Real-time tunneling risk forecasting using vibrations from the working TBM

Abstract

Tunnel Boring Machines (TBMs) are effective tools for excavating tunnels through various soil and rock strata, especially in complex urban areas. During excavating, hazards overlooked during site investigation, such as abrupt geological changes, buried obstacles like boulders and building foundations, fault zones, and water-bearing regions, significantly increase the risks of cutter damage and tunneling accidents. Geophysical methods have been proposed during tunnel excavation to predict these potential hazards ahead of the cutter face. With the existence of a metallic TBM and subsequently installed concrete tunnel lining, the performance of electromagnetic geophysical methods such as ground-penetrating radar and DC resistivity tomography are greatly hampered, despite their success in the traditional open tunnels. In this study, we present a passive seismic method to forecast the tunneling risks ahead of the TBM cutterhead. This method uses the working TBM as the source and predicts the velocity/density changes in front of the TBM by delineating the reflected seismic energy from the noisy recordings. The key to this technology is seismic interferometry, which transforms the chaotic ambient noise into interpretable seismic profiles. We utilize beamforming and semblance analysis to extract the traveltime information from the passive records. After velocity inversion and calibration, we map the arrival-time of a reflection event to a seismic impedance interface ahead of the cutterface. To test the proposed method, we demonstrate the process on a simulated passive seismic record after laying out its theoretical foundations. We present a field application of our method where geological interfaces are predicted in real time as the excavation progresses. The field data results match well with the muck density and TBM parameters measured after excavation and hence demonstrate the applicability of our method in complex urban environments for tunneling risk reduction.