An experiment to detect the stratum disturbance range above the metro tunnel caused by its excavation in Chengdu City using rayleigh surface wave method

Along with the metro development, a series of disasters, such as the ground subsidence etc., occur during the excavation, which bring much trouble to the residents and the transportation. In connection with the specific geological structure of sand and gravel in Chengdu region, some loose area or voids occur above the tunnel under the influence of the construction techniques, the construction load and other factors, such as the tool change etc. The loose area or voids can cause the ground subsidence after a certain period. In the paper, an experiment survey of using Rayleigh surface wave method is adopted to effectively identify the degree and the range of the stratum disturbance (loose area or void) caused by tunnel excavation for the special geological conditions of Chengdu Metro. In the experiment, two different offsets are used during the data acquisition, and the acquisition data are mainly processed to achieve the following two types of profiles: one is the dispersion curve profile of the Rayleigh surface wave using conventional F-K (frequency-wave number) transformation, and the other is the common-offset profile simply by gathering the traces with the same offset from each common-shot record. According to the experimental results, the Rayleigh surface wave method is proved to be a non-destructive, applicable and feasible method to identify the degree and the range of the stratum disturbance caused by the subway tunnel excavation.