Geological factors of rock collapse and shear strength of discontinuous plane on the cut slope of soft rock

Abstract

The gradient of a cut slope is generally decided based on the standard gradient. In the Neogene sedimentary soft rock area in the Tohoku District, however, there are many cases of incipient rock collapse with discontinuous plane.
We have measured the shear strength of various discontinuous planes using a newly devised method of simplified in-situ shear test and reported part of the results.
After investigating the geological factors of collapse and formation of discontinuous plane in the observation results of the rock collapse case, this paper verifies and inquires into the shear strength of discontinuous plane, which was obtained from the investigation results.
The results revealed that the angle of shear resistance on the bedding plane, which mainly forms Neogene in Tohoku District, shows smaller value than the value of the cut slope gradient of the soft rock in Neogene, which is used for design guidelines.
The above study is considered to be an important index to quantitatively plan the excavation of soft rock in Neogene System in Tohoku District.