The advantage of using geogrid reinforcement for the mitigation of swell during first flooding has been previously established. The performance of reinforced expansive clay undergoing cycles of wetting and drying is presently investigated using an apparatus principally similar to an enlarged oedometer. Soils of different plasticity indices were subjected to full swell-full shrink as well as full swell-50% shrink cycles until reaching the steady state. The geogrid has been found to continue functioning throughout the cycles of swell and shrink. Increasing the geogrid stiffness reduced both the swell and shrink and the steady state band. Excluding the first wetting and drying cycle, the improvement factors for swell and shrink are almost the same and they may exceed 80% depending on the grid stiffness.

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Among various soil densification techniques, the sand compaction pile (SCP) has been one of the most frequently used methods to improve loose deposits of sandy soils encountered in Holocene or reclaimed lands. In this method, columns of densely compacted sand are created in the ground by imparting vibration to the sand at the bottom of a pipe which is lifted stepwise while supplying sand from the ground surface. Because of noise and vibration produced during its installation, the SCP is losing its popularity and an alternative technique employing a static driving force is being exploited to install columns of dense sand. To study the degree of soil densification due to such static sand pile installation, multiple series of large-scale hollow cylindrical torsional shear tests were conducted in the laboratory on clean fine sand, simulating stress changes conceived to be occurring in a soil element in the vicinity of the pile being penetrated. To determine the stress changes in the field during the pile penetration, analysis was conducted based on the classical theory of elasticity. The sequence of stress changes thus established was applied to saturated sand specimens prepared in a torsional hollow cylindrical shear test apparatus. This process allows complex stress paths to be reproduced in the specimens, including the rotation of a principal stress direction. In the course of the tests, shear stresses were applied first undrained on loose and medium dense fine sands and induced pore water pressure was dissipated by opening the valve of the drainage system, thereby monitoring the volume decrease of saturated samples. Particular attention was drawn to the influence of the amount of shear strains imposed undrained on the soil specimens on the subsequent drained volume changes. It was found that a volume change of 5∼10% was observed in the test samples, which is considered sufficiently great to bring about substantial densification in the sand. The experimental results of the tests were shown to provide a basis for the assessment of soil densification due to static sand pile penetration. On this basis, a diagram was provided to facilitate the evaluation of the degree of soil densification. In addition, case studies were carried out by taking advantage of soil improvement projects which have recently been implemented at three sites in Japan. These are considered to provide field verification on the effectiveness of soil densification due to static sand pile installation. Finally the degree of in-situ densification as evidenced by increased SPT N-value was interpreted in the framework of the conception established through the laboratory tests.

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The purpose of this study is to verify the ability of a numerical method to predict earthquake-induced deformation of soil embankments. The method is dynamic response FE analysis that incorporates a cyclic elasto-plastic constitutive model for sand, a cyclic elasto-viscoplastic model for clay, and Biot's two phase mixture theory. This method was applied to two sets of case records : 1) a series of dynamic centrifuge model tests of soil embankments resting on liquefiable sandy soils, and 2) a set of river embankments that were damaged and undamaged during the 1993 Hokkaido Nansei-oki earthquake. For the model tests, comparisons between measured and computed model responses were made. The results demonstrated that pore pressures within the foundation soil and vertical settlement of the embankment were qualitatively predicted. In addition, the mechanism of embankment settlement, even though the embankment foundation did not attain zero effective stress state, was explained. In the case of river embankments, the model parameters were estimated from the field and laboratory test data of foundation soils. A difference in the magnitudes of damage at two embankment sections was reproduced well, although it may not be quantitatively satisfactory. The results showed that the foundation sandy soils would have been liquefied. The analyses also indicated that a cohesive soil layer in-between sand layers that existed only at a non-damaged site prevented the liquefied soil beneath the embankment from spreading out lateraliy, and thus minimized the settlement of overlying soil.

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Studying freezing and thawing mechanisms is important to understand rock slope failures in cold regions. This study investigated crack development in rock specimens by freezing and thawing action. Rock specimens were prepared from welded tuff. A freezing test was conducted in a temperature-controlled chamber where the temperature varied from +5°C to -18°C. The frozen rock specimens were thawed in distilled water. The test lasted 3.5 hours for each freeze-thaw cycle, including 2 hours for freezing and 1.5 hours for thawing. The P-wave velocity, acoustic emission, and porosity were measured, and observations on the change in appearance of the outer surface and section of the specimen were made. The results obtained are summarized as follows : (1) cracks in rock developed during the freezing of pore water ; (2) the initial cracks in rock caused by freezing and thawing action formed at a weak position of the surface layer and then propagated to the inside to the specimen ; (3) the patterns of cracks produced by freezing and thawing cycles depended on the rock structure, such as its geometry, porosity and pore diameter ; and (4) the rate of crack development was affected by the degree of water saturation and mechanical properties of the rock.

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The paper presents a theoretical and experimental study of the effects of non-coaxiality or non-coincidence of the principal stress and the principal plastic strain increment directions on the behaviour of granular materials. Experimental results from hollow cylindrical tests on sand involving principal stress rotation which support previously published results on non-coaxiality are presented. These results imply that constitutive relations cannot be sufficiently formulated in the principal stress space unless the deviations between the principal stress and plastic strain increment directions are taken into consideration. It is shown that plasticity formulations with plastic potentials that are scalar functions of the stress invariants alone implicitly assume coaxiality and cannot be used for loading involving principal stress rotation. The paper presents a comprehensive analysis of the effects of non-coaxiality on the energy dissipation of sand. The paper shows that energy dissipation calculated from the principal stresses and the principal plastic strain increments or from the stress and plastic strain increment invariants, would be erroneous and would over-estimate the amount of dissipated energy during loading in the case of non-coaxial flow. A non-coaxiality factor is introduced in order to account for the effects of non-coaxiality on the energy dissipation equation and in a stress-dilatancy relation for granular materials. Explicit expressions of the non-coaxiality factor for two-and three-dimensional loading conditions are given at the end of the paper. Experimental results are presented to show the validity of the proposed energy dissipation and stress-dilatancy equations.

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An elastoplastic anisotropic model for soils is developed on the basis of thermomechanics and derived from two equations : the free energy and the dissipation functions representing the elastic and plastic behaviour of the material, respectively. Additionally, the dissipation function is integrated by two independent equations representing the flow rule and the plastic criteria. The model so derived presents a rotating yield surface, the rate of rotation depending on the induced anisotropy. The shape of the yield surface may be modified and depends on the value of a single parameter. Furthermore, the flow rule is of the non-associated type and can also be modified with a single parameter. Theoretical and experimental comparisons yield the conclusion that the model is able to adequately simulate the soil behaviour for different stress paths.

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A series of drained stress path plane strain tests was performed on saturated dense specimens of Toyoura sand with precise stress and strain measurements. It is shown that all the strain increments (i.e., axial, lateral, shear and volumetric) that occurred by loading between two stress states were dependent on the intermediate stress paths. It is suggested that the use of a strain quantity as the hardening parameter that is independent of stress history in an elasto-plastic model for sand may not be relevant. Based on the test results, one form of energy function is shown to be stress history-independent. This quantity is a state parameter, being a unique function of one form of stress parameter for different stress paths. How this function can be used as a stress history and stress path-independent hardening function in an elasto-plastic model briefly discussed. The effects of stress history and instantaneous stress path on the friction angles at the failure and residual states are negligible. The effects of stress path on stress-dilatancy relationships based on plastic strain increments are found to be small but noticeable.

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The superloading yield surface concept is newly introduced to the original Cam-clay model in order to describe some aspects of the mechanical behavior of highly structured soils, in which destructured soils are assumed to follow the original Cam-clay model. Following are typically those aspects : (a) structured soils are always "bulky" compared with destructured soils, and if they are in the normally consolidated state they always take their state variables outside the "Roscoe surface" of the Cam-clay model (b) when void ratios are the same, structured soils exhibit strengths higher than those of destructured soils ; (c) for the same stresses, the void ratios of structured soils are greater than remolded soils. The structured state of a soil is simply defined as the size ratio of the original Cam-clay yield surface and the superloading yield surface that should lie above the Cam-clay yield surface. On the basis of "unconventional plasticity" theory, the superloading yield surface concept, together with Hashiguchi's subloading yield surface concept, describes the degradation processes from both an overconsolidated state to a normally consolidated state and a structured state to a destructured state. These degradation processes continue gradually with ongoing plastic deformation. Since plastic deformation is irreversible, the decay of soil structure is also irreversible : The degraded state can not come back to the original state again through elasto-plastic mechanical operation alone. Chemical and/or thermal effects with "aging", that are said to newly generate both overconsolidated state and structured state without any change of stresses, are beyond the scope of this study.

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X-ray computed tomography scanners, better known as "X-ray CT scanners" have been widely used for medical purposes. But recently, this machine has for engineering purposes been developed as a nondestructive method for any kind of material. In this paper, the application of X-ray CT scanners for characterization of failure in soils is examined using undisturbed soil specimens under unconfined compression. This CT scanner was developed for industrial purposes so that the x-ray capacity is much higher than that used in a medical setting. Here, the change of density under the deformation in the soil specimen can be investigated without any destruction. Not only 2-D cross sectional images but also 3-D images of the deformed soil specimen were obtained without any destruction of the specimen and the behavior in the soil not only visualized but also evaluated quantitatively. Thus, the effectiveness of the X-ray CT method for geotechnical engineering was confirmed.

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