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This paper presents a soil-water retention curve (SWRC) model for fine-grained soils. Compared with existing studies, the proposed model accounts for the distinct roles of the volume change of soils on capillarity and adsorption mechanisms. The capillary water is described by a relation that includes the characteristics of the pore-size distributions as parameters, while the absorbed water is modeled by a novel proposition that both considers the phenomenon of capillary condensation and allows for the decoupling between the degree of capillary and adsorptive saturation. Based on this feature, the void ratio effects are considered in a way in which they only affect capillary water, i.e., consistent with how volume change influences soil microstructures. The relative contributions of void ratio effects and hydraulic hysteresis on the path- and history-dependence of a SWRC in Sr–s–e space, where Sr is degree of saturation, s is matric suction, e is void ratio, for deformable unsaturated soils are examined. The significance of discriminating the effects of volume change on capillary and adsorptive water is illustrated by applying the SWRC model to computing the shear strength of unsaturated soils with different void ratios. The model performance is assessed by comparing against test data reported for four types of fine-grained soils in the literature and those tested for natural loess in this work.

How to accurately estimate the permanent deformation of soils due to large number cycles of cyclic loads is essential in many engineering projects, such as offshore wind turbine foundation and high-speed railway. It is still a challenging issue, although many works have been carried out. In this study, a series of cyclic triaxial tests with different levels of initial mean effective stress, relative density, initial static stress ratio, and cyclic stress ratio were performed on saturated Toyoura sand. Test results clearly showed that the vertical permanent strain increased with increasing cyclic stress ratio and initial mean effective stress. Moreover, the shear strain amplitude during the initial loading of the first load cycle was used to describe the intensity of soil particles rearrangement which is the mechanism of permanent deformation. A unique relationship between the shear strain amplitude and the vertical permanent strain was established. A four-parameter model was subsequently proposed for estimating the permanent strain of sand under cyclic loading, which inherently considered all the main influencing factors. Finally, the comparison between the estimation and the measurement in laboratory tests convincingly illustrates the adequacy of the proposed model in estimating the permanent deformation of sand under cyclic loading.

Recently traffic induced geotechnical problems have been receiving increasing attention. Yet the silty clay from the third layer of Shanghai, which is within the main influence depth of traffic loading and overconsolidated, had not been sufficiently studied. A series of monotonic and cyclic undrained triaxial tests were thus carried out on reconstituted Shanghai silty clay with various overconsolidation ratios (OCRs). The silty clay specimens showed phase transformation behavior, which was characterized by stages of initial contraction, temporary phase transformation and later dilation. Such behavior could be termed as a transitional mode between the behavior seen for typical clays and that for sands. The dilation behavior in the later stage was shown more significant for overconsolidated specimens. Six different failure criteria were employed to study the friction angle. There was no observable effect of OCR on the friction angle for this silty clay as if it was a granular material. The undrained shear strength could be normalized with only a small deviation. The normalized shear strength increased with OCR and it behaved more as typical silts than clays at high OCRs. The development of permanent strain under cyclic loading was shown to depend significantly on the cyclic stress ratio and OCR. A higher OCR resulted in smaller permanent and resilient strains, and the decrease of the permanent strain with OCR was remarkable for lightly overconsolidated specimens. Such effect of OCR would be more significant in real soil condition where the soil is at least partially drained.

The evolution of toppling deformation of anti-dip slope is essentially a process of energy dissipation and transformation. Aiming to study the characteristics of energy evolution in different stages, the DEM (discrete element method) software PFC (Particle Flow Code) was utilized to establish a two-dimensional numerical model for a bank slope in Chongqing based on geological background data and field investigation. The DEM model was proven to be reliable not only because the deformation discrepancy between the numerical model and actual bank slope was not large but also because some obvious fractures in the actual bank slope can readily be found in the numerical model as well. In this article, content about displacement in the shallow layer was analyzed briefly. Special effort was made to analyze the energy field and divide the toppling deformation process into three stages. (1) Shear deformation stage: this is an energy accumulating stage in which the strain energy, friction energy, and kinetic energy are all small and the deformation is mainly shear deformation in the slope toe. (2) Stage of main toppling fracture surface hole-through: all three kinds of energy present the increasing trend. The shear deformation in the slope toe expands further, and the toppling deformation also appears in the middle and rear parts of the bank slope. (3) Stage of secondary toppling and fracture surface development: strain energy and friction energy increase steadily but kinetic energy remains constant. Deformation consists mainly of secondary shearing and a fracture surface in the shallow layer. Secondary toppling and fracture surface develop densely.

The deformation of the gravel soil is an important component of the total settlement of the pavement under traffic load. The traffic load induces a heart-shaped stress path in the deviatoric stress space characterised by the rotation of principal stress. However, cyclic triaxial tests are usually carried out to investigate the deformation of gravel soil, which cannot account for the effect of principal stress rotation. In this study, discrete element method (DEM) is used to investigate the response of granular soil subjected to a heart-shaped stress path, and the results are compared with cyclic simple shear tests and biaxial tests. The results indicate that the accumulated volumetric strain of the soil increases with increasing cyclic stress ratio and initial confining pressure. Meanwhile, the accumulated volumetric strain changes from contractive to dilative when the cyclic stress ratio increases from 0.3 to 0.7. At the particulate level, the degree of fabric anisotropy increases and the princip...

Influence of particle angularity on the suffusion in gap-graded granular soils remains unclear up to now. In this study, systematical numerical simulations that consider the particle shape as quasi-spherical polyhedra in different angularity are performed with the coupled discrete element method (DEM) and the computational fluid dynamics (CFD) approach. The suffusion of six gap-graded soil samples with 25% fines content is examined by imposing an upward seepage flow. Conventional triaxial tests are also conducted on the pre-eroded and post-eroded specimens to study the coupling influence of angularity and suffusion on the mechanical characteristics of granular soils. Fines loss, vertical displacement, soil strength, volume flow rate, microstructural analyses of force networks, the cumulative percentage of contact force, and the anisotropy of contact are investigated. Results turn out that the angularity intensifies the internal erosion resistance as the fines loss decreases significantly with the increasing angularity. The soil peak strength and friction angle are approximately linearly correlated with angularity. Erosion-induced particle redistribution reduces the degree of anisotropy of contact normal and contact normal force. This study may improve our understanding of the effect of particle angularity on suffusion with both microscopic and macroscopic evidence.

Since cross-anisotropic sand behaves differently when the loading direction or the stress state changes, the influences of the loading direction and the intermediate principal stress ratio (b = (σ 2 − σ 3)/(σ 1 − σ 3)) on the initiation of strain localization need study. According to the loading angle (angle between the major principal stress direction and the normal of bedding plane), a 3D non-coaxial non-associated elasto-plasticity hardening model was proposed by modifying Lode angle formulation of the Mohr–Coulomb yield function and the stress–dilatancy function. By using bifurcation analysis, the model was used to predict the initiation of strain localization under plane strain and true triaxial conditions. The predictions of the plane strain tests show that the major principal strain at the bifurcation points increases with the loading angle, while the stress ratio decreases with the loading angle. According to the loading angle and the intermediate principal stress ratio, the true triaxial tests were analyzed in three sectors. The stress–strain behavior and the volumetric strain in each sector can be well captured by the proposed model. Strain localization occurs in most b value conditions in all three sectors except for those which are close to triaxial compression condition (b = 0). The difference between the peak shear strength corresponding to the strain localization and the ultimate shear strength corresponding to plastic limit becomes obvious when the b value is near 0.4. The influence of bifurcation on the shear strength becomes weak when the loading direction changes from perpendicular to the bedding plane to parallel. The bifurcation analysis based on the proposed model gives out major principal strain and peak shear strength at the initiation of strain localization; the given results are consistent with experiments.

This paper compares the pre-failure instability of sands under axisymmetric proportional strain and axisymmetric proportional stress conditions. Based on a deviatoric hardening plasticity model, th...

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.

The aim of this study is to numerically investigate the influence of particle breakage on the mechanical behavior of granular materials using a discrete element method (DEM). To enable particle crushing, non-crushable elementary particles are boned together to represents the granular aggregates which can be crushed when the external force exceeds its strength. The flaw of the aggregate was also modeled by randomly distributed void. Single particle crushing tests were carried out to determine the distribution of particle strength. The results of single particle crushing tests illustrate that the simulated single particle fracture strength and pattern agree well with the Weibull’s distribution equation. Conventional oedometer tests, drained monotonic and cyclic triaxial tests were also carried out to investigate the crushing of the aggregates and the associated mechanical behaviors. The effect of confining pressure on the crushing of aggregates and the mechanical behavior was also analyzed. It was found that the peak stress and dilation decrease significantly when particle crushing was considered. The deformation behavior of the specimen is essentially controlled by two factors: particle rearrangement - induced dilation and particle crushing - induced contraction. The increase of permanent strain and the reduction of dilation were observed during cyclic loading and they tend to reach a stable state after a certain number of cycles. The crushing of aggregate is most significant in the first two cycles. The results also indicated that for the same axial strain the volumetric strain and the bound breakage in the cyclic loading tests are significantly larger than that in the monotonic loading tests, especially at high cyclic stress ratio.

Cast in situ and grouted concrete helical piles with 150-200 mm diameter half cylindrical ribs have become an economical and effective choice in Shanghai, China for uplift piles in deep soft soils. Though this type of pile has been successful used in practice, the reinforcing mechanism and the contribution of the ribs to the total resistance is not clear, and there is no clear guideline for the design of such piles. To study the inclusion of ribs to the contribution of shear resistance, the shear behaviour between silty sand and concrete slabs with parallel ribs at different spacing and angles were tested in a large direct shear box (600 mm × 400 mm × 200 mm). The front panels of the shear box are detachable to observe the soil deformation after the test. The tests were modelled with threedimensional finite element method in ABAQUS. It was found that, passive zones can be developed ahead of the ribs to form undulated failure surfaces. The shear resistance and failure mode are affected by the ratio of rib spacing to rib diameter. Based on the shape and continuity of the failure zones at the interface, the failure modes at the interface can be classified as "punching", "local" or "general" shear failure respectively. With the inclusion of the ribs, the pull out resistance can increase up to 17%. The optimum rib spacing to rib diameter ratio was found to be around 7 based on the observed experimental results and the numerical modelling.

In situ cast concrete piles with helical ribs have been used in Shanghai, China for uplift piles. The piles are about 600 mm in diameter with half-cylindrical ribs of 75 to 100 mm in radius. Pull-out tests on model helical ribbed nylon piles of different rib spacing have been carried out in a large pull-out box under different confining pressures. Pull-out resistance and displacement of the piles revealed that the resistance varies with rib spacing and is at a maximum when the ratio of rib spacing to pile diameter is 1, and the resistance is twice that of a smooth pile with the same outer diameter. Three-dimensional finite-element analysis of the model piles revealed that the inclusion of ribs changed the soil and pile interaction mechanism. Shear wedges may form ahead of the ribs. The size of the wedges varies with rib spacing, and is the largest when the ratio of rib spacing to pile diameter is about 1. The wedges may connect to each other to form larger shear zones around the piles to increase the pile resistance. It is suggested that rib spacing is one of the most important factors that affect the pull-out resistance of the piles with the designed geometry.

In this paper, the shakedown behavior of flexible pavement on saturated subsoil subjected to moving vehicle loading is studied. Based on Biot’s dynamic theory of saturated porous media, dynamic stresses of layered saturated subgrade are derived by the transfer matrix method. Then, the lower limit shakedown theory is introduced to estimate the shakedown state of the pavement system. The study focuses on the effects of vehicle’s velocity, saturated soil’s permeability, pavement and subsoil’s strength and modulus ratios on the shakedown behavior. Theoretical results show that for the pavement system on the saturated subgrade, the growth of stiffness ratio or strength ratio leads to the transition of the critical shakedown state from the upper to the lower layers. It is also found that there exists a set of optimal modulus and strength ratios so that the upper and lower layers can reach the critical shakedown state simultaneously.

With the expansion of existing networks of underground system, many metro tunnels are excavated in a pair undercrossing existing tunnels. The new excavations will affect the stress distribution and settlement behavior of the existing tunnels, particularly when the clearance is small. Generally, the construction of the second undercrossing tunnel induces more settlement in the upper tunnels compared to that caused by excavating the first one, due to the interaction between the new excavations. The degree of interaction makes it difficult to predict the settlement of existing tunnels. In this paper, a simplified two-stage analysis method considering the interaction between the undercrossing twin tunnels is proposed to estimate the induced response of existing upper tunnel. In the first stage, the greenfield settlement caused by the first excavation is estimated using Peck’s empirical function, and that caused by the second excavation is modified with a modification factor to consider the influence of the first tunnel. In the second stage, the additional stress calculated using the greenfield displacement is imposed on the existing tunnel resting on Pasternak foundation model. The calculated settlement profile and bending moment of the existing tunnel using the two-stage analysis method are compared with those obtained from finite element analysis and historical field monitored data. The effects of the characteristic parameter of Pasternak foundation model, the spacing between the new twin tunnels, and the modification factor of greenfield settlement, on the settlement profile and bending moment of the existing tunnel are also investigated.

For tunnels built in the saturated silty sand ground, fine particles may be migrated into tunnels through seams of tunnel segmental joints and then seepage erosion is triggered, which may induce ground settlement. However, the process from fine particles erosion to the stress redistribution and soil properties’ change surrounding the tunnel and ground settlement has not been clarified up to now. For this purpose, five numerical tests of seepage erosion in granular soils around the tunnel are conducted using the Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) coupling method. The influences of buried depth and grain size distribution (GSD) of gap graded soils (mainly controlled by the fines content and mean particle size ratio from coarse to fine) on the seepage erosion around the tunnel are investigated. Eroded mass, fines loss mode, surface vertical displacement, stress redistribution, fabric anisotropy, soil behavior and water pressure around the tunnel during the seepage erosion process for five tests are presented and compared. The following results can be upscaled to the practical tunnel engineering, such as: (1) the number of fines loss, the eroded zone and the ground settlement increase with buried depth and mean particle size ratio; (2) the earth pressure near the crack significantly increases due to the stress redistribution induced by fines loss, and the stress redistributed area expands with buried depth; (3) the strength and stiffness of granular soils around the crack are significantly reduced by the seepage erosion. All results revealed that the CFD-DEM simulations provide a new sight on understanding the mechanics of tunnel seepage erosion from a microscopic perspective.

Traffic loading causes the rotation of principal stress axis and generates a heart-shaped stress path in the deviatoric stress space. Using dynamic hollow cylinder apparatus (DHCA), a series of cyclic heart-shaped and cyclic triaxial undrained tests were performed on Shanghai clay through simultaneously varying the torsional shear stress and the normal stresses. Experimental results show that the accumulated undrained responses of clay at different stress levels can be described by the shakedown approach. The permanent strain and energy dissipation were used to classify the accumulated deformation patterns, including plastic shakedown, cyclic plastic creep and ratcheting (incremental instability). For the plastic shakedown, the IPC (increment per cycle) of permanent axial strain becomes negligible and the IPC (area of hysteretic loop per cycle) of energy dissipation tends to reach a low constant level after cycling. For the cyclic plastic creep, the IPC of permanent axial strain (plastic strain increment per cycle) and the IPC of energy dissipation become nearly constant after a large number of cycles. For the ratcheting, the IPCs of strain and energy dissipation increase after a limited number of cycles. Based on these results, a new method is proposed to define the boundaries of shakedown behaviors using the effective cyclic stress ratio.

The deformation and the degradation of ballast significantly affect the performance of the railway track. In this study, the discrete element method (DEM) was used to investigate the deformation behavior and degradation characteristics of ballasts under monotonic and cyclic loading. The ballasts were simulated as crushable aggregate which is modeled by bounded discrete elementary particles together and it can be crushed under external forces. The flaw of the aggregate was also modeled by randomly distributed void. Conventional monotonic and cyclic triaxial tests were carried out to investigate the crushing of the aggregates and the associated mechanical behaviors. The effects of confining on the crushing of aggregates and the mechanical behavior were also analyzed. It was found that the permanent deformation of the aggregates significantly increases when particle crushing is considered. The crushing of aggregate is most significant in the first two or three cycles.

The grouting induced enlarged-base pile is an uplift pile combining conventional enlarged-base pile with post-grouting technique. This type of uplift pile has great value in engineering practice for its higher uplift bearing capacity and less material used. This paper will discuss the uplift bearing capacity and failure mechanism in depth from two aspects, including the model experiment and finite element modelling (FEM). Considering the grouting induced enlarged- base pile, experimental model was developed to investigate the uplift capacity of the enlarged base. The load-displacement curves for enlarged bases of different sizes were obtained under the overburden pressure corresponding to a certain depth. Meanwhile, the experiment was numerically investigated via FEM and the results are consistent with the experimental data. It is found that the degree of increase of uplift bearing capacity becomes greater as the length ratio of the enlarged base to the pile or the diameter ratio of enlarged base to pile increase.

To satisfy the increasing power demands, lots of transmission lines are constructed in the middle-west of China. Expansive soil widely distributes in these areas, and the safety of foundation for transmission line tower needs to be evaluated. Based on consolidated drained triaxial tests, the cohesion and internal friction angle of unsaturated expansive soil were obtained. The compressive and uplift bearing capacities of the plate foundation for transmission line tower foundation were analyzed by 3D finite element modeling. The failure mode of the plate foundation under compressive load shows a local shear mode. For the uplift bearing capacity, the failure mode shows an annular apophysis mode around the plate foundation at the surface. The influence of the saturation of soil on the bearing capacity is more obvious than the uplift bearing capacity of a plate foundation.

Sand is the most widely existing granular material for the foundations of extensive constructions which bear various complex cyclic loading such as traffic loading, ocean wave loading and earthquakes. The road pavements and the offshore foundations subjected to cyclic loads tend to produce excessive permanent deformation, which affects the constructions’ normal operation and safety. Nowadays, many engineering projects change from strength controlled criterion to deformation controlled criterion, such as the high-speed railway in China. Therefore, accurately prediction of the settlement due to cyclic loading is essential. Over the years many explicit models have been established to predict the permanent deformation of sand under cyclic loading. Most of these models are more or less empirical and lack of essential analysis of permanent deformation. There are many parameters obtained by fitting the test results in these models which cannot be easily determined in practical engineering use. In this paper, the concept of shear strain amplitude during cyclic loading is put forward to predict the permanent deformation. By introducing the shear strain amplitude, a unified explicit model is established to predict the cyclic permanent deformation of sand, which only contains four empirical parameters. The result shows that the model can successfully predict the permanent deformation under cyclic loading.

In this paper, undrained diffuse instability and strain localization of frictional materials under plane strain conditions were studied. Based on a 3D non-associated Mohr–Coulomb hardening model, the theoretical criteria for diffuse instability and strain localization were proposed by the second-order work theorem and the vanishing of the determinant of the acoustic tensor. The criteria were used to predict the instability characteristics of soil specimen in isotropically and anisotropically consolidated plane strain tests. The studies showed that, when the soil specimen was loaded under strain-controlled loading mode, the soil becomes potentially unstable slightly before the shear stress reaches its peak value. The initiation of diffuse instability accompanies with the peak of shear stress, and strain softening occurs with further loading. Strain localization was predicted by the vanishing of the determinant of the acoustic tensor, and it is shown to occur after the diffuse instability. The non-coaxial plasticity flow rule was adopted to improve the prediction the onset of strain localization, while the inclusion of non-coaxial plasticity flow rule shows no influence on diffuse instability. Both diffuse instability and strain localization occur at the hardening stage of the soil and result in the reduction of the deviatoric shear stress.

This paper presents a micromechanics-based approach to investigate the effects of fabric anisotropy on the behavior of localized failure in granular materials. Based on a micromechanical analysis, the origin of deviatoric stress is decomposed into two components: contact force anisotropy and fabric anisotropy. Using a micro–macro approach, the back stress is interpreted as an contribution to the change of the fabric’s principal direction. The evolution of the back stress is deduced from the stress–fabric relationship and determined with reference to the deviation of the principal directions between the rate of the reduced stress tensor and the actual reduced stress tensor. With this micro–macro framework, a mixed (isotropic–kinematic) hardening model is developed based on the classical isotropic hardening theory. A laboratory simple shear test is first analyzed to validate the proposed model and illustrate the kinematic-hardening mechanism which is usually displayed under non-proportional loading. The analysis further focuses on the anisotropic aspect of localized failure. It has been discovered that the fabric anisotropy can play an important role in the occurrence of shear banding. An increasing degree of fabric anisotropy tends to delay the initiation of the strain localization and result in higher failure strength. The effects of fabric anisotropy have also been illustrated by comparing the theoretical predictions and measured results on the shear band inclination angle, shear strain level and dilatancy at bifurcation.

The initiation of static liquefaction of unsaturated sand was studied by an extended Mohr-Coulomb elasto-plasticity hardening model. The second-order work was used to check the initiation point of static liquefaction under undrained shear. The results showed that the mechanical behavior of unsaturated sands will be changed by the compressibility of gas. The critical stress ratio at the liquefaction instability point decreases with the saturation degree.

The discrete element method was adopted in this paper to numerically investigate the macroscopic behaviors of granular material with initial anisotropic fabric in simple shear tests. The evolutions of the stress-strain relationship, the non-coaxiality referring to the difference of principle directions between the strain rate and the stress increment, and the fabric anisotropy were analyzed during the tests. The results indicated that inherent anisotropy affects the shear strength. The directions of principal strain rate and principal stress are non-coaxial at the start of the shear loading and they become coaxial gradually as the shear strain increases. Both the principle directions of particle orientation and contact normal change towards 45°, but the later is much faster than the other.

Deep-seated landslides in slopes are often induced by rainfall due to pre-existing cracks or weak layers. A series of centrifuge model tests under rainfall conditions were conducted on slopes with different types of cracks. The histories of suction and displacement of the slope were measured during the tests to investigate the infiltration–deformation–failure process of the slopes. The wetting front curved notably near the crack under rainfall conditions. The deformation of the slope was mainly caused by the saturation of soil and crack-affected water infiltration under rainfall conditions. The displacement process of the slopes with cracks can be divided into a small displacement stage, a rapid increase stage, and a stable stage. The influence of the crack on the infiltration and deformation of the slope decreased with increasing distance from the crack. Rainfall induced significant vertical deformation near the vertical crack rather than horizontal deformation. In contrast to the oblique crack, the vertical crack on the slope top was unlikely to lead to global landslide under rainfall conditions. The deformation–failure behavior of the slope with cracks was also affected by the rainfall style and rain intensity.

For coarse grained soils, their principal stress directions may change when the water level of embankment dam varies instantaneously. In this loading case, the principal directions of stress and strain rate will become non-coaxial. In an effort to model non-coaxial behavior, a modified three-dimensional non-coaxial model is developed in the context of vertex yield (tangent plasticity) theory. Discrete Element Method (PFC) incorporating user-defined interparticle contact models is also employed to gain an insight into microscopic mechanism of non-coaxiality. The analysis focuses on non-coaxial behaviors under simple shear condition. It has been shown that the proposed non-coaxial model gives good predictions for non-coaxiality with reference to microscopic observations while the classical coaxial model fails to simulate the non-coaxial behaviors. In general, non-coaxiality as a result of the rotation of principal stress, is large at a small shear strain, and inclined to become negligible with increasing shear strain. For coarse grained soils, their non-coaxiality tends to largely depend on the initial normal pressure, where a larger degree of non-coaxiality can be observed at a higher pressure.

Numerous constitutive models built on coaxial theory and validated under axi-symmetric condition often describe the stress–stain relationships and predict the inceptions of shear banding in sands inaccurately under true triaxial condition. By adopting an elaborated Mohr–Coulomb yield function and using non-coaxial non-associated flow rule, a 3D non-coaxial elasto-plasticity model is proposed and validated by a series of true triaxial tests on loose sands. The bifurcation analysis of true triaxial tests on dense sands predicts the influence of the intermediate principal stress ratio on the onset of shear band accurately. The failure of soils is shown to be related to the formation of shear band under most intermediate principal stress ratio conditions except for those which are close to the axi-symmetric compression condition. Copyright © 2009 John Wiley & Sons, Ltd.