Application And Study On Combined Fiber Optical Sensor For Slope Monitoring

The danger of unstable slope is omnipresent. According to statistics of recent five years, over 170 thousand hazards out of slope unstability occurred in mainland China, leading to casualties of more than 5 thousand and direct economic loss of nearly RMB 16 billion yuan; while only over 2,500 of them were forecasted, accounting for only 1.4% of the total. Acquisition of mechanics information inside the geologic body in a distributed, real-time, and remote manner to conclude a prompt and accurate judgment of abnormal conditions of the rock and soil mass before hazards are the basic guarantee for effective landslide monitoring. In spite of their respective advantages, geophysical method, clinometers,"3S"technology, time domain reflectormetry technology (TDR), optic time domain reflectormetry technology (OTDR), Brillouin optical time domain reflectometry technology (BOTDR), and Brillouin optical time domain analysis technology (BOTDA), etc. are limitedly applied in landslide monitoring, for they fail to meet the requirements of low-cost auxiliary materials, distributed, real-time, and remote telemetry, high initial measurement accuracy, and judgment of landslide movement direction at the same time. Therefore, it is of utmost importance and urgency to explore a new method for slope stability monitoring.Based on optical fiber sensing theory and OTDR technology, and rid of constraints of existing methods, the author presents a combined optical fiber sensor simultaneously meeting the above requirements, conducts systematic researches on slope monitoring method based on the sensor, and establishes a combined optical fiber sensing & monitoring system. The main results are as follows:a. Curvature radiuses corresponding to different core diameter and transmissivity of the single mode fiber (SMF) are discussed based on the theory of ray transmission when the optical fiber is bended, a relation between the critical curvature radius and the wave length, of which the optical fiber microbending loss can be ignored, is established, and the optimal diameter for the optical fiber second coiling is determined to be not less than 200mm.b. Based on the spatial architecture theory of reinforced concrete beam, and according to the test program and results, the low-cost combined optical fiber sensor is improved and designed, whose bending and shearing resistance model principle analysis is conducted, finally contributing to the establishment of theoretical formulas of the bending resistance model at the linear and total cross-section disconnection phases as well as optical fiber stretching of the simple shear model.c. Pulling and shearing resistance tests of bare optical fibers, as well as determination tests on shearing resistance of indented cased optical fiber and bending losses of circle-type and optical fiber sensors with bowknot, are conducted, an assumption of replacing straight optical fiber with optical fiber with bowknot is promoted, and the better effect of the sensor is verified through bending test of the combined optical fiber sensors of different cross sectional dimensions with the base materials of ordinary polyvinyl chloride (PVC) plastic plate and expansile polyester ethylene (EPS) cystosepiment.d. Through bending, shearing, and simple shear tests on the combined optical fiber sensor of different cross sectional dimensions with the base materials of ordinary PVC plastic plate and EPS cystosepiment, where grout with cement plaster and concrete of different strength as the agents are employed, it is found that the sensor with EPS as the base material shows better performance; the vertical load point displacement, as well as relation curve between optical fiber loss and slide of different sensors, are measured, and the optimal bowknot width in the test is provided to be 30-32mm.e. Two indoor and one in-situ simulation tests are conducted targeting the combined optical fiber sensors. The results show that the sensor is of high initial measurement accuracy, maximum sliding distance and dynamic range on the shearing deformation, as well as judgment of the load movement direction; the optical fiber sliding distance and the model movment show significant nonlinear relation, and the loss shows more significant one.f. In-situ direct shear test of the selected project site is conducted as per requirements in relevant codes, accompanied by natural homotaxial compression resistance and triaxial test of the rock sample collected in-situ, whereby c andΦvalues of the in-situ soil mass and rock sample are determined, that is c=18.1kPa,Φ=21.28°representative the soil mass and c=1.83MPa,Φ=37.5°representative the rock sample. The comprehensive internal friction angle is also determined according to the actual condition of the field destructiveness monitoring slope, and its value isΦ0=29.5°.g. Grouting material inside the boreholes is the link between the combined optical fiber sensor and the surrounding rock or soil mass and is one of the key factors influencing the monitoring effect. For effective monitoring of specific slope, the parameters, including the shear strength, friction angle, compacting factor, unit weight, and moisture content, etc. of the rock and soil mass shall be determined first as much as possible, then followed by the selection of matched borehole size, grouting material strength, type and size of base material of the combined optical fiber sensor, and the protection mode of optical fiber.h. A landslide sensing & monitoring system based on the combined optical fiber sensor is established for distributed, real-time, and remote telemetry. The Specification and suitable condition were also introduced. The system was utilized to the field destructive slope monitoring. Results showed that the system is feasible, and the sensor is of high initial measurement accuracy, maximum sliding distance and dynamic range, as well as judgment of the load movement direction. A long-term slope stability field monitoring system based on the combined optical fiber sensor is established for the selected in-situ slope, where the clinometer is employed for contrast monitoring.