Monitoring tunneling construction using distributed acoustic sensing

Underground tunneling with tunnel boring machine (TBM) in Singapore faces challenges related to the rheological behavior of the rock mass and possible changes in the hydrogeological conditions within the tunnel environment. Conventional geotechnical methods only measure surface changes or tunnel deformation that are insensitive to gradual changes of the geological environment at the initial stage of the failure events. Developing in-tunnel geophysical methods that can provide cost-effective imaging of the tunnel surrounding geology is important for reducing the risks for tunnel construction and operation. We implement distributed acoustic sensing (DAS) for tunnel construction monitoring at one stretch of the Circle Line of the Singapore subway network. We analyze the characteristics of the noises in TBM tunneling. The seismic energy generated by the TBM cutterhead is used to estimate the seismic velocity. The estimated velocities along the tunnel are consistent with the borelog data and mud samples. The presented case study shows the potential to use DAS for longterm tunnel monitoring.