Research On 3-Dimensional Geo-stress Sensor Based On Optical Fiber Sensing Technology

Underground rock stress is an important parameter of rock mass properties in the earth,and it is one of the important basis for monitoring earthquake,landslide and volcanic activity.In order to realize the effective monitoring of the magnitude and direction of the spatial stress of the underground rock formation,a three-dimensional geostationary force sensor for monitoring the size and direction of underground space is designed and developed based on the principle of fiber grating sensing and plane stress state measurement.The stress sensor using an anisotropic layered composite material of carbon fiber as the carrier fiber grating package and make the strain sensing unit.According to the requirements of drilling monitoring,the whole structure of 3D inductive stress sensor adopts cylindrical structure design,and nine strain sensing units are composed of three sets of strain flowers.Each strain flowers is placed on a cylindrical monitoring probe.The three probes are assembled into a mechanical structure with a certain spatial angle and form a 3-dimensional geo-stress sensor for fiber gratings that monitor the magnitude and direction of the underground space.Each probe in the geo-stress sensor monitors the stress state in an orthogonal plane in the space,and synthesizes the maximum principal stress in the three orthogonal planes,and finally obtains the size and direction of the maximum principal stress in the space.The paper is based on the principle of fiber grating sensing and the principle of plane stress state measurement.The relationship between the maximum principal strain and the wavelength change is deduced by combining two theories,and the theoretical basis of the application of the optical fiber sensing technology to the geo-stress measurement is given.The indoor temperature calibration experiment and the stress loading experiment were carried out for the strain sensor unit,Temperature calibration test results: the average temperature coefficient in the range of 24.1 to 55°C is 57.3 pm/°C;Stress loading experiment results: The stress sensitivity in the range of 0 ~ 60 Mpa is 58 pm/MPa and the stress resolution is 0.172MPa;The linear fitting coefficient is 0.99986.The three-dimensional geo-stress sensor was embedded in the cement body and subjected to stress loading experiments.Sensor stress loading experimental results: the average relative error of the measured stress of S1 and S2 is 16.31% and 24.36%,respectively,and the average of the direction error is 1.89 °,2.52 °;the S3 probe of the absolute error of 0.0068 MPa.The mean value of the stress space angle error measured by the actual loading stress and the sensor is 1.24 °.