Tomoyoshi Nishimura

Evaluation of Effective Stress of an Unsaturated Soil Under Cyclic Loading

Earthquakes rapidly impose cyclic loading and soils respond in undrained conditions during seismic performance. Geotechnical earthquake engineering historically has focused extensively on evaluation of liquefaction resistance in sandy soils since deformations tend to be large when sandy soils experienced liquefaction. Recently, the role of unsaturated soil mechanics in geotechnical earthquake engineering is required for unsaturated soil structures subjected to cyclic loading or earthquake motion. The shear strength theory of unsaturated soils has been formulated in terms of two independent stress state variables (i.e., net normal stress and matric suction). This study conducted out undrained cyclic triaxial tests for compacted unsaturated soil. Both excess pore-water pressure and excess pore-air pressure were measured in unsaturated soil and described changing of matric suction during cyclic loading. The cyclic triaxial test was performed for saturated soil in order to compare with excess pore water pressure behaviour of unsaturated soil. This study focuses excess pore-water pressure as one component of effective stresses in unsaturated soil and saturated soil. The influence of matric suction on occurrence of excess pore-water pressure was appeared.

Measurement of Swelling Pressure for Bentonite under Relative Humidity Control

Several experimental producers including using suction control to interpret swelling pressure for compacted bentonite have become generally accepted in geo-environmental engineering. This study focuses on both soil-water characteristic curve and swelling pressure of compacted bentonite. Soil-water characteristic curve was measured using vapour pressure technique. Compacted bentonite having two different soil suctions were prepared for swelling pressure tests. This new swelling pressure testing apparatus was developed in order to measure swelling pressure in a constant relative humidity environment. The apparatus was mainly consisted of the triaxial chamber and relative humidity control circulation system. The total volume of compacted bentonite was maintained constant during absorption process. This study demonstrates the influence of soil suction on swelling pressure under permitted adsorbed water into the compacted bentonite. Change of soil suction influenced the maximum swelling pressure of compacted bentonite.

Accumulated Vertical Strain Without Confining Pressure for Compacted Bentonite Due to Hydration Effort

To use high dense compacted bentonite or bentonite-sand mixture material are accepted as engineered barriers for high level nuclear waste disposal waste. The bentonite as engineered barrier is initially in unsaturated condition before pore water coming from natural host rock. The bentonite subjected to thermo-hydro-mechanical actions by natural geological situation and the canisters of high-level radioactive disposal waste. This study conducted out the creep test for compacted bentonite, and the developed relative humidity circulation triaxial apparatus. Two different initial water contents were prepared to make the specimens at constant dry density. As parameter for this creep test, the stress ratio was defined using specified unconfined compressive strength. All specimens were applied without confining pressure. The vapor transfers due to changing of relative humidity induced some hydration efforts that were controlled using vapor pressure technique. Performing the creep test in this study is of couple hydro-mechanical and is applied to identification for two efforts (i.e. strain due to hydration and stain by deviator compression stress). Investigation of creep behavior that accumulated strains associated to couple thermo-mechanical actions are measured at constant deviator compression stress.

Changing of Properties of Unsaturated Compacted Bentonite due to Hydration Effort

Radioactive waste disposal is important facility for human and environment in the world. Compacted bentonite in radioactive disposal engineer barrier design really experience hydration effort as decreasing of suction during long-time. Hydration effort develop macro-micro void structure in bentonite under deeply geological environment. The bentonite occurred uncertainly problems or translation in various experimental interaction boundary conditions such as thermal-hydration-chemical condition. To detect accumulation of deformation or changing of bentonite behaviour due to these processes is important that the modified experimental methods are required. In addition, to interpret laboratory experimental results combine to establish mathematical modelling in possible. The overall investigation or performance of the bentonite have contributed to represent the intrinsic properties of engineer barrier systems. This study focused on changing of properties of unsaturated compacted bentonite related to hydration effort due to increasing of relative humidity. Changing of some properties revealed to become instability or uncertainly problems in practice. Soil-water characteristic curve was measured with considering of various temperatures using vapor pressure technique. Swelling pressure and creep behaviour such as mechanical components were described with hydration effort.

Measurement of Vertical Strain of Compacted Bentonite Subjected to Hydration Effort on Creep Test

Compacted bentonite which is one component of structure framework of barrier system. The hydro-thermal-mechanical properties of compacted bentonite have been investigated in the past decades in the area such as geoenvironmental engineering (Olivella et al. Olivella et al. 1996). This study conducted creep test for bentonite using a modified relative humidity circulation system. Various vertical stresses were applied on basis of unconfined compressive strength that only relative humidity of 98% as hydration effort influenced to creep behaviour in deformation. Thus, either expansion or shrinkage in vertical direction was measured, and all specimens approached to be completely destruction due to apply the combined effort of mechanical effect and hydration effect.