Seismic Stability Influence Of Underground Cavern Complex By Adverse Geological Structure

In recent years, a number of large-scale hydropower stations are constructed in southwest China.Owing to the mountainous in this region, the underground cavern groups are mainly selected as the hydraulic structures. Sometimes even which is the only choice for the powerhouse of a giant hydropower station. These underground cavern groups are typical large span and high side wall. The geological condition is very complex. Meanwhile, southwest China is one of the most active seismic regions with intensive tectonic movements and seismic events, resulting in high requirements on seismic design for these hydropower projects. Thus, the seismic stability of large-scale underground cavern groups is a major scientific geotechnical issue to be addressed during design and construction of these giant hydropower stations. In this paper, based on the underground cavern group project of Baihetan hydropower station, the influence of the Bedding Fault Zone on the seismic stability of surrounding rock of underground caverns are discussed as follows:(1)The occurrence environment of underground powerhouse at the right bank of Baihetan hydropower station is researched from respects of formation lithology, rock mass structure, rock mass mechanics parameters and characteristics, stress field and regional seismic hazard near of field. According to the geological data, rock mechanics test results of related tunnel, corresponding laboratory tests, and code requirement, the shearing strength parameters of engineering rockmass are demonstrated. Based on measured results of geostress and the tectonic movement of Baihetan, the distribution rules of regional stress in underground powerhouse at the right bank of Baihetan hydropower station are studied, which provide the real geostress conditions for numerical simulation. According to the earthquake geology of dam site field, seismic intensity, the seismic mechanism, the rock and soil environmental factors and so on, KOBE acceleration time history and Wenchuan earthquake acceleration time history of Mianzhuqingping stations are chosen as the seismic input condition of numerical simulation.(2)The influence law of station, thickness and strength of the Bedding Fault Zone on the seismic stability of surrounding rock of underground caverns are studied: The distance of Bedding Fault Zone and underground caverns are smaller, the motion stability of underground caverns is disturbed greater. When the distance is bigger than a third of cavern span, the influence of Bedding Fault Zone can be neglected. With the increase of thickness of Bedding Fault Zone, surrounding rockmass plastic zone and the relative displacement of key points after the earthquake increased after the earthquake. The maximal displacements of surrounding rock are more near to Bedding Fault Zone. The directions of displacements are changed adversely. With the strength of the Bedding Fault Zone decreased, surrounding rockmass plastic zone and the relative displacement of key points after the earthquake increased. All these seriously affect the cavern seismic safety stability.(3)Taking the cavity distance of cavities as object, researching the seismic stability of cavities with interlayer faulted zone or not, the following conclusions are gained: The safe space between the main transformer chamber and tail lock chamber that is no interlayer faulted zone is about 1.1 times of cavity characteristic length, while the tail lock chamber and tailrace surge chamber is about 1.4 times of cavity characteristic length. If less than this distance, the stability of cavities is affected. The safe space between the main transformer chamber and tail lock chamber that crosses the interlayer faulted zone is larger than 1.4 times of cavity characteristic length, and which between tail lock chamber and tailrace surge chamber is larger than 2 times of cavity characteristic length. The interlayer faulted zone increases the safety cavity space of cavities, decreasing the seismic stability of cavities. So the static and dynamic security problems of cavities that cross the interlayer faulted zone should be paid special attention in the engineering.(4)A rapid evaluation method of seismic stability of large underground cavern group is studied. According to the above research the influence of the Bedding Fault Zone on the seismic stability of underground cavern group are concluded. Then, a fast evaluation method of the seismic stability of underground cavern group which is on the basis of station, thickness and strength of the Bedding Fault Zone, the cavern distance of underground cavern group is put forward. The seismic stability of underground cavern group profile at the right bank of Baihetan hydropower station is predicted by this method. The dynamic numerical simulation of underground cavern group profile at the right bank of Baihetan hydropower station is studied. Comparing the simulation results with the preliminary forecast analysis, the feasibility of this method is verified.