Abstract:

To address the safety risks and geological hazards encountered during urban subway shield tunneling in soft-hard uneven strata, the applicability and identification characteristics of the surface transient electromagnetic method for advanced detection in such formations were investigated. Taking a metro section in Qingdao as an example, and considering the generally shallow burial depth of urban subways (typically 30-50 m), a numerical model of upper-soft and lower-hard strata was established using COMSOL finite element software. Through simulation of the transient electromagnetic response process, electric field response results and anomalous characteristics at different measurement points were obtained. Combined with field-measured data, the numerical simulation results were compared and validated, and the criteria for identifying soft-hard uneven anomalies in the strata were summarized. The results indicate that soft-hard uneven strata induce significant variations in the electric field response. The response curve at the measurement point directly above the anomalous body exhibits the most drastic changes and the fastest decay rate, demonstrating that the surface transient electromagnetic method can be utilized to identify anomalies in soft-hard uneven strata. The rate of electric field attenuation is jointly influenced by the properties of the medium and the distance from the anomalous zone, manifesting as a "trough phenomenon", with faster attenuation near the anomaly and slower attenuation farther away. In the apparent resistivity contour maps, the tunnel-crossing areas exhibit intense small-scale contour fluctuations, with soft rock showing low-resistivity anomalies and hard rock showing high-resistivity anomalies. Resistivity often transitions from low to high or from high to low, reflecting the heterogeneous characteristics of the strata.

Key words: subway tunnel, soft and hard heterogeneous strata, transient electromagnetic, numerical simulation, advanced detection