A Study Based on Large-Scale Model Tests in a Field for Subsurface Cavity Detectiony Based on Large-Scale Model Experiments in a Field

Abstract

Ground sinkholes caused by subsurface cavities from tunneling poses a significant risk to urban areas. The 2020 Chofu accident highlighted that even deep cavities are dangerous. As surface detection is difficult, in-tunnel exploration is necessary. Seismic surveys might be an effective method to detect cavities, but its propagation mechanisms near tunnels are poorly understood. This study evaluates the applicability of in-tunnel SH-wave exploration for early cavity detection using large-scale model experiments. A large-scale model experiment was conducted. A tunnel structure was constructed on the surface, and two Styrofoam blocks simulating cavities were embedded in the loam. The survey was performed from the concrete surface. A predominantly SH-wave source was generated by horizontally striking embedded rebar. Waveforms were recorded with horizontal geophones. The results showed multiple hyperbolic curves interpreted as reflected and diffracted waves from the cavity edges were successfully captured from both simulated cavities at different depths. Furthermore, reflections from the top surface were also identified from the larger-sized cavity. These findings suggest that in-tunnel exploration using SH-wave sources is an effective technique for the early detection of cavities existing in the vicinity of a tunnel.