Seismic Response And Saftety Assessment Method Of Underground Openings In Rock Under Seismic Loading

In the strategy of Development of Western Areas in China, a lot of large scale hydraulic power plants will be consctructed in western areas which are seismically active zones with high earthquake intensity. The diversion structures are mainly constructed in the mountains beside the rivers. Therefore, how to evaluate the safety of the underground cavern groups when they are subjected to the earthquake is confronted with the engineerers and should have to be solved properly. Facing these problems, the author combines the key project sponsored by the National Science and Natural Funding, Dynamic response of the rock mass and the safety of the rock engineering under seismic loading, and relies on the the construction project of Xi Luo Du hydraulic power plant to study the seismic response of the underground openings in rock and the safety assessment method subjected to the earthquake.First of all, the diffraction of P wave by the circular hole in the infinite elastic medium is stuied. Both analytical and numerical solutions of the dynamic stress concentration fator done by explicit finite difference method are solved. Comparison of two solutions proves the feasibility of the explicit finite difference method to analyze the diffraction of solid waves and accordingly this justifies the application of the explicit finite difference method to study the seismic response of the underground openings in rock and the safety assessment method under seismic loading.Secondly, numerical models both in two and three dimensions of Xi Luo Du underground cavern groups are generated and the seismic response of the underground openings under Tianjin hospital seismic wave arising from Tangshan aftershock is analyzed by explicit finite difference method. It is shown that 2D model can be used to approximately predict the mechanical response of sections in the middle of caverns and 3D model has to be used to analyze the response of the sections near cavern portals. 2D model is much more efficient than 3D model in the generation of the numerical models, computation speed and the data processing, and the memory capacity for 2D model is much less than 3D model. Therefore, when the engineers analyze the seismic response of Xi Luo Du-like large scale underground openings, it will be better to generate 2D model instead of complicated 3D model at the very beginning to make rapid approximate prediction of for the sections in the middle of caverns to understand more about the dynamic response of the surrounding rock mass of the underground openings before 3D model is generated to predict more precise mechanical behavior of the underground caverns.Subsequently, considering the long length of Xi Luo Du underground openings which might indicate that the spatially non-unform seimic ground motion input should be taken into consideration, the influence of the spatial coherence of the seismic loading on the seismic response of Xi Luo Du underground openings has bee investigated. Research shows that spatially non-uniform sesmic laoding results in the phase difference of seismic response along the axial length of the underground openings and has litte imact on the amplitude of the seismic response of Xi Luo Du-like underground openings whose axial length is at the range of 300m~400m. Thereby, when the seismic response of the similar underground openings is analyzed, the spatial coherence of the seismic loading can be reasonably neglected. This conclusion explains why Dowding, Rozen and Sharma, Judd applied PGA as the only indicator to assess the safety of the underground openings subjected to the earthquake.Based on the characteristic of the seismic response of the underground openings in rock, the concept of the representative value of the dynamic stress concentration is proposed which could rationaly reflect the additional dynamic effect on the surrronding rock mass of the underground openings resulting from seismic loading. The representative value is thereby applied as the primary characteristic parameter and the parameter study of the influencing factors on the representative value is carried out. Parameter study shows that representative value appears approximate linear relationship to the accelearton amplitude of the seismic wave, the overburden depth of the underground opeing, rock rate, liner thickness and concrete grade. Based on this understanding, formulas of seismic safety assessment have been developed for both lined and unlined underground openings.Finally, according to the seismic safety assessment formulas, a simple seismic safety assessment method with good universality is developed. The proposed assessment method is therewith applied to evaluate the safety of Xi Luo Du underground cavern groups subjected to Tianjin hospital seismic wave arising from Tangshan aftershock. It is demonstrated that the proposed method draws similar conclusions to Dowding-Rozen and Sharma-Judd's methods. It also reveals that this method gives a relatively conservative assessment results.