Research and Application of an Inner Thrust Measurement System for Rock and Soil Masses based On OFDR

Abstract. For measuring internal stresses in rock and soil masses, specifically on unstable slopes, the fiber Bragg grating (FBG) and optical time-domain reflectometer (OTDR) methods based on fiber sensing technology have disadvantages such as low spatial resolution, low measurement accuracy, and non-distributed measurements. This paper presents a quasi-distributed thrust measurement system based on an optical frequency domain reflectometer (OFDR). First, the optical fiber stress sensing head was designed based on the micro-bending effect of the optical fiber, the cubic spline interpolation method was then used to compensate for the nonlinear effects of the OFDR stress sensing system, the compensation effects of different software methods were compared and analyzed simultaneously, which significantly improved the resolution and spatial-positioning capabilities of the OFDR sensing system, and error calibration was then performed through laboratory experiments of lateral stress. The test results showed that the OFDR sensing system achieved a spatial resolution of 20 cm using a 500 m test fiber (the resolution of an OTDR sensing system is generally approximately 1 m), the maximum measurement pressure can reach 1.059 MPa and the maximum relative error is 8.9 %. Finally, the field engineering application was carried out in the Chenjiagou landslide in Fengjie County, Chongqing City, Three Gorges Dam, China. The application results showed that the system can accurately locate six fiber optic micro-bending stress sensors installed within the landslide body over a range of 0 ~ 420 m and can obtain the pressure values of their lateral thrusts. This system is a quasi-distributed stress monitoring instrument that provides long measurement distances, high spatial resolutions, high sensitivities, and fast responses that can be used for unstable slopes, slope engineering, water conservation and hydropower dams, and tunnel chambers, and thus, has good engineering application prospects in the safety monitoring field.