Stability Analysis Of The High Slope On The Right Bank Of The Yellow River Behind The Cihaxia Hydroelectric Dam

The proposed Cihaxia Hydroelectric Plant project is across the Yellow River valley in the Hainan Tibetan Autonomous Prefecture, Qinghai Province. The concrete faced rockfill dam is designed to be 254 m high and the normal water level in the dam’s reservoir is 2980 m. With an installed capacity of 2.6 million k Wh, the plant is the largest hydroelectric power station above the Longyangxia Dam on the upper reaches of the Yellow River in terms of installed capacity. Situated on the right bank behind the dam, the studied high, steep rock slope contains a large fault dipping towards it at low to moderate angles, and its rear edge is seriously deformed by tension cracks. Any large-scale deformation and failure of the slope will affect the discharge of tail race and the release of water over the spillway at the plant.A field survey was carried out to acquire geological information, and evidence of deformation and failure of the slope on the right bank behind the dam. Based on the survey, the structural properties of rock mass, and types and mechanism of failure were studied. Tests were performed to measure the mechanical parameters of the slope’s controlling boundary conditions. The combination of different controlling boundary conditions was calculated and analyzed to determine the controlling boundary conditions. Last, the slope’s stability was assessed using the general limit equilibrium method and the strength reduction finite element method. The work undertaken and findings of the research are detailed below:(1)Lithology logs were made in many adits within the slope to record the distribution and proportions of different rock formations constituting the slope. The rock mass structures were identified according to the thicknesses of the rock formations and the rock mass integrity obtained by the wave velocity measurement done in the adits. Then a statistical analysis of the rock mass structures was performed, yielding the data about the structural surfaces of rock masses.(2)The properties of the slope’s structural surfaces were studied preliminarily using the data from field survey. It was found that the most developed faults were weak bedding faces dipping towards the slope’s lower section at large angles. The boundary layers between the heavily weathered zones and moderately weathered zones in the adits were determined using the wave velocity ratio indicator. Then the depths of the unloading zones of rock masses in the adits were estimated from a comprehensive analysis of the number and widths of unloading cracks and the wave velocity ratios.(3)The deformation and failure of the slope were classified into several types according to the evidence of deformation and failure. Then the spatial distribution of the large fault dipping towards the slope at low to moderate angles and its influence on the slope’s deformation and failure were analyzed to determine the bottom slip surface below the rock mass.(4)The failure mechanism of the slope was determined to be creep-tension crack failure according to the deformation characteristics of the walls of the F27 fault in the middle part of the slope, the rock mass integrity, and the tension cracks in the rear edge.(5)A strength test was conducted on the fault gouge on the bottom slip surface to obtain the mechanical parameters of the slope’s controlling boundary conditions. The strength parameters of the rock bridge were analyzed using the finite element method according to its deformation characteristics and rock mass class, yielding the mechanical parameters of the controlling boundary conditions. Given the characteristics of the F27 fault, the tension crack zone and the rock bridge, the combination of the controlling boundary conditions were calculated and analyzed and the controlling boundary conditions were determined. Then the controlling boundary conditions were used to construct a model. Based on the model, a slope stability analysis was performed under different working conditions by using the general limit equilibrium method and the strength reduction finite element method. The results showed that, under heavy rain conditions, the slope had poor stability(with consideration of the influence of moisture) and was likely to be destabilized.