Soil Behavior and Geo-Micromechanics

Hansbo's formula for non-Darcy flow was introduced to modify Terzaghi's 1-D consolidation equation, and the modified equation was generalized to account for the consolidation case under time-dependent loading. Numerical analysis was performed by using the finite volume method. The effects of the parameters of non-Darcy flow and the rate of loading on the consolidation process were investigated. The numerical results indicated that non-Darcy flow delays the dissipation of pore water pressure in saturated clay layers; thereby the rate of settlement of these soil layers is less than the solution based on Darcy flow. In addition, the influence of loading rate on the consolidation behaviors is remarkable in the construction period and in the initial stage after its completion of construction.

The accuracy of finite element solutions for the consolidation of porous media is influenced by the number and size of the time increments used in the analysis. A solution algorithm for adaptively selecting time increments for the solution of elastic and elastoplastic coupled consolidation problems in finite element analysis has been developed by Sloan and Abbo (1999). By treating the governing consolidation relations as a system of 1st-order differential equations their algorithm utilized subincrementation to automatically adjust the size of time increments used in the analysis. Unlike other time stepping schemes, the procedure adjusts the time increments in order to control the error due to time stepping to lie near a specified tolerance. The algorithm was shown to be robust and to provide an efficient method for the solution of consolidation problems. In this paper the efficiency of the algorithm is further demonstrated through the analysis of the construction of an embankment on a deep layer of soft soil. The time increments required for the efficient and accurate analysis of the consolidation of porous media are shown to differ by orders of magnitude.

A series of oedometer tests were performed on both undisturbed and remolded samples of Lianyungang clays to investigate the secondary compression behavior. The results of natural clays show that the coefficient of secondary compression, C_α, reaches the maximum dramatically in the vicinity of the consolidation yield stress. When the consolidation pressure is less than the yield stress (termed as pre-yield state), the creep can be neglected. But C_α decreases with the increase in consolidation stress in the post-yield state (i.e. the consolidation pressure being greater than the yield stress), similar to remolded samples. In addition, it is found that there is a unique relationship between the difference in coefficient of secondary compression, (C_α—C_α*), and the difference of void index (ΔI_v) between undisturbed and remolded samples. Furthermore, a simple method is proposed for predicting secondary compression index of natural clays.

The oedometric path of normally consolidated unremolded clay sediments evolves, in the (log σ'_v - I_L) plane, toward a maximum point located above the C_c line (which characterizes the compression of material that has been remolded and reconstituted) highlighting "apparent" overconsolidation. Then, the curve tends to join the C_c line indicating the beginning of destruction (failure) of bonds between particles. This progressive rupture of bonds becomes complete when the path rejoins the remolded behavior in the high stress domain. This gradual transition, from unremolded behavior towards remolded behavior, has been investigated on marine sediments extracted from the Gulf of Guinea. Oedometric tests were performed on both natural (unremolded) clay samples and remolded ones. The study is supported by several microscopic observations with SEM (Scanning Electron Microscope) and mercury intrusion porosimetry.

Experimental data to study the strain softening and instability behavior of loose sand under plane-strain conditions are presented in this paper. K₀ consolidated drained and undrained tests under both deformation-controlled and load-controlled loading modes were carried out using a new plane-strain apparatus. The drained and undrained behaviors of very loose sand under plane-strain conditions were characterized. The test results show that the loading mode affects the post-peak behavior of sand and controls whether strain softening or instability will occur in the post-peak region. The failure line and critical state line are not affected by the loading mode.

The undrained strength is one of the governing soil properties when evaluating stability issues associated with clays. The undrained shear strength is not a unique measure of soil strength; it may be obtained by conducting laboratory tests or by in-situ tests, or a combination of both. This paper presents a case study using piezocone penetration test (PCPT or CPTu) and flat dilatometer test (DMT) to ascertain undrained shear strengths at a site in New Orleans where soft clay deposits are predominant. This paper attempted to correlate the undrained shear strengths obtained from PCPT and DMT to those obtained from laboratory unconfined compression and triaxial unconsolidated undrained tests. A calibration study was performed and a set of site-specific values were suggested to best suit the soils at the exploration site. The suggested parameters or correlations may be valuable to practitioners to evaluate undrained shear strengths in New Orleans area using popular in-situ test methods.

This paper examines the critical state parameters of clays typical to the physiographic regions of Kentucky, a state located in the southeastern United States. The critical state model is unique due to the fact that it links the soil consolidation characteristics with shear strength properties allowing prediction of soil behavior at various loading states. The critical state strength parameters for the clays are presented and well-established engineering material property correlations are translated into critical state representation, particularly those correlations that relate consolidation index and recompression index to liquid limit and specific gravity. The Kentucky clay soil parameters are then compared to these representations. Finally, new relationships correlating critical state parameters to consistency limits and particle size analysis are presented.

In order to study the effects of soil structure on compression and shear behavior, a number of isotropic compression and consolidated-drained triaxial shear tests on Shanghai soft clay in undisturbed and reconstituted samples have been carried out. Comparisons of measured compression curves indicate that undisturbed samples have larger void ratio than reconstituted samples at the same confining pressure. Comparisons of measured stress ratio-strain curves indicate that the confining pressure has no effect on the reconstituted samples but has an effect on the undisturbed samples. The strength of undisturbed samples is lower than that of reconstituted samples when sheared at the same confining pressure. This behavior is explained to be caused by the difference in void ratio between two type samples.

The present paper describes the physical and engineering properties of tropical peat soils from Sarawak, Malaysia. Different physical properties such as organic content (OC), liquid limit (LL), fibre content (FC), specific gravity (G_s) and engineering properties (mainly the standard Proctor test) have been conducted on remoulded peat soil samples. The results show that, the value of LL, FC, Optimum Moisture Content (OMC) increases with an increase in OC. Also the results show that, G_s and MDD decrease with increasing values of OC. Furthermore, the plot of OC against G_s from the current study is compared with other researchers; whereby the correlation shows that R² values ranges from 0.71 to 0.95. This demonstrates that findings on local peat soils from Sarawak are in good agreement with other researchers as similar trends are observed. Consequently, geotechnical engineers can refer these correlations to comprehend the preliminary behavior of peat soil, where the geotechnical data are not readily available.

The moduli of compacted geo-materials are estimated in the laboratory or in the field. Laboratory tests are essential to obtain the parameters that affect the behavior of the materials. Field tests, especially nondestructive field tests, are more practical and desirable because they are rapid to perform, and because they test a volume of material in its natural state. Empirical relations between the resilient modulus and the modulus from in-situ test are sometimes used in the preliminary stages of design and planning. Unlike in situ methods, these relations do not take into account the site-dependent variation in material properties. This paper represents an alternative process based on seismic nondestructive testing technology for relating the results from laboratory and field moduli from a number of chemically or mechanically stabilized geo-materials. Attention is also paid to understanding of the results from seismic tests and incorporating them in the existing design methods, specifications and common practices.

Loess, a wind-blown cemented silt deposit, covers extensive areas of northwest and northern central of China, and is used in most geotechnical constructions including embankments and liners. The purpose of this study is to evaluate the potential application of resistivity measurements for monitoring and assessing the quality of compacted loess soils. The relationships between soil resistivity, water content, pore fluid chemical components, degree of saturation and temperature have been studied based on the measurement of the resistivity of the compacted loess sample. Test results show that soil resistivity decreases with the increase in temperature, water content, degree of saturation and porosity. The higher conductivity of pore fluid, the lower soil resistivity.

Soil anchors are commonly used as foundation systems for structures requiring uplift resistance such as transmission towers, or for structures requiring lateral resistance, such as sheet pile walls. Anchors commonly have more than one plate or bearing element and therefore there is a complex interaction between adjacent plates due to overlapping stress zones. This interaction will affect the failure mode and ultimate capacity. However, no thorough numerical analyses have been performed to determine the ultimate pullout loads of multi—plate anchors. The primary aim of this research paper is to use numerical modelling techniques to better understand plane strain multi-plate anchor foundation behaviour in clay soils.

Natural soil deposits commonly exhibit cross-anisotropy about a vertical axis due to their depositional environment. This paper presents a study of the vertical stress distribution in a cross-anisotropic elastic soil deposit due to a point load. The study is based upon the exact solution derived by Liao and Wang (1998) that uses Fourier and Hankel transforms to solve the governing differential equations of the displacement functions. The solution has been first modified to consider a reduced parameter elastic model for cross-anisotropy as proposed by Graham and Houlsby (1983). The vertical stress distribution in the soil has been studied for a vertical point load with a varying degrees of anisotropy.

Design of many geotechnical problems, such as the stability of slopes and excavations, are strongly governed by the shear strength of soil represented in three dimensions. In literature, a number of failure criteria have been proposed using isotropic invariant of stress tensor in three dimensional stress spaces. For a linearly pressure-dependent material, there are four basic failure criteria which have been commonly used to describe the behavior; Mohr-coulomb, Drucker—Prager (1952), Lade and Duncan (1975), and Matsuoka and Nakai (1985). Mohr-Coulomb criterion is defined in a 2-D stress space and it follows plane strain condition. The other three which are 3-D failure criteria have been considered during this study for analysis. Calibration of such a model is relatively easy when the tests are performed in axisymmetric conditions and principal stresses are applied on the specimen, e.g. triaxial tests. This paper will present the solutions for calibrating such failure criteria assuming associative flow rule and using the shear strength parameter (φ_ps) obtained from the laboratory tests performed under plane strain conditions e.g. Direct Shear and Simple Shear Test.

The thermomechanical approach to establishing elastic/plastic constitutive model for geomaterials not only has a tight mathematical structure, but also satisfies the laws of thermo-dynamics automatically and has a rigorous mathematic and physical basis. The isotropic thermomechanical model proposed by Collins (2002, 2005a) is introduced briefly and analyzed in detail. Aiming at its shortage of hardening law, the unified hardening parameter is incorporated into this model. And then, the model is extended to a unified thermomechanical model for soil which can be applied to both clay and sand. The FEM computer program of thermomechanical model and improved unified model are worked out. The adaptability and validity of improved model can be seen from the comparison and analysis of calculated results.

Based on the UH (unified hardening) model a new anisotropic constitutive model for anisotropic soils is presented in a new anisotropic transformed stress space. An anisotropic peak strength M_α is firstly introduced to represent the inherent anisotropy associated with particles' orientation distribution. An anisotropic transformed stress tensor σ˜_ij^a is proposed by incorporating M_αwith the original transformed stress tensor σ˜_ij. The failure criterion and the stress-strain relationship for anisotropic soils are interpreted in the same constitutive framework. The capabilities of the proposed model are illustrated through comparisons of model simulations with experimental data. The basic soil parameters can be determined from the conventional triaxial compression tests on vertical specimens and horizontal specimens.

An elasto-plastic model with two yield surfaces has been developed for the simulation of granular materials with consideration of grain ruptures. Grain breakage is induced by deviatoric as well as isotropic stresses. One yield function is based on a Mohr-Coulomb criterion with a hyperbolic hardening function of the plastic deviatoric strain. A second yield function is introduced to describe the plastic behaviour under compression, in which a hardening function of the plastic volumetric strain is introduced. The main assumption of this model is that, upon loading, the position of the critical state changes as a consequence of grain breakage. The effect of the grain size distribution is introduced in the relationship between the void ratio at critical state and the mean effective stress. Triaxial and oedometer tests have been performed on crushable granular materials. Comparison of experimental results and numerical simulations shows that the new model can reproduce with good accuracy the behaviour of granular materials subjected to grain ruptures during mechanical loading.

Soft rock ground is widely distributed in Japan and slope failure of soft rock occurs frequently. It is necessary to understand the mechanical behavior of soft rock and to develop a constitutive model which can appropriately express the mechanical behavior of soft rock. In this research, a series of drained triaxial compression tests were carried out to modify the original constitutive model proposed by Nakai and Hinokio (2004). Based on the experimental results, the change of the yield surface during the shearing process was proposed. As the result, the stress-strain behaviors under different confining stresses were reproduced quantitatively by a modified constitutive model.

It is well known that the deformation and strength characteristics of sand are very complicated, and time effects and strain localization are two of the most important aspects. A non-linear three-component rheology model consisting of a hypo-elastic component connected in series to a combination of non-linear inviscid and viscous components connected in parallel is proposed, which can take into account multi-factor influence on the deformation and strength characteristics of sandy soils. The inviscid component is described by an isotropic, energy-based hardening and softening, and non-associated energy model, while the viscous component is formulated based on the strain rate-dependent behavior of sandy soils. A finite element method (FEM) code incorporating the aforementioned model is validated by simulating the physical plane strain compression (PSC) tests on sand. It is shown that the proposed model can much better simulate the deformation and strength characteristics of sand, especially for the rate-dependent behaviors and strain localization into a shear band.

Based on the results from the isotropic loading-unloading tests and a series of plane strain compression tests along different stress path on Toyoura sand, a couple of hardening functions respectively for shear and volumetric plastic strain for sand is introduced in the framework of double hardening elasto-plastic constitutive model. A modified form of plastic compressive strain energy, which is stress path-independent, and a volumetric yielding locus compose the compression hardening model. Another modified plastic shear strain energy, which is also stress path-independent, and a shear yielding locus compose the shear hardening model. Then, the plastic strains can be described as the sum of two independent parts. Based on the above, a double modified plastic work-hardening constitutive model for sandy soils is proposed. It is shown that, based on the direct comparisons between the results from the numerical calculation and corresponding drained plane strain compression tests on Toyoura sand, the above-proposed constitutive model could reasonably simulate the deformation and strength behavior of sand with different stress-path.

Numerical modeling of the behavior of geotechnical structures subjected to cyclic loading requires the consideration of two main aspects: the constitutive relationship and numerical strategy. The constitutive model has to be representative of the clay behavior, whereas the numerical strategy should be efficient in order to reduce the computation time. To simulate undrained triaxial cyclic tests on clay, a method of time homogenization is applied to a bounding surface plasticity model. This method of homogenization is based upon splitting time into two separate scales. The first scale relates to the period of cyclic loading and the second to the characteristic time of the material. Simulations of undrained triaxial cyclic tests on normally consolidated clay under one-way cyclic loading are carried out. The performance of time homogenization is numerically validated. The sensitivity of the time increment is also investigated.

A new viscoplastic model is developed extended from overstress theory of Perzyna. A scaling function based on the experimental results of constant strain-rate oedometer tests is adopted, which allows viscoplastic strain-rate occurring wherever the stress state is inside or outside of the reference surface. The inherent and induced anisotropy is incorporated using the formulations of yield surface and its rotation of S-CLAY1. The bonds are progressively destroyed by plastic straining by incorporating the concept of an intrinsic yield surface. No additional test is needed for determining all model parameters compared to the Modified Cam Clay model, and the parameters determination is straightforward. The experimental verification is carried out with reference to the constant strain-rate and creep tests on two natural clays.

The aim of this paper is to constitute a simple anisotropic elasto-plastic model that can predict the mechanical behavior of saturated structured soils. Undrained triaxial tests and drained stress probe tests under isotropic and anisotropic consolidation modes were performed on natural undisturbed samples of Shanghai soft clay to study the yielding characteristic. The effect of damage to structure caused by accumulated plastic strain is modeled with an inner variable that allows the size of the bounding surface to decay with plastic deformations. An anisotropic tensor and rotational hardening law were introduced to reflect the initial anisotropy and the evolution of anisotropy. Combined with the isotropic hardening rule, rotational hardening rule and degradation law of structure were incorporated into the bounding surface formulation with an associated flow rule. Validity of the model was verified by the undrained isotropic and anisotropic triaxial test results for Shanghai soft clay. The effects of stress anisotropy and loss of structure were well captured by the model.

This paper presents the development of hypoplastic model for simulating deformation characteristics of Bangkok Soft Clay. The results from drained triaxial tests of Bangkok Soft Clay were simulated by the new hypoplastic model with applied stress path of 0–180 degrees in p–q stress space. The proposed hypoplastic model has satisfy performance to predict the deformation characteristics of Soft Bangkok Clay. The stiffness of proposed model varied according to load and unloading condition and different applied stress path.

Bentonite, a smectite clay, is widely used in geotechnical engineering as drilling mud, liner material, and has been proposed as buffer material for high-level nuclear waste disposal. This study focuses on the microstructure formation of unsaturated Kunigel-V1, a bentonite clay, and the effect that the microstructure formation has on its mechanical behaviour. The microstructure was studied with Mercury Intrusion Porosimetry (MEP) and a Scanning Electron Microscope (SEM), while the mechanical behaviour was studied using Constant Rate of Strain (CRS) consolidation testing and Constant Volume Direct Shear (CVDS) testing. Test results indicated that aggregates increased in number and size with the addition of water. However, an increase in applied pressure resulted in the dissipation of air, and a decrease in size of the intra and inter-aggregate spaces. The clay's microstructure had little influence on the friction angle (φ), but did affect the dilatancy of the soil.

This paper presents a stress-strain model for the behavior of structured clays using a microstructural approach. Clay material is considered as a collection of clusters, which interact with each other mainly through mechanical forces. The bonds of structured clays are considered as the bonded forces between clusters in contact. The bonds can also be reduced due to a damage mechanism at a cluster contact during loading. The elasto-plastic stress-strain relationship for structured clays is derived by using the granular mechanics approach which takes into account the behavior at grain level. The model is used to simulate one-dimensional test and undrained triaxial tests. Typical trend of the behavior of structured clay is captured by the proposed model.

Cohesive soil of the interactive marine & terrestrial sediment is deposited along the north side of the Yangtze River estuary of China. The natural deposit has poor physical properties, whose natural moisture content is close to or greater than the liquid limit and the void ratio is close to or greater than one, which is similar to that of the natural soft marine clay, while the former have much higher in-situ strength and sensitivity than the latter. Oedometer tests and corresponding micro-structural tests under different consolidation stresses are performed on both the undisturbed and the remolded interactive deposit soil to investigate the reason which causes the abnormal combination between the poor physical properties and good mechanic characteristic indexes. It is found that the natural interactive deposit soil, which belongs to strong structural soil, shows good uniformity between micro-structural change and macro- mechanical properties.

Some aspects of the macroscopic geotechnical behavior of compacted soils are highly related to their microstructure. This paper presents a qualitative and quantitative microstructure investigation of compacted samples made up of crushed Callovo-Oxfordian argillite. This material is considered as being possibly used as backfill material in radioactive waste disposal at great depth. Scanning electron microscopy and mercury intrusion porosimetry technique were employed to determine the fabric and pore size distribution properties of the samples. The results show that an increase of the compaction effort only results in the reduction of the inter-aggregates porosity. After wetting, the sample displays a relative homogenous microstructure with a decrease in the macropores population.

This paper presents an analytical solution and numerical simulation of a failure mode in over-consolidated clays based on the elasto-plastic description of the material. First, a bifurcation analysis is performed to study the failure mode of the over-consolidated clay specimen along different stress paths. It is shown that the occurrence of either homogeneous failure or localized failure depends on the Lode angle. The bifurcation does not occur in tests along the stress paths at the Lode angle in the ranges of –30° ~ –26.5° and 7.5° ~ 30°. However, bifurcation points are detected in tests along stress paths with the Lode angle varying from –26.5° to 7.5°. Second, the elastoplastic model is integrated into the software ABAQUS. Three-dimensional (3D) finite element simulations along stress paths of the triaxial compression, triaxial extension, and plane strain are performed. The results have validated the capacity of the model to simulate the instability and strain localization phenomena.

The intention of the paper is to compare the calculations of shear zones in granular bodies using two different approaches: a continuum and a discrete one. In the first case, the FEM based on a micro-polar hypoplastic constitutive law was used. In the second case, the DEM was taken advantage of, where contact moments were taken into account to model grain roughness. The comparative calculations were performed for a passive case of a translating and rotating retaining wall.

A two-dimensional Particle Flow Code (PFC2D) was applied to simulate sand liquefaction behavior induced by cyclic loading. The numerical sample was prepared with 4061 discs with its particle size distribution similar to Fujian Standard sand. Based on the theory of strain-controlled undrained cyclic triaxial test, a constant volume numerical test was carried out under cyclic loading with uniform strain amplitude. The macroscopic responses of the sample were obtained and the variation of average coordination number under cyclic loading was analyzed. The effects of cyclic strain amplitude and confining pressure on liquefaction resistance were further analyzed in numerical simulations. It was found that the numerical tests reproduced the general characteristics of liquefaction behavior of saturated sand under cyclic loading. The effects of cyclic strain amplitude and confining pressure on the liquefaction resistance of the numerical sample were comparable with reported experimental results.

The extensive use of granular materials as roadbeds accelerates the study of sand behavior under cyclic loading. While several studies have been done on sand deformation under cyclic loading, little information is available on the explanations for this behavior. By discrete element method (DEM) simulation, this paper offered a preliminary insight into particles interaction of loose sand which controls the material's stress-strain behavior. A 2-D `sample' of 896 `quartz' disks, with diameters of 0.20, 0.25 and 0.30mm, was produced by self-gravity sediment. Then it was K₀ loaded to t' = 127 kPa, at which point, the sample started to be cyclically loaded for 5000 cycles when t' kept constant. Cyclic amplitudes varied from 0.02 to0.04 t'. Increasing strain accumulation with increased number of cycles was observed. For smaller cyclic amplitude, strain accumulation increased smoothly, but for larger one, strain accumulation increased erratically after a large number of cycles. The coordination number variation during cycling load was considered to be responsible for this phenomenon. Induced anisotropy was observed, and it could be explained by deviatoric fabric analysis.

The three-dimensional discrete element method code (PFC3D) was used to model triaxial compression tests on sands and the results were compared with physical laboratory tests. The sand particles were modeled by unbreakable 2-sphere clumps, so that interlocking between particles, which yielded more stable and efficient force chains, could take place. This was confirmed by comparing the results modeled by 2-sphere clumps with the results of tests where particles were modeled by single spheres. In addition to global stress and strain response, void ratios and coordination numbers were monitored at different locations within the specimens throughout the shearing process, and their evolution was evaluated to see whether localization existed within the specimens.

A 3D coupled continuum-discrete hydromechanical model was utilized to simulate the pore fluid flow and solid particle transport during piping in a micro scale. The fluid phase was implemented using the "fixed coarse-grid" fluid scheme based on Navier-Stokes equations and the solid phase which consists of an assemblage of discontinuous particles can be modeled effectively using DEM. Numerical simulations were conducted to investigate the mechanisms of macro-meso response of fluid and grains during piping in the presence of increase hydraulic gradient. The fluid velocity, loss fraction of eroded particles, pathway of moving particles, and etc are traced and recorded. The outcome of these simulations was consistent with experimental observations and revealed valuable information on the micro-mechanical characteristics of piping in sandy soil.

This paper presents a coupled macro-micro method for numerically simulating pile penetration. In order to properly describe a set of particles experiencing large deformation, during the penetration process, Particle Flow Code (PFC), which is based on discrete element method, is introduced here to simulate sands near the pile as interacted balls. Thus the discrete properties of the sands nearly around the pile can well be depicted. In order to overcome the deficiency of particles during the simulation process, the domain containing particles away from the pile is simulated as continuous media by FLAC program, given that this set of particles exhibits linear elastic properties. Thus all the sands around the pile are divided into two domains, and the interface between them is treated as force boundary for the continuous domain and velocity boundary for the other domain simulated as a set of balls. It is shown that the coupled method can well depict the pile penetration process without loosing the discrete properties of the sands near the pile and efficiently reduce particle numbers during the simulation.

This article uses a model test and PFC^2D discrete element simulation for systematic research on the mechanism and behavior of soil arching around passive piles in sand. In the model test, soil arching effects with different pile spacing was studied. In the discrete element simulation, the influence of pile spacing on soil arching was revealed at the meso-level. The model test indicates that pile spacing influences soil arching effects significantly. Numerical analysis also shows that pile spacing is the most significant factor on soil arching surrounding passive laterally loaded piles. From this study, the meso-mechanism and behavior of soil arching are discovered further.

Because of the lack of experimental devices for testing granular materials with large particles, the mechanical behaviour of rockfill is not very well known. This lack of knowledge can lead to improper design of rockfill dams, and as a consequence, to severe accidents including dam failures. A method has been developed by Frossard (2005) for evaluating the shear strength of rockfill based on the estimation of size effect in granular materials, affected by grain breakage. Comparison between experimental results and numerical predictions shows that this method can estimate with good accuracy the strength of granular materials made of large particles.

Granular materials with large particles have been widely used to construct embankments or rockfills dams. However, due to limitations in the size of experimental set-ups, their behavior remains relatively less investigated than the behavior of sands for instance. A new large triaxial apparatus, with a maximum specimen diameter of 1000mm, has been designed to study the behavior of such coarse granular materials. This device is described in this paper. The results of a triaxial test on calcareous particles with a maximum particle size of 160 mm are presented. Special attention has been paid on grain crushing under shear loading.

The paper discusses the main aspects involved in numerical modeling of large deformation, rigid body impacts in sandy soils. The modeling was performed for the study of dynamic compaction of sandy soils induced by repeated drops of a rigid tamper. A two-dimensional discrete element model, written with PFC^2D code (Particle Flow Code in two dimension), has been developed to carry out simulation of dynamic compaction process on a sandy soils. Comparisons were made of the dynamic contact stress of tamper bottom, dynamic stress with depth, displacement fields with depth and crater depth. A preliminary assessment of the reliability of these numerical curves was conducted using results from the laboratory model tests. The results show that the mechanical response of sandy soils induced by tamper dropping could be well simulated with PFC^2D during dynamic compaction, and the change of dynamic stress and dynamic strain with depth can be real-time traced and recorded.

Due to many cases of embankment failures reported in recent years, our society requires high-level stability of embankments. For this reason, it is necessary to make a detailed examination on deformation and strength characteristics of compacted soil used for soil structures. Authors have developed a triaxial testing apparatus for the rotated specimen method (RSM), thereby developing a system that enables us to measure volume changes and local deformations of partially saturated soil. In this paper, we examined the effects of variations in degrees of compaction and in compacted soil layers on local deformation and strength characteristics of the compacted soil in a systematic and experimental manner.

In reservoir areas, due to the change of water environment, the properties of rock/soil change. Therefore, it is necessary to study the changed rock/soil properties due to watering and develop necessary forecasting models. In this study, saturation experiments were performed in the laboratory using rock/soil samples from a bank-slope located in the Three Gorges Reservoir and water samples from the Yangtze River. By SEM (Scanning Electron Microscope) analysis, the mini-structures of undisturbed samples were studied. It shows that the mini-structure of rock/soil changes after a 9-day saturation period in flowing water. Before saturation, the soil structure was of good integration and mainly composed of piled and accumulated prothallus. After saturation, the structure changes to be flocculent and there are many scattered prothallus which connect to each other mainly by edges and angularities. Compare to the unsaturated samples, there are more pores in the saturated samples caused by corrosion.