International Journal of Geomechanics

This paper presents a closed-form analytical solution for the consolidation of soil around a laterally loaded pile, assuming the soil skeleton deforms elastically and under plane strain conditions. The problem is idealized as a circular rigid disk surrounded by a deformable soil zone. Drainage is assumed to occur at the boundary of the assumed zone of deformable soil around the pile. The response around the pile is obtained using an uncoupled consolidation analysis. The solution for the governing equation of excess pore water pressure is found by using the separation of variables technique. Expressions for the initial excess pore water pressure distribution are derived from the mean total stress changes given by the elastic solution. Curves showing decay of the excess pore water pressure with time, and also the variation of pile displacement, are plotted in nondimensional form. The parameters that affect the displacement prediction are illustrated and discussed. Comparison with three-dimensional analysis of a laterally loaded pile shows that the two-dimensional analysis can reasonably predict the consolidation process along the shaft of the pile.

Among the many constitutive equations, the moisture-suction relationship, also known as the soil-water characteristic curve (SWCC), is one of the important relationships needed for modeling the dynamics of unsaturated soils using the finite-element method. In this study, a new SWCC equation is developed that uses four fitting parameters including the maximum possible suction. The new model predicts the experimental data of various soils well for a wide range of degrees of saturation (DOS), especially in high suction (above $10,000\mathrm{kPa}$). Predicting the higher suction at low DOS is one of the deficiencies of the widely used Brooks and Corey, van Genuchten, and Fredlund and Xing models. The numerical stability and performance of the proposed, and other widely used models, are investigated by implementing these models within in a simplified finite-element program for the dynamics of unsaturated soil and simulating static and dynamic problems. For the midrange DOS, all of the aforementioned models predict similar responses. However, at low DOS the new model predicts the suction variations well compared with the other models.

Granular materials, such as sand, experience different stress paths in nature; and the behavior of granular materials depends on stress conditions. This paper presents a study of the relationship between the macro- and microscale responses of granular materials using the discrete element method (DEM) under general three-dimensional stress conditions specified by the parameter $b$, which describes the relative magnitude of the intermediate principal stress with respect to the major and minor principal stresses. With this view, a cubical numerical sample consisting of 8,000 spheres was prepared. Six true triaxial tests were conducted with the same numerical sample with no bias in the initial fabric. Different macroscopic results, including the relationship between principal strains, principal deviatoric strains, the evolution of stress and strain increment vectors, and the behavior of strain increment vectors on the $\pi$-plane, are discussed. Good consistency between the numerical and experimental results was found. In addition, the macroscale stress ratio correlated with material fabric evolution, which was quantified using a fabric tensor and a unique relationship, was noticed regardless of $b$ value considering the strong contact networks.

In this paper, a neuro-fuzzy model in conjunction with particle swarm optimization (PSO) are used for calibration of soil parameters used within a linear elastic-hardening plastic constitutive model with the Drucker-Prager yield criterion. The neuro-fuzzy system is used to provide a nonlinear regression between the deviatoric stress and axial strain (${\sigma }_d-\epsilon$) obtained from a consolidated drained triaxial test on samples of poorly graded sand. The soil model parameters are determined in an iterative optimization loop with PSO and an adaptive network based on a fuzzy inference system such that the equations of the linear elastic model and (where appropriate) the hardening Drucker-Prager yield criterion are simultaneously satisfied. It is shown that the model parameters can be determined with relatively high accuracy in spite of the limited insight gained by a single set of data. To verify the robustness of the technique, a second set of data obtained under different confining pressures is then used in a separate run. The outcome shows a close match with the same order of accuracy.

Unsaturated soils are three-phase porous media consisting of a solid skeleton, pore water, and pore air. The behavior of unsaturated soils is strongly influenced by the matric suction (pore air pressure minus the pore water pressure). Soil water characteristic curves (SWCCs) describe the relationship between matric suction and water content in unsaturated soils and, therefore, capture the hydro-behavior of soils. SWCCs show hysteretic behavior that not only depends on the wetting or drying history of the soil, but also on the stress-strain history (mechanical behavior) of a soil. The hydro-behavior of unsaturated soils, on the other hand, influences the mechanical behavior through matric suction. To predict the behavior of unsaturated soils, a hysteretic SWCC model is proposed based on the bounding surface plasticity concept. The model for hysteretic SWCCs is then incorporated into a constitutive model for unsaturated sands and silts in the general stress space. The resulting model is a comprehensive constitutive model that accounts for the coupling effects between hydro- and mechanical behavior of unsaturated sands and silts in the general stress space. The numerical integration of the constitutive equations and model calibration and validation are presented in a companion paper.

Following the comprehensive model formulation given in Part I, a fully implicit integration procedure for the rate equations using a closest point projection method is presented in this paper. The stress-update algorithm and calibration procedure for all the model parameters are discussed in detail. Two sets of laboratory test results, one from unsaturated Minco silt and another one from Toyoura sand, are used to validate the proposed model. The model is shown to have the ability to capture the coupling effects between hydro- and mechanical behavior and predict the laboratory tests reasonably well.

A number of constitutive models exist to model behavior of granular material under cyclic loading. Most of these models are useful to model soil behavior where the number of cycles is very small. Therefore, a semiempirical model is proposed to simulate the cyclic densification and degradation of coarse granular material under a railway environment where the number of loading cycles is very large. A kinematically expanding elastic surface is introduced to separate elastic and elastoplastic deformation of the material with the number of cycles. The reversible response is represented by the pressure-dependent elastoplastic rule. Empirical model parameters are introduced to consider the effect of stress history, stress ratio, number of cycles, and breakage. Model parameters are evaluated based on cyclic triaxial test results. Predicted results are compared with the laboratory data in order to demonstrate the model validity. The proposed model captures the realistic deformation and degradation of ballast at various frequencies and confining pressures for a large number of cycles.

In this paper, on the basis of the micropolar (Cosserat) continua approach, a constitutive model for simulation of granular material (e.g., sand) response in the elastoplastic range has been presented. Numerical analyses on strain localization in this type of material are carried out in the last part of this paper. The influence of the evolution of the internal length-scale micropolar parameters on the formation of strain localization is investigated.

This paper introduces a servocontrolled true triaxial apparatus that has been developed to test cubical specimens [with 7.62 cm (3 in.) sides] of unsaturated soil under controlled-suction states and for a wide range of stress paths not readily achievable in a cylindrical cell. The apparatus is a mixed-boundary-type device, with the specimen seated on top of a 5-bar high-air-entry-value (HAEV) ceramic disk and between five flexible membranes on the remaining sides of the cube. The new cell is an upgraded, more elaborate version of one previously reported by the authors, featuring two independent pore-air and pore-water pressure control systems, as well as a fully computer-driven stress application/control system. Suction states are induced in the specimens via the axis-translation technique. The paper outlines the full development of the apparatus, including its main components; assembling process; and selection of appropriate compaction method, dry unit weight, and constant-suction loading rate for clayey sand. The suitability of the apparatus is demonstrated by repeatable results from two triaxial compression tests performed on identically prepared samples of compacted clayey sand under constant 200-kPa matric suction.

Results from a comprehensive series of suction-controlled hydrostatic compression, conventional triaxial compression, triaxial compression, triaxial extension, and simple shear tests, conducted on statically compacted specimens of poorly graded sand with clay, are presented. The experiments were accomplished in a newly developed true triaxial apparatus to test cubical, 7.62 cm (3 in.) side, specimens of unsaturated soil under controlled-suction states and for a wide range of stress paths that are not readily achievable in a cylindrical cell. The key features of the apparatus and a thorough performance verification test program have been documented in a first companion paper. Results from suction-controlled tests performed in the present work were used to calibrate and further validate two of the most popular elastoplastic critical state-based frameworks postulated to date for unsaturated soils, namely, the Barcelona Basic Model and the Oxford Model. In general, predictions of unsaturated soil response from either model are shown to be reasonably accurate, highlighting their potential for use in computational analyses of geotechnical boundary-value problems that involve geosystems made of compacted soil or resting on unsaturated ground.

This paper describes a simplified analytical method that considers postinstallation normal stress, normal stress due to soil dilation, and grouting pressure when calculating the maximum shear stress of the nail-soil interface. Systematic test results from two previous publications were used to verify the accuracy of the proposed method. The Hong Kong design method was also used to determine the maximum shear stress of the nail-soil interface. These analyses confirmed that the experimental data were more accurately predicted by the proposed analytical method. An extensive parametric study was conducted to examine the effects of several key parameters on the maximum shear stress. It was observed that when there was no grouting pressure, maximum shear stress increased with increasing overburden pressure, failure surface distance, dilation angle, or decreased drill hole radius. Analysis using the proposed model also demonstrates that grouting and overburden pressures share an interactional effect. The larger the grouting pressure, the greater the influence of the overburden pressure; and, consequently, the larger the maximum shear stress at the nail-soil interface.

A consolidating clay layer can be singly or doubly drained. The degree of consolidation is a function of the initial excess pore pressure distribution and the drainage conditions. Traditional geotechnical engineering practice assumes a uniform initial excess pore pressure distribution, for which the singly drained and doubly drained solutions to Terzaghi’s one-dimensional consolidation theory are available in most textbooks. In this paper, several symmetric and asymmetric initial excess pore pressure distributions are studied using a series solution method. Excess pore pressure isochrones and average degree of consolidation plots are generated for all cases, assuming an impervious boundary at the base of the clay stratum. These plots enable users to determine the average degree of consolidation and pore pressure at a specific depth for a variety of loading scenarios. Pore pressure redistribution is also clearly more prevalent in singly drained clays, where pore-water pressures at certain depths increase at certain times, a phenomenon not generally expected during consolidation.

This study investigates the drained stress-strain-volume change behavior of Delhi silt under triaxial compression and extension loading conditions using two different existing constitutive models, the hardening soil (HS) model and a hierarchical single surface (HISS) model. The HISS model is shown to be able to capture the stress-strain-volumetric response in both compression and extension loadings with higher accuracy as compared with the HS model.

Geotechnical centrifuge modeling provides a means of accelerating the transport of contaminants through a soil matrix. Contaminant transport mechanisms occurring in the geotechnical centrifuge model can be used to verify various mathematical and analytical models. This technical note presents modeling of chloride (Cl) diffusion through soils in a geotechnical centrifuge. Acceleration level, $N$, of 100 g was used for centrifuge modeling of Cl migration through 7.5-m-deep soil strata for a period of 150 and 600 days. From the centrifuge test results, the Reynolds number, $\mathsf{R}$, and Péclet number, $\mathsf{P}$, were found to be several orders less than unity. This indicates that the dominant Cl transport mechanism in the soil is diffusion. Finite-element modeling was performed, using SEEP/W and CTRAN/W, to validate the experimental results, and excellent matching has been obtained. The study also highlights that geotechnical centrifuge modeling can be used as a viable alternative to field scale experimentation.

The horizontal pullout capacity of vertical anchors embedded in sand has been determined by using an upper bound theorem of the limit analysis in combination with finite elements. The numerical results are presented in nondimensional form to determine the pullout resistance for various combinations of embedment ratio of the anchor ($H/B$), internal friction angle ($\varphi$) of sand, and the anchor-soil interface friction angle ($\delta$). The pullout resistance increases with increases in the values of embedment ratio, friction angle of sand and anchor-soil interface friction angle. As compared to earlier reported solutions in literature, the present solution provides a better upper bound on the ultimate collapse load.