Stability Evaluation Of Toppling Rockmass On The Left Bank Of Ci Zhong Rebuild Bridge Of Lancang River Wunonglong Hydropower Station

Over the vast areas surrounding Qinghai-Tibet Plateau,located at southeast of China, there are deep valleies, steep topography, complex regional geological structures and frequent seismic activity. High and steep descentslope generally lead to large geological disasters and massive damage, especially in the regions where widely distributed sedimentary rocks or stratified rocks. Along bank slope, rockmass toppling and deformation are serious, resulting in lots disasters. The large toppling rockmass, locates at leftbank of Ci Zhong rebuild bridge of Wu Nonglong hydropower station in Lancang River, which poses a serious threat to the project safe construction and normal operation of Ci Zhong rebuild bridge. Therefore, it is very significant to develop the systematic study of the stability of toppling rockmass. In this paper, taking the large toppling rockmasslocatingat leftbank of Ci Zhong rebuild bridge as research object.On basis of geological conditions, the structure of rock slope, the space-time characteristics, characteristic of toppling rockmass deformation and failure was analysed with the innovated geological exploration technology, and the stability of large toppling rockmass was calculated by rigid limit equilibrium method and finite element method. The conclusions are as follows.(1) The distribution characteristics and range of toppling rockmass are obtained by the geological surveying mapping with different precisions. The border conditions of toppling rockmass is determined in three-dimensional form, by many integrated exploration, combining with inclined drilling hole inside imaging, surface seismic wave tests, borehole acoustic and elastic modulus tests.(2) With the technology of 45 ° angle inclined drilling, the borehole imaging and elastic modulus was tested, and undisturbed sample is collected during drilling process.And then the shear strength parameters are obtained with three-axis testa on the advanced MTS testing machine.(3) According to specifications of “Design Specification for Slope of Hydropower and Water Conservancy Project”(DLT5353-2006) and “Code for Hydropower Engineering Geological Investigation”(GB50287-2006), accurate layer measurement and survey method are adopted to divide the structure types of toppling rockmass and to determine the failure mode combined by structure planes. In according with the detailed division of rock structure, the structure of toppling rockmass at left bank of Rebuild Bridge is considered as Thin-interbeds, with characteristics of parallel bedding.(4) Based on borehole imaging technology,the data such as the tension position and aperture of toppling rockmass structure planes is obtained and then, the rockmass integrity variationas different depth, zoning rock and the tracking feature of cracking deformation of toppling rockmass to existing structure plane is studied, Meanwhile, the deformation direction, degree, type, depth and boundary conditions of large toppling rockmass is analysed on the basis of statistical data.(5) The causal mechanism of large toppling rockmass is analysed from many aspects including topography condition, rockmass structural characteristics, lithology change rule, fracture development situation and valley stress field; and the characteristics of toppling deformation in dipslope of anti-dumping rock layer is studied withnumerical analysis method and so it concluded that the main reason to form toppling rockmass is less flexural rigidity of Thin-interbeds rock, steep slope structure and highly steep topography.(6) The 3D geological model of large toppling rockmassis established and the stability of large toppling rockmass as a whole by selecting representative profiles and the reasonable strength parameters is calculated with the limit equilibrium method and finite element method. The results show that the toppling rockmass is stable or basically stable before and after impounding. The instability possibility of whole toppling rockmass is small, but it may occurs some slips in upstream area, and caused small collapse and surface block falling in excavated slope. Then the relative treatment measures and suggestions about the large toppling rockmass are put forward in this paper.