Studies in the past showed that the laboratory tests on dense sand specimens obtained by the conventional tube sampling method may exhibit lower liquefaction resistance than the field strength observed in situ. This phenomenon would result in excessive over-design of important infrastructures on such dense sandy ground. Therefore, in this study, a series of cyclic undrained hollow cylindrical torsional shear tests were conducted on sand specimens that were prepared by air-pluviation method with high initial relative density (around 80%), with several attempts to reach high liquefaction resistance. During these torsional shear tests, different kinds of pre-shear histories were applied before cyclic loading. The test results showed that specimens with pre-shear using small strain amplitude could exhibit significantly high liquefaction resistance, as compared to the one without the pre-shear.

Soil is often subjected to complex three-dimensional stress paths in geotechnical engineering. It is difficult to achieve such complex stress paths in laboratory tests. To this means, discrete element numerical simulations are carried out on cross-anisotropic granular material by using PFC3D in this study. The response of granular soil under three-dimensional seismic load is simulated and compared with results from corresponding pure principal stress magnitude change or direction change scenarios, illustrating the importance of principal stress value and direction combination on the mechanical behavior of soil. Simulations of three kinds of idealized stress paths are then conducted and discussed. In the idealized stress path simulations, the deformation and micro-scale fabric evolution is observed to be most significant under pure direction change.

Liquefaction can be considered to occur in saturated sand widely under seismic load, which will cause serious disaster, including road damage, ground subsidence, cracking of houses. Thus, it is quite necessary to study the liquefaction characteristics of saturated sand. For this aim, the undrained simple shear test on saturated sand under cyclic loading were simulated by three-dimensional distinct element method (DEM), where the stress-strain relationship, excess pore pressure ratio, mechanical coordination number and contact normal direction were analyzed. The results show that the liquefaction of saturated sand is manifested by the accumulation of excess pore pressure ratio. In addition, the mechanical coordination number gradually reduces and the sample anisotropy slightly fluctuates before the saturated sand reaches initial liquefaction. When the specimen approaches initial liquefaction, the mechanical coordination number drops abruptly and the sample anisotropy obviously increases.

This paper presents a three-dimensional (3D) numerical simulation of bonded sand in direct shear tests using the distinct element method (DEM), by employing a 3D contact model for bonded sand considering the size of bond. Firstly, series of direct shear tests with different vertical pressure were simulated by DEM. Then, the macroscopic characteristics of sample were analyzed along with sand sample without cementation. Finally, the bonding effect on the mechanical response of sand were studied, and the bond breakage information were studied within the shear band. The results show that the bonded sand demonstrates higher peak and residual stress as well as dilatant behavior due to cementation, and intensive bond breakage and high local void ratio are found inside the shear band.

When a long-term consolidation test is conducted for a soft soil sample collected from a coastal area in Ryukyu Islands, Okinawa and Kagoshima Prefectures, Japan, a noticeable delayed settlement may occur during secondary consolidation, which sequentially occurs after primary consolidation. Such behavior is apparently similar to the “delayed consolidation” generally observed in long-term consolidation under a consolidation pressure between the overburden effective stress and the consolidation yield stress for slightly overconsolidated clayey soils. The “delayed consolidation” for clayey soils can be explained by the isotache concept that is derived from strain rate dependency of compression curves. However, because the soils collected from the coastal area in Ryukyu Islands are cohesionless and consisting of fine coral soils without viscosity, the isotache concept which is strongly related to viscosity should not be applicable. Therefore, the delayed settlement during the secondary consolidation is thought to be caused by factors other than viscosity. In this study, a series of long-term consolidation tests was conducted for reconstituted soil specimens with different grain-size distribution curves collected from Nakagusuku Bay Port in Okinawa. As a result, it was shown that delayed settlement during secondary consolidation for soft soils in the coastal areas can be explained by the effect of particle crushing.

Apparent preconsolidation pressure is an important parameter for soft rock. In this paper, investigated red bed soft rock was taken from Dingxi region, Gansu province, China. In order to obtain the apparent preconsolidation pressure of the soft rock, firstly, a new YS-1 high pressure oedometer was redesigned and used in the step loading confined compression experiments. Due to the soft rock’s low compressibility, the compressive curve was flat. Numerical mapping method was proposed considering curvature variation of the curve was not obvious. Secondly, the actual boundary condition which is the void ratio should monotonically decrease with the vertical pressure in the given domain was introduced. The existing 7 mathematical models were compared using the new defined conditions. The results showed that the Harris model was in good agreement with the compressive experimental results. Finally, the apparent preconsolidation pressure of the soft rock was calculated using the equations established in this research.