Measurement Of In-situ Stress By ASR Method And Analysis Of Stress Field Inversion In A Tunnel Of Sichuan-tibet Railway

A tunnel section of Sichuan-Tibet Railway is located at the intersection of two canyons,the terrain is complex,up to 5100 m above sea level.In order to accurately obtain the initial in-situ stress field of the complex terrain in the canyon region,by summarizing the in-situ stress measurement method and research status,the inelastic strain recovery method is used to measure the in-situ stress of four directional horizontal exploration holes.In order to increase the success rate of the experiment,the strain gauge is changed from the traditional horizontal sticking method to the vertical sticking method.It saves time for data collection and is more convenient for field experiment operation.The flexibility ratio of the rock was studied by double-lever uniaxial experiment in the laboratory,the relation equation between Poisson’s ratio and Poisson’s ratio is found.The debugging method of the influence coefficient of temperature difference is put forward,it is in good agreement with experimental results.Thus,the method of selecting the influence coefficient of core temperature difference which cannot be tested is made up.According to the principle of anelastic strain recovery method and viscoelastic theory,a fast estimation method of ASR based on the anelastic Poisson’s ratio is proposed,the calculated results were compared with those calculated by ASR based on the flexibility ratio and verified,The result error is within the allowable range.The calculation results of measured principal stress are analyzed and summarized.Finally,the average value of the ratio of horizontal and vertical principal stresses of the borehole is 1.56.It indicates that the horizontal principal stress is dominant in this region.By defining the ratio of horizontal principal stress of different horizontal holes at different depths,it is found that the horizontal principal stress at the canyon junction is greater than that in other areas,the horizontal small principal stress is larger than that in other areas when the buried depth is shallower,as the depth increases to 1000 meters,the horizontal small principal stress is smaller than in other regions,and the optimal direction of the principal in-situ stress is from NE30° to NE70°,it is consistent with the principal stress trace in Tibet,China.The finite element 3D model of the study area was established accurately through the advanced line,boundary load method was adopted,and give full consideration to the boundary conditions,combined with the measured data,the intersecting topography of the two canyons and the initial in-situ stress field in the middle section of the canyon are inverted.It can be found that there is a stress relaxation zone within 150 m close to the surface,roughly in the range of 150～450m is the stress transition zone,it gradually enters the primary stress zone after 450 meters,the in-situ stress value gradually increases with the increase of depth.Under the condition of the same buried depth,the dominant stress at the canyon junction is larger than that in other regions,there is an obvious stress concentration phenomenon under the surface of the valley floor,the inversion results show that the direction of stress field in this region is NE30°～NE60°.According to the inverse stress component of the tunnel location,the stress field distribution law is calculated,it provides a reference for the scientific design of the project.