Back Analysis Of Mechanical Parameters Of Hard Rock In Deep Cavern Based On Displacement And Fracture

In recent years,a large number of hydropower projects have been built in China,among which underground hydropower station has become the main form of hydropower project construction because of its unique advantages.Restricted by the distribution conditions of water resources in China,underground hydropower stations are mainly distributed in mountain and canyon areas,which are characterized by significant deep burial,high ground stress and high rock strength.There are obvious deformation and failure phenomena in the construction of underground power house,among which the rock mass failure is the most obvious phenomenon,such as the spalling,rockburst and collapse of surrounding rock.Therefore,in order to analyze the engineering stability,it is necessary to determine not only the deformation parameters of rock mass,but also the strength parameters of rock mass.However,the commonly used methods to determine the strength parameters of rock mass,such as experimental method and engineering analogy method,are quite different from the actual engineering;the developed displacement back analysis method is no longer applicable because the strength parameters in deep hard rock engineering are not sensitive to deformation information.Therefore,in view of the characteristics of deep hard rock engineering,this paper puts forward a comprehensive back analysis method of displacement and fracture,then discusses the application of this method in detail combined with an example of underground powerhouse of a hydropower station,and finally discusses the method and basis for determining the weight of multivariate information.Specific contents include:(1)According to the characteristics of deep hard rock engineering,the steps of comprehensive back analysis method of displacement and fracture are summarized.Firstly,the method to determine the deformation parameters and strength parameters to be inverted is introduced;Secondly,based on the sensitivity principle of back analysis method,the principle of selection of monitoring means,monitoring section,monitoring points/holes are explained;Thirdly,combined with the deformation and fracture characteristics of deep hard rock engineering,the method and basis for the establishment of numerical model and mechanical model are specified;Express the mapping relationship between rock mass parameters and excavation response with efficient relation model and select the back analysis algorithm suitable for multi parameter global optimization.Finally,the method of verifying back analysis results is described in detail.(2)The main works of back analysis of rock mass parameters of a deep underground powerhouse are as follows:the back analysis target is determined according to the in-situ deformation and failure conditions and single parameter sensitivity analysis;the layout of monitoring section and measuring points/measuring holes on the section are determined in combination with the engineering practice and numerical simulation,so as to obtain the back analysis information;Design orthogonal experiment and construct neural network learning sample by numerical simulation;Through cubic spline interpolation,the exact correspondence between the field measurement points and the numerical model measurement points is realized,and the numerical simulation results are extracted based on MATLAB programming.On this basis,genetic algorithm is used to optimize the initial weight and network structure of BP neural network,and a neural network model of mechanical parameters with displacement and relaxation depth of rock mass is established;the mechanical parameters of rock mass are obtained by back analysis using genetic algorithm;finally,the back analysis results are verified from the perspective of sample test and non sample test.(3)Based on the sensitivity ranking of the mechanical parameters of rock mass to the displacement and relaxation depth determined by the variance analysis of orthogonal experiment,and the convergence of the parameters with the evolution process under different weights,the influence rules of different inversion information on the inversion results of different parameters are discussed,which shows that it is necessary and reasonable to apply the back analysis method of comprehensive information of displacement and fracture in deep hard rock engineering.Finally,the method and basis of determining information weight in multivariate information back analysis are given.