Deformation Characteristics of a Saturated Cohesive Soil Subjected to Increase in Pore Pressure

Abstract

Landslides resulting from rain and melting snow are generally known to be caused by an increase in the ground water level within the slopes. In these cases, an element on a slip surface before failure is subjected to the increase in pore water pressure, and deforms under a drained condition. It is very important, therefore, to estimate the drained deformation characteristics of clay during this increase in pore pressure. A drained test of cohesive soils, however, requires a long-term period to complete, and the drained deformation of cohesive soils during the decrease in mean effective stress has never been made clear. In this paper, deformation characteristics of a saturated cohesive soil subjected to the increase in pore water pressure were investigated and were compared with its undrained deformation characteristics. State points (p', q) at which a given value of equivalent shear modulus Ge(= Δη / Δεs, where εs is shear strain and η is stress ratio, q / p') is the same locate in the same very narrow zone for both of the two tests stated above. Any zone for an other Ge value locates in parallel with the other zones. Therefore, the Ge-value at a given stress state is independent of stress paths. Stress points showing the same εv-value forms a contour line in the pore pressure increase tests, and the line can be uniquely defined for a given εv-value. The distribution pattern of these lines is similar to the effective stress paths obtained from the consolidated undrained tests on overconsolidated specimens with different OCR-values. As a result, drained deformation characteristics in the pore pressure increase tests may be estimated with little error from the results of rapid undrained shear tests. In the final, to investigate the deformation behavior under general stress conditions, the effects of direction of principal stress and stress induced anisotropy on the above deformation characteristics were examined using the triaxial extension and torsional shear test results, and using the results on anisotropically consolidated specimens.