Research interest in the thermo-mechanical behaviour of unsaturated soils is growing as a result of an increasing number of geomechanical problems involving both thermal and unsaturated effects. In this framework, this paper addresses a unified thermo-mechanical experimental study of saturated and unsaturated states and, in so doing, contributes to the understanding of the non-isothermal mechanical behaviour of unsaturated soils. The present experimental program has been carried out on a sandy silt called “Sion silt” using two thermo-hydro-mechanical (THM) cells, one isotropic and one oedometric. The characteristics of these two cells are briefly presented, as well as the THM paths followed. The main results are presented and interpreted in the light of a suitable THM constitutive framework. The compressibility of the soil tested appears not to be affected by the temperature but decreases with a suction increase. As far as the apparent preconsolidation stress is concerned, the results show a decrease of the yield limit with increasing temperature, while a suction increase tends to enhance this limit. Finally, an analytical expression is proposed to describe the evolution of the apparent preconsolidation stress with respect to temperature and suction.

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The fundamental characteristics of strength for the hydraulic fracturing of highly compacted bentonite were studied. Firstly, the constant pressurize rate tests were carried out for the material having various specifications. Secondly, the cyclic pressurize test was carried out to examine the self-sealing function as a buffer material. Thirdly, the constant pressure test was carried out to observe the change in strength during seepage. The observed phenomena were analytically examined. As a result, it was found that the strength for hydraulic fracturing of the buffer material increased with the increase of initial dry density, decrease of sand-mixture ratio and decrease of water content. The swelling pressure of the buffer material worked as a constraint stress for the strength for hydraulic fracturing. The fracture made by hydraulic fracturing was fixed through the supply of water. However a long period of low-pressure supply was needed to recover the strength at the failed parts. While the tensile failure was dominant, the specimen having a low dry density might be failed initially by the shear failure. When the water content became large during seepage, the strength for hydraulic fracturing reduced.

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The rate-dependency of the stress-strain behavior of EPS (Expanded Polystyrene) geofoam with densities of 19.3 and 28.0 kg/m³ was investigated by performing unconventional unconfined compression tests. A set of monotonic loading (ML) tests were performed at different constant values of vertical (axial) strain rate, ε_v. The ε_v value was stepwise changed many times and several sustained loading (SL) tests were performed during otherwise ML at a constant ε_v in other tests. A number of SL tests were performed during global unload and reload cycles to infer the stress-strain relation when ε_v=0. The elastic properties were evaluated by applying minute unload/reload cycles during otherwise ML. The rate-dependent stress-strain behaviour observed in these tests was described by an elasto-viscoplastic model (i.e., a non-linear three-component model), for which the vertical (axial) stress, σ_v, consists of inviscid and viscous components, σ_v^f and σ_v^v, while ε_v consists of elastic and irreversible components, ε_v^e and ε_v^ir. It is shown that the viscous property of EPS geofoam is of Isotach type in that, under the loading conditions where ε_v^ir is always positive, the current σ_v^v value is a unique function of instantaneous ε_v^ir and ε_v^ir, therefore the strength increases with ε_v. This viscous property was quantified based on the test results and incorporated into the model. The rate-dependent stress-strain behaviour, including the creep behaviour, observed in the experiment is simulated very well by the proposed model. In particular, the fact that the creep strain becomes significant when the sustained σ_v value becomes larger than the inviscid yield vertical stress is well simulated.

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The consolidation characteristics of clay, based on the isotache concept in which the strain rate effect is considered, have been studied by many researchers. Most of these studies are aimed at calculating the secondary consolidation with high accuracy in order to evaluate the long-term settlement of large structures. In this study, as the first step toward improving the accuracy of the evaluation of long-term settlement at the Kansai International Airport, the consolidation characteristics of Osaka Bay clay are examined and organized based on the isotache concept. This study proposes a simplified model based on the isotache concept by using a compression curve and the relationship between the consolidation yield stress and the strain rate. The former and the latter are obtained from the constant rate of strain consolidation (CRS) tests and long term consolidation (LT) tests, respectively. The latter is expressed by an equation with three isotache parameters. This model is very practical because it requires a minimum of only one CRS test and one LT test. It is widely applicable to the Osaka Bay clay. The isotache parameters used in this model can be commonly determined for the Osaka Bay clays retrieved from various depths at the Kansai International Airport.

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Soil can experience cyclic or repeated loadings in a range of situations and prediction of cyclic soil response is obviously important to geotechnical engineers. The particulate nature of soil results in highly complex, non-linear response characteristics, and developing models that can capture soil response under cyclic loading is non-trivial. The distinct element method (DEM) can be used to study the fundamental, particle-scale mechanics of granular materials, and offers much promise as a tool to advance understanding of soil response. The first stage in adopting DEM to model cyclic soil response is to quantitatively demonstrate that a DEM model can replicate physical test data, as analytical validation of DEM models for random assemblies of particles under repeated loading is not viable. This paper describes a series of strain-controlled cyclic triaxial tests on an ideal granular material (steel spheres) that were used to validate the capability of an axi-symmetric DEM model to analyse cyclic loading. The DEM model was then used in a parametric study to examine the particle-scale mechanics of the response of specimens of uniform spheres to 50 cycles of loading with various strain amplitudes. The distribution of contact force orientations and magnitudes during testing was examined. The simulations indicate that both the fabric anisotropy and coordination number continued to evolve over the 50 cycles considered. While the variation in the macro-scale response was less marked, there is a clear relation between the micro-scale parameters and the overall specimen response.

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The inherent anisotropic behaviour of sand and the effect of adding different amounts of non plastic silt are studied in this paper. A series of undrained torsional shear tests with constant α and b-values were conducted by hollow cylindrical torsional shear (HCTS) apparatus on Firoozkuh sand containing a wide range of silt percentage. The dry deposition method was selected in the tests. The principal stress direction or inclination angle with respect to the depositional (vertical) direction has a considerable effect on the response of sand that is related to inherent anisotropy in sand fabric during sedimentation. In addition, adding different amount of silt to the host sand causes dramatic strain softening response in the behaviour even at high relative densities, whereby silty sands present higher tendency to flow compared to pure sands. The results show that adding silt to sand skeleton decreases the anisotropy effect up to a certain silt content and then it starts to increase. This phenomenon is attributed to the mixture's unstable structure. Sand-silt-sand contacts cause the meta-stable and unstable fabric. Flow potential increases by increasing the silt content till thirty percent and it has a direct relation with inclination angle α. Also, brittleness index is influenced much more by silt content at thirty percent compared to inclination angle.

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The objective of this study is to empirically clarify the scale effect in long-term consolidation behavior. Frictional pressure loss arising at the interface between the consolidation ring and the specimen must be considered, when the specimen thickness increases. This study uses a special oedometer that can evaluate the quantity of the frictional pressure loss. Incremental loading oedometer tests are carried out in order to evaluate the frictional pressure loss in the normal oedometer test using a specimen with a height of 20 mm. The frictional pressure loss ratio generally exceeds 0.2 when the consolidation pressure is less than approximately 300 kPa, rather than the overconsolidation range. Consequently, a series of inter-connected type consolidation tests, in which the frictional pressure loss can be minimized by limiting the thickness of each specimen element, are carried out in order to investigate the scale effect of the specimen thickness in the range of 20-200 mm. The end of primary consolidation (EOP) can be clearly identified in the excess pore pressure dissipation. The law of squared H is essentially valid for the pore water pressure dissipation. However, the EOP becomes unclear in the strains when the specimen thickness decreases. The compressive strain at the EOP stays constant or slightly increases with the specimen thickness.

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The static stability of sheet pile quay walls on a thick clay deposit against horizontal loads was studied through a series of centrifuge model tests. In the tests, an overconsolidated Kaolin clay layer was prepared over a layer of dense Toyoura sand in a rectangular container. The model quay wall was set to the bottom of the sand layer. The sea-side area adjacent to the quay wall was improved with cement-treated Kawasaki clay. Under 50 g centrifugal acceleration, the clay deposit was consolidated and horizontal line loads of about 0 to 70 kN/m were applied to the quay wall. The width and the depth of the improved area were varied and its performance was compared with that of a quay wall embedded in unimproved ground. Results of the study indicated that the improved ground provided significant resistance against horizontal loading. In addition, a numerical model to estimate the mechanical behavior of the sheet pile quay wall is presented. The outcomes of the numerical model show good agreement with the centrifuge test results.

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Performance of band shaped prefabricated vertical drain (PVD) installed into soft Hai Phong clay with a 110 cm triangle arrangement is reported together with the engineering properties of the clay investigated by field and laboratory tests. Stationary piston sampling was carried out to obtain high quality undisturbed soil samples for laboratory tests and reliable engineering characteristics of the clay. It was assumed for the design of PVD spacing and preloading that the ratio of apparent value of horizontal coefficient of consolidation c_h(ap) to vertical coefficient of consolidation c_v is equal to 1.0. The settlement monitored in the field, which clearly showed that the actual settlement was faster than expected, resulted in the c_h(ap) value 1.5 times as much as c_v determined by the laboratory test.

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Vegetation can significantly contribute to stabilise sloping terrain by reinforcing the soil: this reinforcement depends on the morphological characteristics of the root systems and the tensile strength of single roots. This paper describes an investigation on the reinforcing effect of soil-root matrix in the laboratory using a modified large shear box apparatus (300 mm×300 mm). Four different species of plant namely Vertiveria zizanoides, Leucaena leucocephala, Bixa orellana and Bauhinia purpurea were planted in special boxes containing residual soil compacted to a known density. The results show that roots significantly contribute to the increase in soil shear strength. The presence of the roots only affects the apparent cohesion of the soil and no significant change in angle of friction is observed. L. leucocephala shows the outstanding increase in its root strength in which the strength varies with depth and time e.g., under soil suction-free condition (matric suction=0), the roots have increased the cohesion by 116.6% (0.1 m), 225.0% (0.3 m) and 413.4% (0.5 m) after six months of growth. In twelve months, it is observed that the increase in cohesion is more than three-fold of the six months growth period at 0.1 m depth. The results also indicate that shear strength is influenced by root profile and to some extent, the physiological parameters of the plants.

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It is verified that the linear relation of logarithmic volumetric strain vs. logarithm of pressure derived from the linear relation of both logarithms of volume and pressure (Hashiguchi, 1974) for isotropic consolidation of soils is exactly compatible to the multiplicative decomposition of deformation gradient (Lee, 1969) which would be one of the fundamental requirements for constitutive equations describing the finite elastoplastic deformation. On the other hand, it is emphasized that the adoption of the linear relation of void ratio vs. logarithm of pressure is impertinent for the constitutive equation describing the finite deformation although it is most widely used for constitutive equations of soils.

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The behavior of sand crushing will appear when the confining pressure is up to a certain value, which results in disappearing of the positive dilatancy of sand. Adopting a new hardening parameter with the crushing stress, an elastoplastic constitutive model considering sand crushing is proposed. Comparing the conventional triaxial compression test results with the model prediction, it shows that the proposed model can reasonably describe the dilatancy of sand from positive to negative.

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