Rock Planar Slide—A Case Study at Tien Yen–Mong Cai Expressway, Vietnam

This paper presents results of a study on sliding of a rock slope, which occurred at km 108 along the Tien Yen–Mong Cai expressway in Vietnam. Collapse of the rock slope caused the movement of large volume of rock deposit toward the expressway, covering half of the way, which can be classified as a planar slide type. The investigated rock slope is 16 m high and 200 m wide. Most of the slope is weathered jointed siltstone, which is covered by the 0.5–2.5 m thick topsoil layer. The in situ rock can be divided into two sublayers, which are the 9b-1 layer with RQD = 0–36% and the 9b-2 layer with RQD = 0–92%. Results by the stability analysis showed that the overall factor of safety of the slope is lower than 1.0, which indicated the instability of the slope. Besides, the countermeasures have been put forward, at which part of slope will be removed to create a slope angle of 45° and ground anchor system will be installed to retain the remaining slope. The overall factor of safety of slope after treatment was greater than 1.30.