OPTIMAL DESIGN OF MINIATURE FBG SOIL PRESSURE SENSOR AND ITS APPLICATION TO GEOTECHNICAL MODEL TEST

Soil pressure is an important parameter in geological engineering and related model test applications. Although many currently soil pressure sensors can meet most of the requirements for soil pressure monitoring of practical engineering, limitations still exist in measuring soil pressure for geological model test, especially for those model test containing underground water. Fiber Bragg Grating(FBG) is small in size, easy to encapsulate into sensor, waterproof, non-corrosive, and resistive to Electric-Magnetic Interference(EMI).It is a promising sensing element for monitoring key parameters of geological model tests. In this paper, a miniature FBG soil pressure sensor is customized for geological model test to counter the issues mentioned above.It is developed utilizing short-gauge FBG.Finite Element Simulation is adopted for the optimization of sensor's performance. Some specifications including size of diagram and the thickness-diameter ratio of the sensor are designed based on the simulation result so as to get better sensitivity and reliability. The simulating results indicate that a cylinder flat diagram with the diameter of 20mm and the thickness of 0.2mm can provide higher sensitivity for the sensor and minimize the FBG spectrum chirp induced by non-uniform of strain distribution. Moreover, the sensor with the thickness-diameter ratio of 0.16(H/D(4mm/25mm)) can contribute to less interfere to the medium to be measured. It will provide more reliable soil pressure measurement results. To verify the performance of designed sensor, both pressure calibration experiments and temperature experiments are conducted. Air pressure calibration system consisting high accuracy digital pressure gauge and air pressure pump is used to calibrate the sensor in the pressure range of 0~1MPa and to verify the repeatability and consistency. The results demonstrate that the designed sensor exhibits desirable repeatability and consistency. The temperature experiments show that although temperature sensitivity of the designed soil pressure sensor is slightly higher than the bared FBG,the sensor's linearity is good and the temperature induced wavelength variation can be compensated with extra strain-independent FBG sensor. Finally, the designed soil pressure sensors are utilized in a subsea tunnel model test to monitor the stress of surrounding rock during the tunnel excavation. Stress measured by the sensors fluctuates slightly at the beginning of excavation, and then decreases rapidly when the tunnel face is passing the stress monitoring section. After the tunnel face passes the monitoring section, the stress stabilizes gradually. The tendency indicates that the tunnel becomes instable rapidly due to the excavation and then gradually stabilizes. The variation of stress during the whole process is traced by miniature FBG soil pressure sensor, which verifies its feasibility for model test.