Elasto-plastic behavior of a material depends on the past loading history, and for clays, the most important process representing the past loading history is overconsolidation. The deformation behavior of clays including the general stress-strain relation, shear strength, and compressibility all depend on overconsolidation. A soil is generally referred to as overconsolidated soil when the existing stress is smaller than the maximum stress experienced by the soil in the past (i.e., preconsolidation pressure); otherwise, the soil is referred to as normally consolidated soil. In many circumstances, even when the soil is normally consolidated by this definition, the soil may behave like an overconsolidated soil exhibiting an "apparent overconsolidation" Various factors such as cementation desiccation, aging, bonding, etc., have been suggested as possible causes of "apparent overconsolidation". In this paper, using a particle-1evel numerical simulation technique known as the discrete element method (DEM), a study is performed to examine the microscopic mechanism responsible for overconsolidation in clays. The stress-based overconsolidation, and a leaching induced "apparent overconsolidation" are numerically simulated and studied. It is shown that there are many similarities at the microscopic level between stress-based and leaching-induced overconsolidations. It is further found that the change in void ratio during the process of leaching-induced overconsolidation is negligible.

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Effects of wet conditions (air-dried or saturated), pressure level and dry density on the viscous properties of sand were evaluated by performing unconventional triaxial compression tests on Toyoura sand. Effects of stress path were studied to a limited extent. During otherwise monotonic loading at a constant strain rate, the strain rate was changed stepwise and/or creep loading was performed after different strain rate histories. The test results were analysed in the framework of a three-component model, called the TESRA model, that had been developed based on the results from plane strain compression tests on dense Hostun and Toyoura sands at a confining pressure of 400 kPa. The manner of viscous stress change by a change in the irreversible shear strain rate observed under the various test conditions could be represented by the same viscosity function representing the viscous properties. The viscous stress decayed with an increase in the irreversible shear strain in all the tests, while the rate of decay was noticeably larger with air-dried sand than with saturated sand. The TESRA model using the same viscosity and decay functions simulates very well the effects of viscous properties observed in the whole tests.

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Effects of viscous properties on the yielding characteristics of sand under general stress conditions were evaluated by performing unconventional drained triaxial compression tests on saturated dense Toyoura sand. Shear yield stresses evaluated by applying unload/reload cycles of deviator stress at a fixed confining pressure were noticeably affected by a strain rate change between primary loading and reloading as well as creep deformation at the maximum deviator stress before unloading due to the viscous properties of sand. It is shown.that the double yielding concept, describing the yielding of soil in terms of shear and volumetric yielding mechanisms, is relevant to sand. The shear yield loci on the q -p' stress plane were obtained by decreasing and then increasing the deviator stress respectively at different confining pressures. The shape of shear yield loci was also affected by the viscous properties. With an increase in the effective mean principal stress, p', along a given inviscid shear yield locus, the deviator stress, q, increases while the stress ratio, q/p', slightly decreases. It is demonstrated that it is necessary to incorporate the effects of viscous properties when developing a realistic constitutive model for sand. An interaction between shear and compression yielding mechanisms is discussed based on experimental results.

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A new piezocone was developed and applied to Osaka Pleistocene clay layers as deep as 250 m. Its geometry is the same as that of the reference procedure defined by the ISSMFE and standard of JGS: i.e., the projected cross sectional area is 10 cm²; the angle of the cone is 60°; the pore water pressure is measured at the shoulder. This piezocone does not measure the skin friction. Due to great depth at the investigated site, the capacity of the point resistance (q_t) and the pore water pressure is as large as 30 MPa and 20 MPa, respectively. The test was carried out, using a borehole drilled prior to the penetration, because of stiff sand or gravel layers and large skin friction between the rod and the ground. The q_t measured by the cone penetration test (CPT) was correlated to the yield consolidation pressure (p_y) measured by the Constant Rate of Strain (CRS) oedometer test for the soil sample recovered near the point of the CPT investigation. The cone factor for the p_y value (N_pc) was defined by (q_t -p_vo)/p_y, where p_vo is the in situ total overburden pressure. The range of observed N_pc value is relatively narrow and between 2.5 and 2.8, which is in the middle of the range of N_pc factors measured in Holocene clays in the various areas in the world as well as Japan. The overconsolidation ratio (OCR) was also derived by CPT. Variation of the OCR estimated by the CPT is nearly equivalent to that measured by the CRS oedometer. It may be concluded from this investigation that the consolidation properties, especially the p_y value, derived from samples recovered from great depths, are quite reliable as design parameters.

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Pore Size Distribution (PSD) of naturally deposited clayey soil samples recovered from various parts of the world and at varying depths was measured by Mercury Intrusion Porosimetry (MIP). In addition to naturally deposited samples, the artificially mixed soils, namely Singapore clay with Toyoura sand or diatomite, are studied in order to know the influence of grain size component on the PSD. Characteristics of the PSD were represented by the mean pore size (D_p50). As an application of the PSD characteristics to practical geotechnical engineering, an attempt has been made to relate the hydraulic conductivity (k) with the PSD. The following interesting findings are obtained: 1) With an increase in the consolidation pressure (p'), D_p50 also decreases in the same manner as that of the e-log p' relation. 2) The range of D_p50 for naturally deposited soils is relatively narrow, namely between 0.03 and 2 μm, except for London clay which is an old deposit clay, i.e., Tertiary. 3) It is found from the artificially mixed soils that D_p50 generally increases with an increase in silt and sand content. 4) There is a unique relation between the hydraulic conductivity (k) and nD²_<p50>, where n is the porosity.

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A conventional direct shear apparatus was modified to provide drained simple shear mode during cyclic loading. Several tests were performed in order to investigate the seismically induced deformation, stress-strain behavior and cyclic residual shear deformation of unsaturated and relatively dense sandy earth structures. This paper reports the effects of different parameters such as number of cycles, initial static shear stress, soil type, degree of compaction, consolidation time and stress, creep time and aging, amplitude of harmonic and irregular cyclic loading and loading rate on the cyclic stress-strain behavior and residual deformation accumulation of the samples. Based on the test results, concepts of Masing's rule are modified to model the stress-strain behavior of the material during the cyclic simple shear.

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The evidence of earth structure damages due to earthquakes and the need for study on relatively dense and unsaturated earth structures such as river dikes and road embankments motivated this research work. A series of shake table tests were performed in order to provide the needed experimental data for this study. During the experimental program, two types of soils, i.e. Toyoura sand and a sample from natural deposits of sandy material were used. A rectangular laminar box was mounted over a rigidly fabricated slope and subjected to horizontal shaking. Sweep tests and Swedish sounding tests were also performed in order to measure the material properties. The results of these tests show the effect of different parameters such as amplitude of cyclic loading, number of cycles, initial static shear stress, confining pressure and shear stress history (loading-unloading, over-consolidation and creep time) on shear deformation of unsaturated slopes during and after cyclic loading. The outcomes of the test results are used as a basis to develop a new single degree of freedom model of mass and nonlinear spring over a slope. Stress-strain behavior of the soil is defined as a nonlinear stiffness function of a spring. Time dependent input acceleration is considered as the base excitation and the induced inertia force is superimposed on the initial static shear stress. The numerical solution of the equation of motion of the model predicted the residual deformation of the system at the end of shaking. Several cases were analyzed and compared with the test results.

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The reconnaissance results and a preliminary analysis of soil liquefactions, which occurred during the Chi-Chi earthquake are presented. The study focuses on: (1) the characteristics of ground motion; (2) major liquefaction locations and characteristics of the soil at these locations; (3) liquefaction phenomena and damage; (4) damage to buildings and restoration of this damage; and (5) liquefaction analysis by using various empirical SPT-N methods commonly used in Taiwan. These results should provide extensive documentation of liquefaction that was observed during the Chi-Chi earthquake.

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A series of tilting tests and shaking table tests was conducted with a geotechnical centrifuge apparatus to determine the dynamic active earth pressure of cement treated clay ground by investigating the seismic stability of cement treated clay ground having an unsupported vertical slope. In the tilting tests, the horizontal seismic inertia force was simulated statically by tilting the whole model ground in accordance with the horizontal seismic coefficient k_h. The tilted model ground was subjected to monotonically increasing centrifugal acceleration until it failed due to the enhanced selfweight. In the shaking table tests, on the other hand, the model ground was subjected to dynamic excitations under a high centrifugal acceleration field. The magnitude of the dynamic excitation was increased stepwise until the model ground failed. It was found from both test results that the model ground failed with a combination of a vertical tensile crack and a straight shear failure plane, which was much different from the failure phenomenon of sandy or clayey ground. It was also found that the conventional pseudo-static limit equilibrium method such as Mononobe-Okabe's earth pressure theory was not able to evaluate the critical height and the failure zone of the model ground accurately. Based on the model test results, a modified pseudo-static limit equilibrium method incorporating the tensile force along the crack plane was proposed in this study. In the method, the critical height and the failure zone of the cement treated ground were influenced by the strength ratio of the tensile strength to the compressive strength of the treated soil. It was found that the proposed method with a suitable strength ratio was able to predict the tilting test results with fairly high accuracy. Also, the proposed method seemed to be able to reasonably explain the shaking test results if the dynamic response acceleration of the model ground was carefully taken into account. Dynamic active earth pressure of cement treated clay ground was also discussed in which the importance of incorporating the failure pattern was emphasized.

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In Osaka Bay of Japan, a number of large-scale reclamations have been executed. The settlement due to those reclamations has been serious enough to provoke investigation. The compression of the stiff Pleistocene clay deposits has caused the half of this settlement, and it makes the quality control of the reclaimed lands very difficult. According to the geological history, the Pleistocene clays in Osaka Bay are thought to be normally consolidated aged clays that exhibit seemingly overconsolidated behavior due to the nature of quasi-overconsolidation. This makes the problem of settlement more complex. In this paper, characteristics of the long-term settlement of the reclaimed islands in Osaka Port are discussed based on the in-situ measured data. Long-term compression is found to occur even in the deeper Pleistocene clay layers, stress conditions of which remain close to or less than p_c. Discussion is extended to the applicability of conventional procedure in terms of elasto-viscoplastic finite element method to the long-term settlement measured in the Maishima Reclaimed Island in Osaka Port. The conventional procedure to assess the settlement is found to function well for normally consolidated clays, however, serious limitation exists for describing the timedependent behavior of the Pleistocene clays, in which stress states remain close to or less than p_c. A series of long-term consolidation tests is done to assess the property of the quasi-overconsolidated Pleistocene clays. Laboratory experimental results also provide remarkable time-dependent behavior even in the region less than p_c. It is found that the rational modeling for compression in the region of close to and less than p_c is indispensable to describe the actual time-dependent behavior of the quasi-overconsolidated Pleistocene clays in Osaka Bay.

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A new construction method, called "the preloaded and prestressed reinforced soil method", proposed in this paper, aims at making reinforced backfill structures very stiff and stable. To make the deformation of a reinforced backfill nearly elastic, sufficiently large preload is first applied by introducing tension into metallic tie rods that penetrate the reinforced backfill and are connected to top and bottom reaction blocks. High tensile force in the tie rods functions as prestress, increasing the confining pressure in the backfill and thus keeping the stiffness and shear strength of the backfill soil sufficiently high. In 1996, in northern Kyushu, Japan, a prototype pier of preloaded and prestressed geogrid reinforced backfill was constructed for the first time to support a pair of simple beam girders for a temporary railway bridge. An abutment of geogrid-reinforced soil retaining wall, which was neither preloaded nor prestressed, was also constructed for the same bridge by otherwise the same construction method. The behaviours of the pier and the abutment were measured during the construction and the service period of about four and a half years and subsequently full-scale loading tests were performed. It is shown that the geogrid-reinforced backfill pier became substantially stiffer against static and dynamic load by having been preloaded and being prestressed when compared to the geogridreihforced backfill abutment.

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This paper presents a new earth reinforcement method by soilbags ("Donow" in Japanese) and its applications to such earth reinforcements as reinforcement for ballast foundations under railway sleeper, reinforcement for soft building foundations and construction of retaining walls. The strength properties of soilbags are studied theoretically and verified through the biaxial compression tests on 2D model soilbags and the unconfined compression tests on real soilbags with various kinds of materials contained. It is found that the high strength of the soilbag is produced due to the tensile force of the bag when the soilbag is subjected to an external force and the bag expands. This reinforcing mechanism contains a "reversal idea" of making use of an external force, which is the "enemy" against the foundation. The earth reinforcement using soilbags has the advantages of improving greatly the bearing capacity for soft ground, resisting flexibly against deformation and being friendly to our environment with less noise during construction and no use of any chemical agents. It also has the effect of reducing traffic- or machine-induced vibration and preventing frost heave.

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