The Regional Stress Characteristics And Fault Activity Analysis Of The Mid-segment Of Tan-Lu Fault Belt

The hollow inclusion hole wall strain gauge based on the overcoring method is one of the most widely used insitu stress measuring instruments in engineering practice.The traditional hollow inclusion hole wall strain gauge is a hole wall strain measuring instrument based on a resistance strain sensor.There are still deficiencies such as few measurement points and being easily affected by environmental factors.In this paper,a new quasi-distributed fiber grating three-dimensional hole wall strain gauge is designed with reference to the structure of the traditional hollow inclusion hole wall strain gauge.The strain gauge uses a Fiber Bragg grating strain sensor as a strain measuring element,26 Fiber Bragg grating sensors are installed on the surface of the strain gauge according to the method of double-region quasi-distribution winding,which improves the accuracy of hole wall strain measurement and realizes quasi-distributed measurement of hole wall strain.This paper deduces the inversion equation of in-situ stress matching with the Fiber Bragg grating sensor layout scheme for the strain gauge,and designs a stress inversion optimization algorithm to reduce influence of abnormal value of strain measurement on the inversion stress results by eliminating the measurement data with abnormal residual absolute value multiple times.In addition,the linear relationship between the wavelength of the Fiber Bragg grating strain sensor and the sensor strain(within 1000με)was determined by the calibration test of the Fiber Bragg grating strain sensor and the elastic-optic coefficient required to convert the measured wavelength data to the strain value was inverted,Pα=0.4369.In order to analyze the feasibility of the strain gauge in the stress measurement,the study used Abaqus numerical simulation software to simulate the process of the geostress measurement by overcoming method in the in-situ stress environment whereσ1,~σ2,and~σ3 are 10MPa,5MPa,and 3MPa,respectively(Orientation of~σ1 is15°North-East in the horizontal plane).Extract the difference between the linear strain of each sensor’s arrangement position on the inner wall of the epoxy resin layer before and after the overcoring as the measured strain data of the Fiber Bragg grating sensor to invert stress,the stress inversion results show that the error of the three principal stresses and the azimuth of the maximum horizontal stress are all less than10%.Extract the difference between the linear strain of each sensor’s arrangement position on the inner wall of the epoxy resin layer before and after the overcoring as the measured strain data of the Fiber Bragg grating sensor to invert stress.In order to simulate the abnormal measurement value in the actual measurement,randomly select and adjust three strain measured values in the simulated strain data of the Fiber Bragg grating sensor and perform stress inversion through the stress inversion optimization algorithm.After two optimizations,the measured strains of the three adjusted strain measured values are all screened out,and the error of the inverse stress results is greatly reduced compared with that before optimization,the error of the three principal stresses and the azimuth of the maximum horizontal stress are all less than10%(Before optimization,error of~σ2 and~σ3 reached 46.47%and 50.58%).The simulation analysis results of stress measurement show that it is feasible and reliable to measure the in-situ stress using a quasi-distributed Fiber Bragg grating three-dimensional hole wall strain gauge.The stress inversion optimization algorithm can eliminate the influence of a small number of abnormal values of strain data on the inversion stress results.