| The newly built Gong Jue tunnel of Sichuan-Tibet railway is a special tunnel in the Tibet Autonomous Region and one of several key research projects along the whole line.As it is located in the eastern part of Tibet,the mountains and rivers are densely distributed,the regional tectonic activity is strong,and the characteristics of high geostress are significant.The distribution characteristics of the in-situ stress field in the tunnel crossing area will directly affect the major technical issues such as the selection of route schemes and construction method planning.Due to the severe traffic and terrain conditions in this area,it is not suitable for carrying out all-round large-scale in-situ stress measurement.Therefore,based on the regional engineering geological conditions,this paper first analyzes the structural characteristics and evolution of the structural stress field in the area;based on on-site investigation and data collection,comprehensive analysis of the area of the tunnel site using methods such as focal mechanism solution and statistical analysis of measured in-situ stress data Stress field characteristics;based on the principle of multiple linear regression and measured data,a three-dimensional in-situ stress field simulation is carried out on the tunnel site and the tunnel axis.The simulation results are in good agreement with the comprehensive analysis results.Based on this,the stress field around the fault in the tunnel area and its influencing factors were studied in detail.The main conclusions and results of this paper are as follows:(1)Using the seismic source solution and the stress environment database and other method data,the direction of the structural stress field in the tunnel site area is mainly NE.Using the statistical analysis of the measured direction data of the multi-tunnel hydraulic fracturing method in the area,the dominant orientation of the maximum horizontal principal stress near the tunnel is N20°E～N40°E,which is in good agreement with the direction of the regional tectonic stress field.At the same time,this article linearly fits the maximum horizontal principal stress,minimum horizontal principal stress,vertical principal stress and burial depth in the five sets of measured data in the tunnel site area,and quantitatively analyzes the tunnel site area from the surface to the burial depth of 800m The distribution of in-situ stress in China shows that the measured stress distribution along the depth of the borehole is consistent with the overall stress distribution in Tibet.(2)The simulation results show that the maximum horizontal principal stress(σ_H)at the axis of Gongjue Tunnel reaches a maximum value of 32 MPa at 10.69 km extending inward at the tunnel entrance,and the minimum horizontal principal stress(σ_h)extends 15.49 km at the entrance of the tunnel.The maximum value is 9.11MPa,and the vertical principal stress(σ_v)reaches the maximum value of 30.0MPa when the tunnel entrance extends inwards at9.29km.The angle between the maximum horizontal principal stress direction and the tunnel axis direction is about 0°～44.9°.(3)Due to the influence of the physical and mechanical properties of the fault rock mass,the structural surface of the fault and the angle between the fault strike and stress,the magnitude and direction of the principal stress near the fault will change to varying degrees,and this change is mainly distributed around the fault.Within the range,as the observation point moves away from the fracture,the principal stress will gradually be consistent with the recovery of the regional stress field.The maximum horizontal principal stress and vertical principal stress have a slight stress concentration phenomenon at the lower wall of the fault,while the minimum horizontal principal stress is very obvious at the boundary of the lower wall.And there will be a large area of stress relief outside the stress concentration zone,about500m in length along the fault boundary,and then the minimum principal stress will gradually return to the regional average.(4)On the basis of the existing fault model,the influence law of Poisson’s ratio,elastic modulus,and the angle between the fracture trend and the maximum principal stressαof the fault rock mass on the distribution of the in-situ stress field near the fault is further studied.In both the elastic modulus and Poisson’s ratio,the elastic modulus has a greater influence on the stress field near the fractured structure area,and the range of the stress weakening zone increases with the decrease of the elastic modulus.At the same time,the maximum principal stress inside the fault increases as the angle between the fault strike and the regional principal stress increases,and the deflection angle of the surrounding principal stress direction decreases as the angle increases. |
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