Mechanism on apparent dip sliding of oblique inclined bedding rockslide at Jiweishan, Chongqing, China

Because of the existence of a front stable rockmass barrier, the failure pattern of an oblique inclined bedding slope is conventionally recognized as a lateral rockfall/topple, and then a transformation into a rockfall accumulation secondary landslide. However, the Jiweishan rockslide, Wulong, Chongqing, which occurred on June 5, 2009, illustrates a new failure pattern of massive rock slope that rockmass rapidly slides along apparent dip, and then transforms into a long runout rock avalanche (fragment flow). This paper analyzes the mechanism of the new failure pattern which is most likely triggered by gravity, karstification, and the processes associated with mining activities. A simulation of the failure processes is shown, using the modeling software, FLAC3D. The results show that there are five principal conditions for an apparent dip slide associated with an oblique inclined bedding slope are necessary: (1) a block-fracture bedding structure. The rockmass is split into obvious smaller, distinct blocks with several groups of joints, (2) an inclined rockmass barrier. The sliding rockmass (i.e., the rockslide structure before movement) exists along a dip angle and is barricaded by an inclined stable bedrock area, and the subsequent sliding direction is deflected from a true dip angle to an apparent dip angle; (3) apparent dip exiting. The valley and cliff provide a free space for the apparent dip exiting. (4) Driving block sliding, which means the block has a push type of effect on the motion of the rockslide. The “toy bricks” rockmass is characterized by a long-term creeping that induces the shear strength reduction from peak to residual value along the bottom soft layer, and the sliding force is therefore increased. (5) The key block resistance and brittle failure. The pressure on the key block is increased by the driving rockmass and its strength decreases due to karstification, rainfall, and mining. The brittle failure of the karst zone between the key block and the lateral stable bedrock occurs instantaneously and is largely responsible for generating the catastrophic rockslide–rock avalanche. If there was not a pre-existing key block, the failure pattern of such the inclined bedding rockmass could be piecemeal disintegration or small-scale successive rockfall or topple. The recognition of catastrophic failure potential in such inclined bedding slopes requires careful search for not only structures dipping in the direction of movement, but also key block toe-constrained condition.