Feasibility study of applying SOFO optical fiber sensor to segment of shield tunnel

The feasibility of applying SOFO optical fiber sensor to the segment of shield tunnel is presented in this paper. A concrete beam with two optical fiber sensors is used to simulate the future application in segment of shield tunnel. The concrete deformed behavior during the curing process of steam and water is monitored. After the curing process is completed, a pair of optical fiber sensors are installed on the beam surface at the same distance away from neutral axis the same as the embedded optical fiber sensors. Then the cyclic two points load testing is conducted and two LVDT are used to monitor deflections at two sections. The deformation of surface sensor is smaller than the embedded sensor which can be shown the actual deformation of concrete beam. The theoretical contraction of sensor is very close to the embedded sensor, however, the difference between theoretical extension and sensor measurement is gradually enlarged with loading level. This reflects that the neutral axis moves upwards when concrete tensile stress exceeds strength. The results of LVDT cannot be used as a basis to evaluate the results from optical fiber sensors. According to experimental results, SOFO optical fiber sensor not only can survive in the harsh curing process, and but also can reliably monitor the concrete deformation. The accuracy of strain measurement by SOFO optical fiber sensor is also verified in two points load testing. Besides, the appropriate location at which the optical fiber sensor can be embedded in the segment is evaluated. The finite element method is used to analyze the shield tunnel in the soft clay, hard clay, loose sand as well as dense sand. The bending, hoop and shear stress distributions are evaluated for each soil condition. Based on the length of sensor and the results of numerical analyses, the best location for sensor is at the spline of segment, because the gradient of bending stress at spline is minimum. The application of SOFO optical fiber sensor in shield tunnel for long-term warning system is promising and is adopted in a shield tunneling project now. (C) 2008 Elsevier Ltd. All rights reserved.