Failure of a Cut Slope and Deterioration of Shear Strength due to Weathering

Abstract

In this paper, strength distribution of a cut-slope of stiff clay and strength deterioration leading to a failure are investigated. A slope with a gradient of 1:1 measuring 10m in height and 50m in width was cut for a new local road through hills constituted of stiff clay. Four months after cutting the slope, a 25m wide portion of the slope failed. On the site, two borehole tests for sampling and SPT were done. In-situ sounding tests and sampling by the bench cut method were carried out. From results of sounding tests at the site, the strength distribution of the clay in the slope is found from the surface of the original slope to the depth, which is considered to be traces of weathering evident during a long period of time. Then, five series of shear tests on the stiff clay, test series A to E, were carried out using the direct shear apparatus, which was improved by Mikasa (1965), in order to observe strength deterioration of stiff clay. The test conditions were programmed to simulate different degree of weathering conditions by controlling the water absorption into the samples. It was found that samples with a lager amount of water absorption showed lower shear strength. Therefore, it was concluded that deterioration of stiff clay at the site induced the slope failure. It should be noted that the most effective and useful method to simulate different degree of weathering is not by way of triaxial compression tests, but direct shear tests, and that this approach, which simulates weathering of stiff clay on samples in a shear box, follows the concept of 'Advanced Total Stress method'.