Yasuhiro OYAMA

Stress Control Test of Landslide Clay by Ring Shear Apparatus

Satoru OHTSUKA

Landslide is a phenomenon that slope slowly moves downward along slip line while keeping the shape of slope. There are two types of landslide, one is a virgin type failure and the other is a re-slide type failure. It is well known that shear strength of re-slide type landslide is much lower than that of virgin type one and it causes landslide repeatedly. However, the shear strength of landslide clay has been investigated by strain control test and it is still not clear due to different stress conditions between landslide and testing method. In this study, the stress control test to reduce a normal stress under constant shear stress condition was conducted to investigate the shear behaviors of both virgin and re-slide type landslides. The following conclusions were obtained.

1. The testing clay was sampled at the Okimi landslide. Stress control tests with ring shear apparatus were conducted for virgin and re-slide type failure conditions. Yield point was defined as the increase point in displacement velocity. It was found to locate on the shear strength line by strain control test for normally consolidated clay. It expresses the conventional shear strength line indicates the threshold to yield displacement.

2. In case of virgin type failure tests, shear displacement continuously increased with the decrease in normal stress. After passing the yield point shear displacement increased greatly and the specimen got to the failure point where shear stress decreased due to large displacement. Failure point was obtained to locate on the parallel position of shear strength line by strain control test.


3. In case of re-slide type failure test, the specimen was observed to fail brittle. There was almost no displacement before yield point and the specimen rapidly deformed. Yield and failure points were obtained coincident and located on the residual shear strength line by strain control test.

4. The effect of OCR at the initial state just before stress control test on shear strength was investigated. It corresponds to the effect of seasonal ground water change. Yield points were obtained constant for OCR in both virgin and re-slide type failure tests. Failure points were obtained greater with the increase in OCR for both testing conditions. In re-slide type failure test, the soil condition attained to the residual state and the consolidation effect was not expected prior to tests. Both stress and strain control tests were conducted to investigate the consolidation effects on failure points for re-slide type failure condition. The increase in failure points for OCR was obtained almost identical between both tests.