Monitoring of metro-tunnel freezing construction using fiber sensing technology

In order to grasp the temperature and strain fields of frozen soils during freezing construction of the metro-tunnel cross-passage, a method for real-time monitoring of freezing construction using fiber Bragg grating (FBG) sensing technology is proposed. Under the condition of low temperature and high humidity, FBG sensors have the advantages in terms of accuracy, sensitivity and stability in comparison with the traditional electrical sensors. In this connection, the design scheme and installation method of a novel freezing monitoring device integrated with FBG sensors is addressed. FBG sensors are deployed in freezing monitoring holes at both sides of the frozen soil wall of the cross passage to timely monitor the temperature and strain of the frozen soils in the process of freezing construction. The original temperature and strain monitoring data are analyzed based on the FBG sensing principle by use of the wavelet transform de-noising approach. The distribution pattern of the temperature and strain of the frozen soils in thickness and depth directions are recognized, which will provide a basis for further digesting the formation and expansion mechanism of frozen soil walls.