Stability Study On The Right Bank Dam Crest Slope Of Dagangshan Hydropower Station In Daduhe River

With large-scale energy exploitation and other large projects in China, the engineering geological problem in high-steep slope stability has been widespread, and become one of today's major engineering geological topics. The high-steep slopes, encountered during the water resources exploitation in China's southwest and northwest regions, are mostly located in China's north-south trend of earthquake tectonic belt and its western area. The complexity of rock structure, and the lower part of valley slope to a higher stress have formed the complex rock mechanics environmental conditions of the high-steep slopes in this area. To build high dams and large reservoirs under these conditions, the engineering geological problem in high-steep slope stability is not only an important element in the feasibility study of the project, but also of great practical significance in design, construction and safety during operation. Moreover, it will have a direct impact on investment returns.Dagangshan River Hydropower Station is one of the large-scale hydropower projects recently developed on the mainstream of Daduhe River. It is located in the middle reaches of this river in western Sichuan Province. This hydropower station is planned to be concrete double-curvature arch dam and to be installed capacity of 2.6 million KW. The material composition and the structural characteristics of the right bank are very complex, and there is a long and large developing crevice belt at the bottom of the slope, whose stability has a direct impact on the safety of the dam. These factors are the top priorities of Dagangshan hydropower project. With the backing of the scientific research project on "The stability study on the high slope of Dagangshan River hydropower ", this paper conducts thorough and painstaking research on the stability of right bank top dam slope. The right bank top dam slope of Dagangshan Power Station, with an excavating height of 150 ~ 200 m, is high and steep, and there are two long and large fissure zones, XL-316 and XL9-15, developing at the deep slope. Moreover, as this region is of high earthquake intensity, the occurrence of earthquake will be a serious threat to the stability of the dam top slope and to the building and operation of the whole hydropower station. As a result, this will not only delay the project, but also bring about huge economic losses and serious social impacts. Therefore, the analysis of engineering geological conditions of the slope, the study on the cause mechanisms of the long and large crevice belt and the stability of the slope, the guidance to the optimized design and reasonable construction of the project, and the guarantee of slope safety during the construction and operation period are of important practical significance to the construction and normal operation of the hydropower station project.Based on field geological surveys and detailed prospecting data, this paper makes in-depth analysis on the environment geological conditions of the right bank's dam-crest slope, to probe into the basic characteristics of slope's rock-mass structure, and to study the cause mechanism of the fissure zone at the deep slope. This paper adopts quantitative calculation, combined with other methods as qualitative analysis and numerical simulation, to comprehensively evaluate the dam's stability. It also analyses those sensitive factors that affect the slope's stability, thereby offering some supporting suggestions. Main achievements are as follows:Generally speaking, the material composition and structural characteristics of the right bank dam crest slope are complex, and there are long and large fissure zones developing at the deep slope. So its stability has a direct impact on the security of the whole dam.(1)Located in the north sector of the Sichuan Yunnan meridional tectonic belts, Dagangshan hydropower station occupies the convergence position of multiple tectonic belts, with a minimum earthquake intensity of VIII degree, and relatively poor regional structure stability.(2) The dam crest slope at the right bank is high and steep, which is basically consisted of granite. As the grantie rock has strong unloading and rock-mass weathering, some places have exposed diabise dikes and granite aplite, resulting in poor rock mass integrity. Moreover, this hydropower station is located in high earthquake-intensity area. All these conditions go against the project security and stability of the slope. The steeper it becomes, the lower the slope's stability will be.(3) According to field investigations and analysis, the slope's deformation mode is preferred to be slip pressure-induced fracturing. As there is no secondary mud filling the cracks and the fractural plane is relatively fresh, it is inferred that the formation of fissure zone is of the remote past, and the slope is basically at a steady state.(4) The rock mass is of poor integrity resulting from a number of dykes and faults developing at the slope. Moreover, there are two fissure zones exposed at the deep slope with complex spatial distribution and large-scale dimension. After excavation, the crevice belt will combine with dikes or faults to form unstable blocks separately taking crevices XL9-15and XL316-1 as the potential bottom slip surface.(5) Once the dam crest slope is excavated, the front part will become steeper. If the earthquake acceleration is 0.251g, the excavated slope is in an unstable state. Thus the original design cannot meet the project's safety standards.(6) The earthquake acceleration and the connectivity rate of potential slip planes are the two major factors which have obvious impact on the slope stability. What's more, the connectivity rate and the slope stability are basically in a linear relationship.(7) The results of numerical simulation overall accord with the general distribution laws of the slope stress field. And they keep in line with the analysis results of limit equilibrium analysis method, that is, the excavation has an apparent influence on the slope stability. Though the existing supporting system helps to improve the slope stability to some extent, it still cannot completely eliminate the impact of excavation with the limit of the supporting conditions.