International Journal of Geomechanics

This paper presents the finite-element simulations of the behavior of strip footings resting on sand beds, with different density of soil, reinforced with geocells of different dimensions. The strength and stiffness of sand confined with geocells is represented by an equivalent composite model developed from triaxial compression tests. The additional confining pressure due to geocells, calculated using hoop tension theory, is used to obtain the apparent cohesive strength imparted to sand due to geocells. The elastic modulus of the geocell encased sand is related to the elastic modulus of the unreinforced sand and the tensile modulus of the geocell material using an empirical equation. Load-settlement response of strip footings on geocell reinforced sand beds obtained from the numerical simulations are compared with the corresponding experimental results and the match is found to be good. In addition, numerical results showed that with the provision of a geocell layer, the mobilized shear stress contours become horizontal and shift downwards, indicating that the geocell mattress transmits the footing load to a deeper depth, thereby bringing about a higher load carrying capacity.

This paper investigates the influence of anisotropy and destructuration on the behavior of Haarajoki test embankment, which was built by the Finnish National Road Administration as a noise barrier in 1997 on a soft clay deposit. Half of the embankment is constructed on an area improved with prefabricated vertical drains, while the other half is constructed on the natural deposit without any ground improvement. The construction and consolidation of the embankment is analyzed with the finite-element method using three different constitutive models to represent the soft clay. Two recently proposed constitutive models, namely S-CLAY1 which accounts for initial and plastic strain induced anisotropy, and its extension, called S-CLAY1S which accounts, additionally, for interparticle bonding and degradation of bonds, were used in the analysis. For comparison, the problem is also analyzed with the isotropic modified cam clay model. The results of the numerical analyses are compared with the field measurements. The simulations reveal the influence that anisotropy and destructuration have on the behavior of an embankment on soft clay.

It is well known that all three principal stresses play a role in the stress-strain-strength-volume response of solids and granular materials, yet conventional triaxial compression tests and direct shear tests are typically used for the determination of design parameters for granular materials, even when field conditions may be plane strain (e.g., behind a long retaining wall). The effects of loading conditions on the macroscale response of granular materials have been studied extensively using experimental methods and it is relatively straightforward to quantify the variations in material macroresponse using continuum constitutive equations. However, these methods provide relatively little insight into the driving micromechanics that govern macroscale behavior, particularly in soils that fail via regions of high localized strain (e.g., shear banding). Thus, it is desirable to develop a set of models that reproduce the expected macroscopic behavior and allow insight into the governing microscale mechanics. To this end, a series of numerical experiments has been performed to assess the effects of loading (i.e., boundary) conditions on particulate material response. Three-dimensional discrete numerical specimens, with the same material and model properties but different problem geometries, were assembled to similar void ratios and confining stresses and then subjected to plane strain compression, conventional triaxial compression, and direct shear loading conditions. Macroscale responses, including deviatoric stress (plain strain and conventional triaxial compression) or stress ratio (direct shear), volumetric strain, friction angle, and dilation angle of the specimens are investigated. Shear strength relationships of the three different loading conditions are explored. Small-strain response and the volume change response are studied. Simulated material response is generally in good agreement with previously published results from laboratory investigations though some significant differences are noted and discussed. Given that the models reasonably produce boundary condition effects, it may be possible in the future to exploit the discrete nature of the simulations to observe changes in material microstructure under different boundary conditions and to infer how micromechanics influences the macroresponse of the assemblies.

Controlled delivery of water to plant root zone remains a challenge in the exploration of plant growth under zero gravity, in which soil water retention characteristics are difficult to predict due to possible soil particle rearrangement and pore restructuring. This paper aims to provide an understanding of the effect of soil particle rearrangement on the water retention characteristics in zero gravity. A three-dimensional pore network model is developed to predict water retention characteristics under two forms of particle packing—cubic packing (most porous) and rhombohedral packing (most compact). The pore-throat distributions, as input parameters to the network model, are derived from particle size distribution under the two forms of particle arrangement. The model simulations reveal the quantitative variation of water retention characteristics due to particle rearrangement. Soil particle rearrangement that occurs during the drying process is shown to significantly change the water retention curve when a soil is near saturation. When the soil is near residual water content, the particle rearrangement has little effect on the water retention curve. The model results indicated that an increase in packing density results in decreased water content at saturation, increased bubbling pressure, and accentuated water retention hysteresis. The model also reveals nonuniform spatial distribution of pore fluid in porous medium in zero gravity. Although this paper studied the soil-water characteristics from the microgravity perspective, its results also contribute to the understanding of the soil-water characteristics due to soil compaction on Earth.

The erosion of the fine fraction of granular soils due to seepage is a possible cause of the settlements of shallow foundations. Here the problem is tackled with reference to deep pumping wells to be installed in an urban area. First, the results of laboratory tests on reconstituted soil samples are illustrated that provide the quantity of eroded particles during time depending on the imposed hydraulic gradient. Based on the experimental data, an incremental erosion law is proposed that has been implemented in a finite-element code for erosion-transport analysis. After describing the steps of the numerical solution procedure, the results of two- and three-dimensional finite-element calculations are discussed. They provide an estimation of the quantity of eroded material in the vicinity of the pumping wells and of the possible consequent settlements of nearby buildings.