System Reliability Analysis Of Soft Rock Tunnels With Complex Support Structure Conditions

The ZC section of the tunnel of the return line interval of Dahedong Station of Qingdao Metro Line 4 is a typical soft rock tunnel.Taking this tunnel as an example,finite element method（FEM）simulation of tunnel construction was carried out to analyze the deformation and force characteristics of the support structure of the soft rock tunnel.The limit state function of the tunnel system is established,and the reliability analysis method of the tunnel system based on Monte-Carlo simulation（MCS）and surrogate model is applied,and the 2D FEM model and 3D FEM model of the soft rock tunnel are used as the basis for the reliability analysis of the tunnel system,respectively.The system failure probability(P_f,sys)of the tunnel and the single failure probability of the failure mode corresponding to each single limit state function were solved,and the correlation between each failure modes was analyzed by screening the major failure modes of the tunnel system.The effects of support structure parameters and support measures on the failure probability of the tunnel system and the failure probability of the major failure modes are also studied.The main contents are as follows.A tunnel system reliability analysis method based on MCS and surrogate model is proposed for the tunnel 2D FEM model.The sensitivity of Monte-Carlo sampling number and coefficients of variation for rock masses on the failure probability of tunnel system is conducted.The primary failure modes in tunnel system are identified by reanalyzing the failure samples.The simulation results demonstrate that the failure probability of a tunnel system within soft upper and hard lower surrounding rock mass is mainly attributed to the soft upper part of the surrounding rock.The coefficients of variation of the elastic modulus E₁ and the internal friction angleφ₁ of the pebble layer（soft upper part）have significant effect on the failure probability.The failure probability of tunnel system increases as the coefficients of variation of E₁ andφ₁ increase.Two primary failure modes are found to contribute to the tunnel system reliability.The effect of rock bolt length L and pipe-roof thickness H on tunnel system reliability and two primary failure modes as well are investigated.The simulation results indicate that both the enhancements in L and H tend to be more effective than the enhancement in either L or H if a small target failure probability of tunnel system is expected.The supporting structures design can be performed based on the potential sets of（L,H）satisfying target failure probability.Based on the tunnel 3D FEM simulation,the tunnel deformation and support structure force characteristics and the support effect of support structure（diaphragm wall,pipe shed）.Simulation results show that the settlement deformation of the tunnel is mainly manifested as arch settlement,the settlement values of the vault,left and right arch shoulders are:1.18cm,1.32cm,1.24cm,the settlement value of the left arch shoulder is the largest of the three;Surface settlement is characterized by the following:the surface settlement is the largest at the top of the tunnel centerline,and it expands to both sides with this position as the center point,and the settlement value becomes smaller and smaller,the maximum value is 5mm;The deformation of the tunnel working face is mainly characterized by extrusion deformation,and shows that the deformation in the center is large and small around,the maximum value appears in the center of the pilot tunnel II working face,which is 4.99cm;The center diaphragm effectively controls the settlement deformation of the tunnel,which ensures the stability of the surrounding rock and shares the support pressure of other support structures,which reduces the probability of failure of other support structures to a certain extent;the pipe-roof support has a better control effect on the deformation of the tunnel structure,and can effectively share the support pressure of rockbolts and other support structures.According to the deformation characteristics of the working face,g₇ is added as a limit state function of the stability of the working face.When g7 is not considered,the P_f,sys of the tunnel based on the 3D FEM model is smaller than that based on the 2D FEM model,and the failure probability of the original major failure mode f₅（failure of the tunnel system due to insufficient rockbolt support capacity）is greatly reduced,which is due to the different deformation laws of the 2D and 3D models.The failure modes of the tunnel system can be replaced by three major failure modes:f₂（failure of the tunnel system due to excessive settlement of the left arch shoulder）,f₇（failure of the tunnel system due to excessive extrusion deformation of the working face）and f₆（failure of the tunnel system due to insufficient support capacity of the center diaphragm）when g₇ is considered.The study of the effect of grouting reinforcement on the failure probability of tunnel system found that grouting reinforcement can effectively reduce the P_f,sys,and the failure probability of each major failure mode and the P_f,sys decreased significantly with the increase of reinforcement depth.