In order to understand the mechanism and conditions leading to failure of sandy slopes due to the infiltration of rainwater, constant shear stress drained triaxial tests were conducted on three sets of soil samples-normal sand, gravelly sand and silty sand-taken from natural slopes where large-scale landslides have occurred in the past. Water was infiltrated from the bottom of an initially unsaturated soil specimen under constant shear stress drained condition until failure occurred. Such a loading pattern simulated the stress path followed by a soil element on a potential failure plane on a slope subjected to rainwater infiltration. The effects of various parameters reflecting the initial condition, such as relative density, principal stress ratio, degree of saturation and infiltration rate, on the development of deformation during the infiltration process were investigated. The test results confirmed that the development of pore water pressure within the soil is the main reason for the failure of slopes during heavy rainfall. The results obtained can serve as guidelines in developing warning systems against impending rainfall-induced slope failures.

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Rainfall-induced slope failures often occur within a short period of time during or immediately after heavy rainfall. To analyze the mechanism and conditions leading to such failures, constant shear stress drained triaxial tests, which mimic the field stress path, were conducted on three sets of sandy samples obtained from sites of previous disastrous landslides. The effects of various parameters, such as initial relative density and principal stress ratio, on the temporal development of deformation were examined. Behavior after initiation of failure was significantly affected by the mode of deformation, i.e., whether compressive or dilative. Compressive soil failed rapidly due to increased pore-water pressure associated with the shearing process, while dilative soil exhibited slow failure due to loss in pore-water pressure during dilation that increased the shearing resistance of soil. Moreover, conventional triaxial drained and undrained tests were also performed to illustrate the difference in soil response under different stress paths. The results demonstrated that in analyzing the stability of slopes under rainwater infiltration, the use of strength parameters associated with failure initiation in constant shear stress drained tests may be more appropriate than those obtained from conventional triaxial tests.

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The aim of this paper is to evaluate the influence of the interaction between the piles of a group fixed in a rigid head on both its bearing capacity and the stiffness of the group. For this purpose, a parametric three-dimensional nonlinear numerical analysis was carried out for different pile group arrangements. The responses of the pile groups are compared to that of a single pile established from pile test results in conjunction with numerical assessment. The bearing capacity efficiency factor is then evaluated, while the stiffness efficiency factor is determined as a function of the deformation level. The influence of the number of piles, the spacing and the deformation level to the group response is discussed. Furthermore, the contribution of the piles constituting the group to the total group resistance is examined and its variability with the settlement level is established. Finally, a relationship is proposed allowing a reasonable prediction of the response of fixed pile groups in similar soil profile conditions.

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The strength properties of undisturbed Japanese, Korean and United Kingdom deposits and their remolded soils are examined through K₀ consolidated-undrained triaxial compression and extension tests by a new precision triaxial test apparatus (PTA) using small specimens 15 mm in diameter and 35 mm in height. The effect of sample disturbance on the coefficient of earth pressure at rest (K₀) under K₀ consolidation was quantitatively examined. The K₀ value can be explained by the equation, K₀ = 0.003 + 0.302I_p, under the normally consolidated region (NC). The K₀ values under the effective overburden pressure (σ'_VO) were (6∼15)% smaller than those of the NC. The rate of strength increase (c_u/p) under σ'_VO were in the range of 0.4 to 0.7 and those under the NC were 0.3 to 0.4. The effective angle of friction (φ') obtained from the strain rate of 1%/min was 10% smaller than that of 0.05%/min. The K₀ values for the undisturbed soils were 0.07 greater than those using Jaky's formula and the K₀ value can be explained by the equation, K₀ = 0.912-0.012φ'. Brooker and Ireland and Yamanouchi and Yasuhara's equations can explain the K₀ values for the remolded soils. The PTA effectively measured the engineering strength properties of natural soil deposits and their remolded samples. Therefore, it is a valuable tool in diagnosing soil samples and analyzing triaxial strength properties, thus leading to improvements in the accuracy of engineering design.

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An isotropic hardening elastoplastic model for soil, which can describe typical deformation and strength behavior of normally and over-consolidated soils in general stress conditions, is presented. This model can take into consideration the influence of density and/or confining pressure on the deformation and strength characteristics of soils. Further, it also takes into account the influence of intermediate principal stress on the deformation and strength of soil and the stress path dependency on the plastic flow, as did other previous versions of this model. For considering the intermediate principal stress on the deformation and strength, the concept of modified stress t_ij is employed in the same way as the previous models. With respect to the subloading surface concept, the previous models are revised and extended into one in which the influence of density and/or confining pressure on the deformation and strength of soil can be considered. Furthermore, the influence of stress path dependency of the direction of plastic flow is considered by dividing the plastic strain increment into two components - i.e., a component satisfying associated flow rule and a component of isotropic compression. In the present model, only one material parameter a for representing the influence of density and/or confining pressure is added to the parameters of the previous model, which are fundamentally the same as those of the Cam clay model. The validity of the present model is checked by monotonic and cyclic loading tests not only on normally and over-consolidated clay but also on loose and dense sands in three-dimensional stresses.

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Two-dimensional model tests of tunnel excavation and the corresponding numerical analyses were carried out. Numerical analyses were performed with the finite element method using the elastoplastic subloading t_ij model, which can simulate typical soil behavior. Model tests and numerical analyses were performed for three series of tunnel excavations. To investigate the influence of tunnel excavation on surface settlement and earth pressure surrounding a tunnel, both the model tests and numerical simulations were conducted considering the ground as green field. It is found that earth pressure decreases at the place of excavation, while increasing adjacent to the excavation due to an arching effect. The results confirm the validity of the constitutive model including the initial condition of the ground. To investigate the effects of building loads in tunnel excavation, model tests and numerical analyses were carried out as well. In this series, the initial dead load was applied adjacent to the tunnel. It is demonstrated that the building loads control surface settlements such that the maximum surface settlement does not always occur above the centerline of the tunnel, but at the position of the existing building. The deformation mechanism and the earth pressure during the tunnel excavation for the ground disturbed by the building loads are different from those for the greenfield situation. The results of this series validate the model regarding the boundary condition and initial state of the ground. In a third series of tests, two different excavation sequences were considered for a tunnel with a large cross-section. In the first case, the middle section was initially excavated and later two side-sections were excavated. In the other case, two side-sections were excavated before excavation of the middle section. The results of this series show that final surface settlements vary slightly with excavation sequences, whereas final earth pressures differ considerably for different excavation sequences. The results of this series prove that the model can consider construction sequences perfectly in finite element analyses. Numerical simulations agree not only qualitatively but also quantitatively with the model tests results for both deformations and earth pressures.

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Compacted soil is often employed in geotechnical constructions. A set of triaxial compression tests was executed on fully saturated specimens of a medium-fine compacted soil to investigate the influence of preparation water content on the stress-strain behaviour. Specimens were prepared by compacting at different water contents on the dry and wet sides of the optimum and around the optimum for different compaction energy levels. A state-of-the-art triaxial apparatus, which allows investigation into soil behaviour from very small strains up to failure, was used. It is shown that, as previous researches have indicated, compacting soil at different moulding water contents produced materials with different microstructures. The test results showed that the effect of soil microstructure was noticeable at small strains but it decreased with an increase in the strain and had little or no effect on the shear strength envelope in terms of effective stresses. Particularly, it is shown that, even after removing the effects of compacted dry density, the specimens compacted at the optimum water content presented not only the lowest isotropic compressibility, but also the highest initial stiffness and damping ratio at small strains. As the strain level increased, the stress-strain behaviour tended to be more uniquely controlled by the instantaneous dry density. It was also the case of the dilatancy characteristics, which controlled the pattern of effective stress path.

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A new three-dimensional stress-strain model of sand is proposed by using the concept of multiple shear mechanisms which previously has been developed in a two-dimensional manner. The original model exhibited excellent capability for simulating the principal stress axis rotation. By extending this approach with a new dilatancy model, the present model makes it possible to describe a general stress state in three-dimensional space with the whole six degrees-of-freedom of stress taken into account. Modelling of sand behaviours under general drained and undrained conditions was achieved by applying a stress-dilatancy relation to all individual shear mechanisms. It was demonstrated that the model predicts well soil behaviour in rather complex three-dimensional shear loadings as well as in more fundamental two-dimensional ones.

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Reclaimed ground of Kobe Port Island liquefied during the 1995 Hyogoken-Nambu Earthquake. That ground had been created by reclaiming decomposed granite called Masado. In this research, shear and liquefaction characteristics of Masado were examined by conducting monotonic and cyclic triaxial tests. A similar experiment was also conducted on Toyoura sand for comparison. Differences in shear and liquefaction characteristics between Masado and Toyoura sand were demonstrated on the basis of triaxial test results. Three types of Masado were artificially created to represent different physical properties of Masado obtained from Kobe Port Island. Effects of fine content, breakage of particles, and grain size distribution on liquefaction behaviour were discussed to explain the peculiarity of Masado in a loose state. Accurate stress-dilatancy relationships of Masado and Toyoura sand, which are necessary to make a numerical model, were also demonstrated.

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A case study is presented of an application of the cone penetration test for construction control of a seawall construction on soft ground improved by sand drain. Effective construction control of such a soil structure requires a systematic way to collect accurate information about the progress of consolidation and gain in soil strength. An empirical correlation is proposed concerning cone resistance measured by a cone penetration test and the undrained shear strength of the ground. The relationship between cone resistance and the undrained shear strength is not affected by the state of consolidation of the ground. Based on the empirical correlation found in this study, gain in undrained shear strength during construction can be accurately evaluated by the cone penetration test. This paper also demonstrates the usefulness of cone penetration test to monitor the degree of consolidation of the improved ground. Also estimated by the cone penetration test are aging effect, classification of clay layer and strength increment ratio in normally consolidated state. Aspects geotechnical behavior of the ground are discussed such as the process of strength increment, aging effect, sequence of various shear tests to examine change in strength and considerations on shear strength of improved ground after deformation. The results of this study clearly show that the cone penetration test is a very useful investigation method and has high applicability for construction control.

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