Research On Formation Mechanism And Dynamic Response Of Counter-tilt Rock Slopes

The Guantan landslide, which is located in the right bank of the Jushui river at Tuanjie hydropower station in Jushui town of Anxian country in Sichuan province. The landslide integrally failed at 22 to 23 in May 12, 2008 and clogged the Jushui river, then formed the Anxian country Guantan quake lakes. The landslide was occurred 8 hours after the earthquake and is a typical landslide that reacted to after-earthquake effect. The landslide is happened in counter-tilt rock slope with weak rock base, which forms the boundary of dolomite rocks in the upper part and the mudstone in the lower part. So for the counter-tilt rock slope with rigid upper layers and weak lower layers, it is significance to research its formation mechanism and dynamic response reacted to after-earthquake effect. Based on the geological conditions, basic characteristics and slope structures characteristics, combined with the formation mechanism qualitative analysis by the characteristics of deformation damage of the slope, this paper simulated the failure process using the 3DEC numerical simulation software and comprehensive- ly and quantitatively analysed the formation mechanism, then obtained the dynamic response to earthquake and failure process under rain. All the research contents and achievements as follows:(1) Guantan landslide is located in counter-tilt slope with weak base and on the upper plate of the Jiangyou-Guanxian Fault. The orientation of the slope is parallel to the fault. Taking the branch of the Jiangyou-Guanxian fault F_3-1 as dividing line, the upper layer is T₂j+l dolomite with weak weathering and medium thickness seam and the lower layer is T1f+t interbed purplish red mudstone and muddy siltstone of which the rock mass is broken. The elevation of the leading edge of the landslide is 708m, and the posterior edge is 1180m. So the lacks is 472 meters. The length along the river is about 400m, and longitudinal length is about 800m. The slope angle is about 45 degree. the leading edge angle is about 65 degree due to excavation. The glide direction is 99 degree, and the area of plane is 18.2 million square meter. Its volume is up to 4.68 million m3, the storage capacity of the lake is about 1.45 million cubic meters, and the backwater is about 2 kilometers.(2) The signs of deformation and failure manifested as tension in the trailing edge and compression-dump in the heading edge. In the trailing edge, several tension cracks distributed. In the heading edge, the rock, exposed by the lake excavation, has the same sequence with bedrock, but the dip has significant change. This showed that the slope has the tendency toward the river valley.(3) The formation mechanism and development process could be separated into 3 phases: compression-dump deformation of the nature slope; potential landslide face formed but not completely transfixed under earthquake, the landslide face completely transfixed and the whole slope failed under rain.(4) The numerical simulation result showed that the dynamic response manifested vertical amplification and free face amplification to the seismic wave, and the vertical amplification effect is magnified more obvious. The amplifying effect of the acceleration is that horizontal acceleration amplification coefficient is greater than vertical acceleration amplification coefficient, it means the horizontal acceleration amplification effect is more obvious, speed amplification effect also have the same law.(5) The numerical simulation showed that, under the earthquake effect, the minimum principal stress appeared and the range is bigger than in nature condition. That means new tension fracture appeared under earthquake and old expanded deeply. Seeing the displacement under earthquake of the slope, the maximum displacement of the X-direction is 7.5cm, the Y-direction is 21cm, but the plastic zone of the slope is not completely transfixed. So the landslide is not happened. Under the rain effect, the simulated result showed that the plastic zone completely transfixed and the slope developed large-scale failure.Based on the research results, the survey and the analysis indicated that the slope structure with rigid upper layers and weak lower layers is the basis of the landslide, the strong tectonic activity is the important condition and the intensity ground shaking and strong rain is the root.