Takashi KAWANO

Hirofumi TOYOTA

Fundamental study on strength estimation for unsaturated sandy soils

Shear failure is the most important failure mode in the geotechnical problems. Therefore, it is indispensable to understand shearing properties considering ground deformation problems including failures. The strength parameters of soils are important on the design of foundations or the stability of the ground. The strength parameters depend on soils, their state and the environment. Therefore, it is difficult to decide the soil parameters uniquely in those problems. Then, experiments were conducted using Toyoura sand or other soils to generalize the strength mechanism of unsaturated sandy soils. The aim of this research is that index, by which the strength parameters are easily estimated from the physical properties of soils, are established using the experimental results. In the research, sandy soils are mainly used because the saturation degree of the soils easily changes by lowering the underground water level. Materials used are Toyoura sand that is pure sand, DL-clay that is non-plastic silt, and the soil obtained form the Sanbongihara tunnel construction site.
The saturated and unsaturated triaxial tests, where suction is controlled by axis translation technique, were conducted to clarify the mechanism of strength increase due to drying.

The results obtained in the research are follows:
1. In stress-strain relationships, the more obvious peaks appeared in the case of larger density and larger matric suction, and the strengths showed the softening behaviour.
2. In volumetric strain, larger dilatancy appeared in the case of larger density and larger matric suction.
3. The inclination of the failure lines between the saturated and the unsaturated sands, which have the same matric suction, almost became parallel, when the densities were equal.
4. Cohesions of unsaturated soils were increased with an increase of fine contents, density and matric suction.
5. Cohesions increased with matric suction by the angle of an internal friction angle of saturated soils when matric suction is smaller than AEV (Air Entry Value). When matric suction is larger than AEV, cohesions increased gradually with matric suction. The shape of this curve resembles the grading curve or the soil-water characteristic curve.

From the above-mentioned result, when AEV and internal friction angle of the soil are obtained, cohesion can be roughly estimated without unsaturated triaxial tests.

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