Abstract
The first paper of this series described the slips on the ground water surface in the homogene ous sand layers which can not be explained by the Mohr-Coulomb law. The second paper stated the interpretation of vane tests in sand. This third paper is to interpret the mechanism of the slips on the ground water surface from the in-situ shear strengths measured by vane tests.
From the vane tests and the examination of their results the followings have been found.
1) The safety factor at the ground water surface is usually smaller than that above the water surface, because of the existence of capillary suction.
2) The safety factor at the ground water surface can be smaller than that below the ground water surface, because the effectiveness of interlocking increases with depth in the compacted sand layer. The effectiveness of interlocking was proposed in this paper and tentatively termed here, it means the magnitude of how effectively sand particles are interlocked at the same void ratio of the same sand.
3) The shear strengths measured by vane tests in the model sand layers were those which the safety factor took its smallest value at the ground water surface
