numerical method and numerical stepwise procedure. The proposed approach can be reduced to Fahimifar and Soroush’s (Tunn Undergr Space Technol 20:333–343, 2005) solutions for special cases. The proposed method was validated by field monitoring data and FLAC results of Yanzidong tunnel. Examples under the M–C and generalized H–B failure criteria for rock mass are generated through MATLAB programming. Moreover, parametric studies are conducted to highlight the influence of rockbolts effectiveness in combination with seepage force on the stress and displacement of very good, average, and very poor surrounding rock. Results show that in this case, stress confinement is higher and tunnel convergences are lower than the corresponding stresses and displacements obtained in non-reinforced tunnels. Displacement and plastic radius are also higher than those without considering seepage force.