Seepage-stress Coupling Analysis Of Hydraulic Tunnel And Statistical Prediction For Surrounding Rock Stability

With the rapid development of China's urbanization,the current hydropower industry has shown a new situation that hydraulic tunnel is playing an increasingly significant role in the pumped storage power station project.However,in the event of complicated geological conditions,the problems such as gushing water and instability of surrounding rock of hydraulic tunnel seriously threaten the construction safety and adversely affect the ecological environment.In the study of the stability of the surrounding rock of the hydraulic tunnel,the Biot consolidation theory is a classical seepage-stress coupling one which does not take into account the effect of the stress on the seepage,that is,the permeability does not change with the pore.Therefore,the influence of the stress field on the seepage field is analyzed by considering the damage value in the coupling equation.Thus the seepage-stress-damage coupling model of the hydraulic tunnel is established.At the same time,the influence of various parameters on the stability of the surrounding rock is analyzed by the statistical forecasting method,which provides a strong support for the design of the tunnel.The main contents of this paper are as follows:Based on the principle of effective stress of porous media,considering the dynamic evolution characteristics of permeability coefficient during the excavation of rock mass,the seepage-stress-damage coupling model of hydraulic tunnel is established.Utilizing the general finite element software ABAQUS platform,the secondary development of its subroutine USDFLD is carried on,and the calculation program of the damage coupling model of rock mass is compiled.The seepage-stress-damage coupling process of rock mass during excavation is simulated to study evolution characteristics of the seepage,the stress and the damage.It turns out that the surrounding rock yields and the damage zone develops due to the stress release caused by the excavation unloading effect during the construction of the hydraulic tunnel.This results in the change of the permeability coefficient accordingly.With time going by,the scale of damage area in surrounding rock enlarges,and the degree of damage enhances.The permeability coefficient of surrounding rock where damage develops continuously increases,and the seepage field redistributes.The stress field interacts with the seepage field and finally both reach an equilibrium.Compared the Biot theory with the seepage-stress-damage coupling,the distribution range of the plastic zone is wider;the plastic strain is larger;the shrinkage deformation is greater;the pore pressure is less;and the seepage velocity is higher.On the foundation of the seepage-stress-damage coupling model,the sensitive analysis of influence factors such as the tunnel buried depth,the initial permeability of the surrounding rock,deformation modulus and initial groundwater level is conducted.The maximum vertical shrinkage,the maximum horizontal shrinkage and the seepage flow are determined as characterization results.Furthermore,the main factors that affect the stability of surrounding rock is ordered by their degree of influence.The four parameters that affect the stability of the surrounding rock are sorted descendingly by the magnitude of the sensitivity:the deformation modulus of the surrounding rock,the groundwater level,the impermeability of the surrounding rock and the buried depth.According to the LIB-SVM and PLSR statistical analysis methods,the stability of the surrounding rock of the hydraulic tunnel is predicted and the numerical calculation results are applied to verify the accuracy of the model.The results show that the SVM and the PLSR method show high accuracy on the prediction of the stability of the surrounding rock.In the practical engineering,the explicit quadratic PLSR analysis model is recommended for regression analysis firstly.When the analysis accuracy can not meet the engineering requirements,the LIB-SVM model can be used as a supplemental method to modify the regression results.Finally,according to the regression analysis model,the design envelope diagram along the line of the tunnel is drawn,and the results can be easily applied to the preliminary design work of the initial tunnel design.