Study On Monitoring Technology And Application Of Surrounding Rock Stress Change In Cross Active Fault Tunnel

In tunnel engineering crossing active fault zone,stress changes will occur in surrounding rock after earthquake,thus affecting the safety of the tunnel engineering.Therefore,stress monitoring is particularly important.Taking Daliang Tunnel in Menyuan County as the engineering background,this paper realizes long-term monitoring of the three-dimensional stress changes in surrounding rock after earthquake based on Fiber Bragg Grating(FBG)sensor technology.Based on Fiber Bragg Grating(FBG)sensing technology and hollow inclusion stress measurement principle,the stress inversion principle for 3D stress change monitoring is derived and summarized,including the establishment of spatial coordinate system,the derivation of stress inversion equation and observed value equation,and the construction of the coordinate transformation relationship between the local coordinate system and the global coordinate system.In order to verify the feasibility of stress monitoring by a three-dimensional stress change monitoring sensor,a comparison experiment was carried out between stress measurement by a three-dimensional stress change monitoring sensor and stress measurement by a strain gauge.The results showed that axial strain measurement results and radial strain measurement results were basically consistent with each other using these two measurements.Meanwhile,a three-dimensional stress change verification experiment was also carried out.By comparing the stress value with the applied stress,the results show that the inversion stresses based on the stress inversion principle and measured data from FBG sensor have the same trend with the applied stressed.The relative error of the stress inversion results is 2.90%～22.60%,which meets the practical requirements.Ansys finite element numerical simulation software was used to simulate the stress measurement of three-dimensional stress change monitoring sensor,and the theoretical feasibility of FBG strain measurement and stress inversion method was tested again.The results show that The calculated results of the simulated stresses are basically consistent with the designed stresses,and the relative error is between 3.07%-15.55%.Adding epoxy resin layer would increase the relative error of stress inversion.Moreover,the relative error of stress inversion is affected by the elastic modulus and thickness of the FBG strain barrel.In order to reduce the error caused by the elastic modulus and thickness of the FBG strain barrel,the optimal selection range of elastic modulus of the FBG strain barrel is 10～20 GPa and the optimal thickness range of the FBG strain barrel is 4～7.5 mm.The three-dimensional stress monitoring sensor based on FBG sensing technology has been used to monitor the stress change of tunnel surrounding rock for nearly one year.The results show that the strain change is relatively stable in the past year,except that the maximum strain monitored by some sensors in DYLSPZ-3-1 is 311.1 με,and the maximum strain value in other boreholes is no more than 165 με.The results of stress inversion show that the stress change range of DYLSPZ-1 is-4.047～0.2562 MPa,the stress change range of DYLSPZ-2-3 is-6.328～0.5695 MPa,and the stress change range of DYLSPZ-3-1 is-11.42～0.589 MPa.The stress change range of DYLSPZ-4-1 is-2.265～0.953 MPa.The initial stress conditions of the study area were predicted based on the modified Sheorey model.Combined with the initial stress conditions and the monitored stress change data,the stress accumulation parameters of the study area were analyzed.The results show that the stress accumulation trend of borehole DYLSPZ-1 decreases.The other boreholes showed an increasing trend of stress accumulation degree with time,and the fault slip and seismic risk would increase with time.