| As the city continues to build the subway, many surface settlement problems become more and more serious because of subway construction. As an efficient shallow geophysical techniques, GPR has a good effect in the monitoring of surface settlement.It has special geological background in Cengdu Metro surface, which contains the water-rich sand and gravel stratum. After using shield construction, it is prone to surface subsidence by ground mechanical vibration, underground gravel loss, etc. GPR data from Chengdu Metro Line 1 shows that surface subsidence in Chengdu Metro has a time delay feature. It causes inconvenience to us in prevention and cure because this settlement has great unpredictability.As a mature electromagnetic numerical method, finite difference time domain (FDTD) method has high precision, accurate calculation and other characteristics. This paper build three numerical models, which are characterized with void ratio, void size and fluid medium, to simulate underground media feature in not dense band. We use FDTD method to simulate these three models and MATLAB to display graphics, remove direct waves and reunite color values. It obtains GPR field response characteristics under different parameters by analyzing and comparing ground penetrating radar field results of each group model.Because of the time delay feature in Chengdu Metro, it will take a long duration in GPR monitoring. It obtains variation of surface lag settlement in Chengdu Metro by analyzing and comparing results of numerical simulation and four phases GPR monitoring, according to establishing lag settlement monitoring program of GPR probing in Chengdu Metro Line 2. |
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