Optimization Of Protective Layer Thickness For Blasting Excavation Of Deep Underground Powerhouse

The key for the development of abundant hydropower resources in southwestern China is the construction of large scaled underground powerhouse group caverns.The characteristics of large underground caverns are often described as large spans,large buried depths,high side walls and complex structures,etc.During the construction,the deformation and stress concentration of the surrounding rock will occur,and it even cause the deformation and destruction phenomena such as rock burst,surrounding rock damage and burst deformation.At present,underground powerhouse of hydropower station usually excavates layered and blocked,of which the excavation procedures for retaining side wall protective layers are most widely used.It is essential to select the reasonable protective layer thickness of the side wall,according to the increasing requirements for the deformation and stability control of surrounding rock during the underground cavern excavation.However the existing design of the protective layer thickness of the side wall is mainly based on experience,it is necessary to carry out further study on the determination and optimization of the protective layer reserved for the side wall in the excavation process of underground powerhouse under complex conditions.This paper concentrates on the excavation of the underground powerhouse.The calculation model of the excavation of underground powerhouse is built,under the consideration of both the explosion load and the excavation load transient unloading during construction.It discusses the effects of blasting excavation of powerhouse under excavation schemes with different protective layer thickness,and studies about the changes of surrounding rock disturbance under different in-situ stress and different excavation schemes.This research has certain significance to the choice and optimization of excavation program of underground powerhouse.Firstly,the excavation procedures of the underground powerhouse of the hydropower station are summarized and analyzed in detail.The factors affecting the thickness of the protective layer of the side wall,such as the section size,surrounding rock stability factors and in-situ stress level,are analyzed,in order to study the optimization of the excavation procedure for the reserved protective layer of side wall.Combined with the blasting excavation of the underground powerhouse of Baihetan Hydropower Station,the excavation schemes of different reserved protective layer thicknesses are numerically calculated to study the effects of different excavation schemes on the surrounding rock damage,deformation and dynamic response characteristics.According to the results,with the increase of the reserved protective layer thickness,the range of damage and dynamic response of surrounding rock both show a tendency of decreasing first and then increasing,and there is an optimal protective layer thickness corresponding to the in-situ stress level.For the underground powerhouse of Baihetan Hydropower Station,the reasonable protective layer thickness of the side wall is 2～4m.Considering the in-situ stress level on the blasting effect of the surrounding rock,the excavation of the underground powerhouse under different initial stress levels are simulated by using dynamic finite element software LS-DYNA.The results show that,under different initial in-situ stress conditions,with the increase of the thickness of the reserved protective layer,the damage and deformation of the surrounding rock firstly decrease and then increase.Based on the degree of surrounding rock disturbance,there is an optimal thickness of the protective layer under different initial stress conditions.Within a certain range of in-situ stress,the optimal protective layer thickness increases with the increase of initial in-situ stress;the reasonable protective layer is 2m when the ground stress is about 10 MPa,and when the in-situ stress increase to about 50 MPa,the optimal protective layer thickness is about 4～6m.Finally,based on the comparative analysis of typical practical engineering examples,the recommended values of protective layer thickness under different conditions are given,when the influence of plant size,in-situ stress level and rock mass quality are considered.It can provide a certain reference for the selection of the protective layer thickness of the side wall during the construction of the underground powerhouse.