Research On Stability Evaluation And Treatment Of Chenjiashanping Collapse In Sichuan Earthquake Disaster Area

5.12 Wenchuan earthquake initiated a large number of collapses secondary geological disasters, causing serious damage to mountain towns, villages and traffic facilities and bringing great difficulty to rescue and reconstruction of disaster areas. Because secondary geological disasters had hysteresis effect, their trend of development would be the main problems before the reconstruction. Earthquake caused a great number of loose deposits. In rainy season, rain infiltration made structural strength of loose mountain continue to decrease and collapses occurred frequently, threatening lives and property of people in disaster areas.The paper used Chenjiashanping collapse in Shifang city in Sichuan Province as the research object. Chenjiashanping collapse which occurred in 5.12 earthquake was the second batch of important geological disasters in Sichuan earthquake disaster areas. The collapse formed about 130,000 m3 accumulations and the upper part of the slope remained unstable rock masses, which were easy to produce rolling stones, collapses and slope debris flow threatening lives and property of 31 residents 108 people of shimen village two groups in rainstorm and earthquake conditions.On basis of collection of relevant information, through on-site investigations and surveys, the paper found out the engineering geological conditions, the basic characteristics of unstable rock masses and accumulations and took the required samples to do physical and mechanical tests. Formation conditions and processes, influencing factors and failure modes of collapse were deeply analyzed. Limit equilibrium methods including Sarma method, Janbu method and transfer coefficient method and the finite element method were used to evaluate the stability of dangerous rock masses. The stability coefficient and contour map of the stress and displacement were obtained. Mohr-Coulomb strength theory, Mises theory and judgment of the tensile stress were used to analyze the yield failure and tension crack shape characteristics of slope rock masses. Velocity of falling rock, trajectory, distance of collapse, high jump, distance of bounce and thickness of buffer layer were calculated. Based on the calculated parameters, considering technical feasibility, economic rationality and other considerations, regulation projects of Chenjiashanping collapse were designed. The main research results were as follows:1. The engineering geological conditions and the basic characteristics of unstable rock masses and accumulations were found out. The formation conditions and processes, influencing factors and failure modes of collapse were deeply analyzed. The formation processes included three stages: formative stage of potential collapse, peristaltic displacement stage of potential collapse and sudden collapse stage. The influencing factors were analyzed: terrain and physiognomy, characteristics of rock and soil, geological structure, earthquake, groundwater and rainfall, weathering and human engineering activities. The main failure mode of Chenjiashanping collapse was sliding-style collapse. Dumping-style collapse, rupture-style collapse and leap-style collapse could be seen.2. Back analysis of the shear strength parameters was carried out. Calculation parameters of stability were determined comprehensively. Limit equilibrium theory including Sarma method, Janbu method and transfer coefficient method was used to evaluate the stability of dangerous rock masses and the stability coefficient was obtained. The sensitivity of various factors of stability coefficient was compared. By calculating, W1, W2, W4, W5 dangerous rock masses were at stable state in three conditions. W3 dangerous rock masses were at stable state in natural condition and would be unstable in rainstorm and earthquake conditions. Many block stones on the upper part of dangerous rock masses and accumulations were at unstable state, which were easy to produce falling rocks in rainstorm and earthquake conditions, causing damage to people's lives and property.3. The finite element method was used to evaluate the stability of dangerous rock masses and analyze the slope stress. The slope stress vectorgraph, contour maps of maximum and minimum principal stress, contour maps of shear stress and the maximum shear stress were obtained. The slope deformation was analyzed, and slope displacement vectorgraph, contour maps of horizontal displacement and vertical displacement were obtained. Mohr-Coulomb strength theory, Mises theory and judgment of the tensile stress were used to analyze the yield failure and tension crack shape characteristics of slope rock masses. Through judgment of Mohr-Coulomb strength theory and Mises theory, there was no yield damage area in the slope. By judgment of the tensile stress, tensile stress zone of the slope was found out. By finite element calculation, the stability coefficient was 1.437. The dangerous rock masses were stable. The stability coefficient calculated by Limit equilibrium theory was 1.485. Results calculated by finite element method and limit equilibrium method were basically consistent.4. By calculating, velocity of falling rock at the foot of slope was 20.3 ~27.1m/s. The maximum vertical deviation and the maximum horizontal deviation were 2.15m and 4.97m. After rolling stone impacted the road, its high jump and distance of bounce were calculated. The maximum high jump and distance of bounce were 2.21m and 17.59m. Considering calculation results of bounce and vault and according to the terrain of the foot, the minimum distance between retaining wall and the foot of slope was 5m and the height of retaining wall was 2.5 ~ 3.0m. Then thickness of buffer layer was calculated and the thickness of buffer layer was 1.2m.5. According to principles of technical feasibility and economic rationality, treatment of Chenjiashanping collapse was carried out. A-type and B-type retaining walls were set at the foot of the slope. The height and length of A-type retaining wall were 2.5m and 183m. The height and length of B-type retaining wall were 3.0m and 183m. Meanwhile, 0.6m×0.6m drains were set and the length was 66m. W3 dangerous rock masses were anchored. The anchoring force calculated was 2365kN/m. The width of unstable rock masses was 20m. Therefore, the total anchor force required was 47300kN. The anchor was composed of 22φj15.24 (7φs5.0) steel hinge line. The anchoring force was 3200kN. The angle of anchor was 15°and the diameter of drilling was 175mm. The length of anchor segment was 11.6m. 15 plum-shaped anchors were needed in 20m width range of the slope.