Study On The Equivalent Simulation Based On The Volume Convergence Method For Surrounding Rock Of Tunnels And Its Application

The stress and displacement of surrounding rock can’t be released immediately after excavation due to the spatial and time effects of the excavation face,the deformation of surrounding rock in the process of stress release will affect the mechanical properties of supporting structure,and the interaction between surrounding rock and support further affects the stability control of the tunnel.However,it is difficult to quantitatively determine the stress release after excavation in theory.Methods of reversed stress release and reducing the elastic modulus of rock mass in excavation area are adopted to realize stress release simulation.The former controls the stress release through a certain proportional coefficient,which results the difference between the stress release path of surrounding rock and practical engineering;the latter lacks an effective method to determine the elastic modulus reduction parameter.Therefore,in this paper,study on the method of reducing the elastic modulus of rock mass in excavation area and its theory and application were carried out.The coefficient of volume convergence is introduced to characterize the overall convergence of the tunnel.The overall convergence and deformation law of surrounding rock in the process of stress release were analyzed,and an equivalent simulation method of excavation unloading based on the volume convergence of surrounding rock was put forward.The dissertation mainly includes the following contents:(1)Based on the elastic theory of thick cylinder,the equivalent mechanical model of excavation unloading in deep buried circular tunnel was established.Then,analytical formulas of surrounding rock stress and displacement when reducing the elastic modulus were obtained,and its correctness was verified by a numerical example.(2)The volume convergence coefficient was used to represent the overall convergence and deformation.Then,the convergence law of surrounding rock in the stress release process was studied,and the effectiveness of using the equivalent circular method to analyze the unloading of non-circular tunnel excavation.The results show that the volume converges uniformly at the beginning of stress release and the accelerated convergence occurs when the stress release reaches a certain degree.The displacement release path is different at different points around the tunnel.Compared with the displacement release coefficient of a single point,the volume convergence coefficient proposed in this paper can better reflect the overall convergence of the tunnel section.(3)The numerical simulation study on the affecting factors of the convergence of surrounding rock was carried out,and the influence law of ground stress and lateral stress was simulated.The results show that the deformation of surrounding rock occurring at the later stage of stress release takes more proportion when the ground stress is higher.The influence of lateral pressure on the overall convergence of surrounding rock is small,and it mainly affects the deformation occurring at plastic stage.For the hard surrounding rock with mainly elastic deformation after excavation,there is an approximate linear relationship between the volume convergence and stress release.The accelerated deformation appears earlier when the degree of surrounding rock is lower.(4)The relationship between the volume convergence of surrounding rock under different geological conditions and the advance position of tunnel face was studied.The results show that the deformation release becomes more concentrated when the quality of surrounding rock is better,and the time reach to final convergence is shorter.For surrounding rock Ⅱ、Ⅲ and Ⅳ,the convergence deformation of surrounding rock after excavation occurs mainly in the range of R、2R and 3R behind the tunnel face.(5)In order to solve the problem that it is difficult to determine the elastic modulus reduction factor in the simulation process,an equivalent simulation method of excavation unloading based on the volume convergence of surrounding rock was put forward,and the process of determining elastic modulus reduction factor was discussed.It was studied that the transformation law of elastic modulus reduction factor when the support is set at a different distance behind the tunnel face,which takes a tunnel with surrounding rock Ⅱ as example.The results show that the elastic modulus reduction factor is recommended to be 90%-95% when the support is set in the range of 0.5R-1.0R behind the tunnel face,and 95%-97% when the support is set in the range of 1.0R-1.5R behind the tunnel face.(6)The excavation unloading simulation method proposed in this paper was applied to erdaoya tunnel project,the calculation results of initial lining stress are basically consistent with the monitoring data,which verifies the effectiveness of the method.