Development of simple experimental method for evaluation of mechanical properties for unsaturated soils

Superviser: Hirofumi TOYOTA

The soil mechanics has been systematically developed dealing with saturated soils because the mechanism of the saturated soils is simpler than that of unsaturated soils and the design of saturated soils means concept of safety side. However, strength estimation for unsaturated soils is important for the stability problems of slopes or earth structures due to rainfall and cutting face during tunnel excavation. Although testing method of unsaturated soils is being expected its technical progress, there are many problems in the test such as long testing time, expensive apparatus and complicated method. Therefore, development of simple testing method to measure strength parameters of unsaturated soils contributes to the practical matters.
Previous research has elucidated that an apparent cohesion increases with suction as saturated soils up to the air entry value. However, after changing into unsaturated soils, it is difficult to estimate the strength increase because of difference in soil type. The testing method to measure the strength parameters of unsaturated soils was examined using a conventional shearing test without special treatment in the study. Unsaturated specimens were prepared controlling their void ratio by compacting of soils, which have certain water content. Materials used are DL-clay that is non-plastic silt, the soil obtained form the Sanbongihara tunnel construction site and Yoneyama sandy silt that obtained from the Yoneyama nearby Kashiwazaki city in Niigata. The strength parameters obtained from the present research are compared with those from the unsaturated soil testing using pressure plate method.
The major findings are:
1) Concerning about non-plastic soils, the strength parameters of unsaturated soils can be obtained from the proposed simple direct shear test for unsaturated soils.
2) Under quasi-saturated specimens, it is necessary to load the total stress to the specimens considering the negative pore water pressure (suction) of the real ground.
3) Concerning about plastic soils, the strength parameters of unsaturated soils cannot be correctly obtained from the simple direct shear test.
As a result of the study, it is clarified that the strength parameters of unsaturated non-plastic soils are obtained from the simple direct shear test as the same as the unsaturated triaxial test. Although the proposed method is available under limited application, this is useful as the unsaturated testing method to measure the relationships between the degree of saturation and the apparent cohesion.

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