Failure Mechanism And Influencing Factors Of Tunnel Portal Section In Rock Slopes With Weak Interlayers Under High Intensity Earthquake

China has a vast mountainous area due to the collision between crustal plates.There are many fault zones in China that create earthquakes frequently.The Tangshan earthquake in1976 and the Wenchuan earthquake in 2008 caused a great loss.Many bridges and tunnels were damaged during the earthquakes.The stability of the slope of the tunnel entrance has a great impact on the operation of the tunnel.Many seismic damage surveys show that the earthquake-induced landslide is one of the main types of seismic geologic hazard.It is necessary to make a full and scientific analysis on the influencing factors and the failure mechanisms of the slopes of tunnel entrances.To select a reasonable stability analysis method,it is necessary to determine the failure mechanism of rock mass when the slope is unstable.Through the study of the commonly used stability analysis methods of slopes.This dissertation develops a modified Mohr-Coulomb constitutive model to simulate the yield of rock in a continuous medium.A parameter calculation method is introduced to simulate the crack growth in a discontinuous medium.These two methods are developed to analyze the failure mechanisms of rock slopes with weak interlayers.Through the comparison between the numerical simulation models and the shaking table tests,the accuracies,and reliabilities of the modified model and the formula proposed by this dissertation.Both the shape and the internal structure of the slope are proven to be equally important to affect the aseismic abilities of the slope.The main work and conclusions are described as follows:(1)Based on previous studies,it can be found that the classical Mohr-Coulomb constitutive model tends to overestimate the tensile strength of rock materials in analysis.This dissertation introduces the uniaxial tensile strength to improve the tension reaction of the classic Mohr-Coulomb yield criterion.By comparing with the measured data from shaking table tests,it is found that the dynamic response simulation results of the modified MohrCoulomb model are more accurate than the results of the classical model.(2)When using extended finite element technology to analyze rock cracking,some material parameters are not easy to be measured directly.This dissertation introduces the uniaxial tensile strength,the energy release rate of type I and type II faults can be obtained by calculations.By comparing the crack locations and displacements of linings of both the model test and the extended finite element model,the energy release rate calculation equations are proven to be accurate.(3)Based on the improved Mohr-Coulomb model and extended finite element technique,the influence factors of rock slopes with weak interlayers with the horizontal and vertical earthquake are analyzed respectively.To simplify the model,it is assumed that the problem is a plane strain problem,and the slope height,gradient of slopes,and the thickness of the overlying layer are introduced as three possible factors.A total of four quantitative indices are introduced for the slope stability,which are the horizontal and vertical peak accelerations of the slope and the maximum principal stress of the overlying layer and the weak interlayer.At the same time,the extreme variance analysis is introduced to determine the influence of various factors on different indexes and to calculate the reliability of the conclusions.Through the tests,the main influencing factors of each performance index are changed according to the direction of excitation.At the same time,it can be found that the geometrical shape and internal structure of the slope of the tunnel entrance have an extremely close influence on the seismic behavior of the slope.The slope with a thin overlying stratum may be in an unfavorable condition,even the slope is low and gentle when struck by a horizontal earthquake.The high steep slope containing the thin overlying stratum can still be in a stable state under the vertical seismic force.The aseismic design of the tunnel entrance needs to consider both the size and the internal structure of the slopes to minimize the loss caused by earthquakes.