The objective of the paper is to investigate the initiation of the Jiweishan landslide by simulating the field geological conditions around and inside a selected critical rock block and then to study its stability through 3-D discontinuum stress analysis. The study led to the following main conclusions: (i) The mining excavations that occurred underneath the investigated block at the landslide site, the friction angle of T2 discontinuities and sliding plane, and the dip angle of the sliding plane seem to be the most important factors with respect to the instability of the investigated block. (ii) It seems that the friction angle of T2 discontinuities and sliding plane need to drop to a value between 5° and 10° for the investigated block to be unstable. This means that rainfall and karstification have played important roles in reducing the said friction angle and contributed to the failure of the investigated block. (iii) The northern part of the investigated block (key block) had moved out first from the top of the mountain in a direction parallel to the strike of the north boundary discontinuity plane (T2) and had undergone shear failure on the T2 surface as well as on a weak shale layer (the sliding plane) and the remaining part of the investigated block (driving block) had moved in a NNE direction and had undergone shear failure on the sliding plane and separation from southern and western boundary discontinuity planes (T0 and T1) to fill the empty space created by the key block to initiate the Jiweishan landslide
Figures 1a and 1b show the Jiweishan mountain, located in Wulong county, Chongqing City in southwest region of China, before and after the slide, respectively . The Jiweishan mountain extends approximately NS. Its eastern flank is a 50-150 m cliff. The landslide occurred around 3:00 pm on June 5th, 2009. The source area of the rock slide was 720 m long, up to 125 m wide and involved approximately seven million cubic meters of rocks. Limestone blocks slid along a weak interlayer of bituminous and carbonaceous shale. The source mass collapsed from the upper part of the slope rapidly and crossed a 200 m wide and 50 m deep creek in front of it. The rock slide debris changed the course after impacting the opposite side of the creek, following the creek profile (Fig. 2), and traveled about 2.2 km within 3 minutes along the valley and finally formed a large accumulation zone with an average depth of 30 m. This is one of the most catastrophic rockslide events in China in recent years. It buried 12 houses and covered the entrance of an iron mine tunnel in which 27 miners were working at the time. This hazard caused 74 deaths and 8 injuries. Compared to other landslides, the Jiweishan landslide can be characterized by the following features: (i) higher traveling velocity, (ii) longer running distance, and (iii) more difficult forecasting [1, 2, 3].