Research On Mechanism Of Apparent Dip Slide From Oblique Inclined Thick Layered Rock Slope Failure

Oblique inclined thick layered rock slope is a monoclonal slope dipping slightly opposite to monoclina scarp in a small intersection angle and it covers well in Southwestern China which is known as a mountainous area including Chongqing, Hubei, Guizhou, Yunnan, Sichuan, et al. Under influence of natural and human activities, geological hazards from oblique inclined thick layered rock slope failure, such as landsldes, occur frequently and potential hazards exist extensively. The most common failure mechanism is collpes, so that there are rare literatures on apparent dip slide from oblique inclined thick layered rock slope failure. Even if there are, they are also about casuses research basing on geological survey and reduce it to plane-strain problems.This paper takes Jiweishan Landslide as example and reveals casuses of apparent dip slide from oblique inclined thick layered rock slope failure via detailed geological investigation and comprehensive research on rock mass strcutre mechanics, interms of understanding of macroscopic and microscopic geological phenomena. A3D computational model has been built and3DEC is brought in to simulate initial failure process involving gravitational creep, karstification and weak layer soften, mining goaf. Values of parameters adopted in numerical simulation refer to laboratory tests and empirical values. Centrifuge modeling tests have been carried out to investigate intiation and instantaneous failure processes and characteristics of Jiweishan Landslide and validate the unique rear-blocks-driving-key-block-fail-instantaneously failure mechanism of apparent dip slide from oblique inclined thick layered rock slope failure. According the unique failure mechanism and landslide characteristics, a mechanical model for3D limited equilibrium analysis is established. An analytical method for safety factor based on a key block theory is put forward and applied to stability analysis of Jiweishan Landslide and centrifuge model. In the end, statistic and contrastive analysis of classification methods for massive rock slope failure are conducted, so as synthesis and generalization of identification methoeds of rock slope failure mechanism. Then several recongnition characteristics for apparent dip slide from oblique inclined thick layered rock slope failure are proposed. During the research on mechanism of apparent dip slide from oblique inclined thick layered rock slope failure, methods of atablility analysis including numerical simulation, centrifuge modeling and limited equibrilium have been discussed. The research and results are meant to provide technical support for risk assessment and emergenct relief, so as to deepen and expand reaearch on massive rock slope failure. Several results and conclusions from the research are assembaled as follows:1. Confined by geologic structure and topography, failure mode of oblique inclined thick layered rock slope has the character of compound. The slope creeps to the dip initially as usually and is most likely to collapse because of unloading effect. Formed deposits may transform into landslides due to earthquake or rainfall. If the potential slide plane is reveal in the front, a planar rotational could occour when local resistance exists. Apparent dip slide from oblique inclined thick layered rock slope failure is in consequence of key block in the front failure which casuses slide in the rear and then apparent dip slide in the front.2. The reasons of apparent dip slide from oblique inclined thick layered rock slope failure are revealed in terms of topography, lithology, karst, mining activity and geological structure as follows:monocline provides free face for apparent dip slide; thick layered limestone sandwiching weak layers is a susceptible lithology; favorable original terrain for catchment developes into a relative weak belt in apparent dip due to karst; mining goafs generate cantilever effect which accelerates slope deformation and promotes the formation of lateral deep crack ending up to landslide boundary; fissured rock slope with joints and karst is discrete and easy to disintegrate during run-out.3. Centrifuge modeling tests shows that the crack strain gauges are damaged in consequence from back to front as centrifuge acceleration ascends. During the process, the driving blocks in the rear lose stability and crush on the key block in the front, then sudden displacement rise happens to the key block which indicates instantaneous apparent slip failure and triggers entire landslide. The results state that the initial failure is a progressive failure process from the rear to the front of the slope and from stable to instable. Apparent dip slide from oblique inclined thick layered rock slope failure has a character of blocks in the rear driving and the key block destabilized instantaneously.5. During creep process of numerical modeling, displacement of slope in the rear increases gradually while displacement of the key block is small. As strength of weak layer reduces, the key block has a sudden displacement rise and occurs apparent shear failure and triggers the entire landslide. It demonstrates that during the initial failure of apparent dip slide of oblique inclined thick layered rock slope, the key block palys a obvious role of control and resistance. In initial failure process, the blocks in the rear creep to the dip and drive on the key block which ends up with instantaneous apparent sheat failure. The strength reduction of weak layer and instantaneous failure of the key block are main reasons for landslide.5. According to failure mechanism of "blocks in the rear driving and the key block in the front resisting" and comparison with stability analysis in a whole, it is believed that the3D limited equilibrium analysis method based on the key block theory is reasonable and available after being applied toanalysis of centrfigue tests. During this method, the key block is confined by karst belt and weak layer while the drving blocks by lateral crack and weak layer. The concept is imbalance thrust force method and the safety factor of the key block represents the stability of the slope.6. Basing on geological investigation, numerical modeling, physical modeling and statistic analogic analysis, five reconginition chacteristics have been pointed out in terms with geologic structure, topographic and failure behavior which are thick layered fissured rock, stable resistance in the dip, free face for apparent dip slide, driving blocks in the rear, the key block instantaneous failure.