Stability of shale at a slope

Abstract

The road-construction and nature create rock slopes along the Anticlinal Folded Belt in the eastern part of the Chittagong hill tracts, the northern margin of Surma Basin, and some eastern parts of Sylhet, Bangladesh. The total number of death due to slope failure in these regions is 622 from 2000 to 2017. Shale outcrops occur in several parts of the hilly region where rock fragments are on the roads at the slope base, and slope failure is common in these regions. This research aimed to find out the mechanism of formation of the rock fragments and the slope failure at a shale slope by water immersion, slake durability, direct shear, and triaxial compression tests. The rock is very unstable in the presence of water and, wet and dry cycles where water is absorbed along with the clay layer and breaks along with the layer. The gradient of the slaking index is 20 SI%/cycle in the 1st, 2.5 SI%/cycle in the 2nd, and 1.17 SI%/cycle in the 3rd stage which reveals that the rock degrades immediately upon exposure to wet and dry cycle. The ten shale pieces before the durability test became more than 300 pieces after the test. From the image analysis, it reflects that the average area decreased by 98% with increased aspect ratio, circularity, and roundness. The strength decreased to around 75% by wetting having low cohesion and internal friction of 105 kPa and 24.2° respectively. Effective countermeasures for slope failure should be investigated.