The Validation of a Look-Ahead Geophysical System at the Rondout West-Branch Bypass Tunnel

Although geophysical investigations have been conducted in the past with the goal of characterizing rock structure from both the ground surface as well as vertical boreholes, no published studies have successfully demonstrated the capabilities of down-the-hole surveying through horizontal probeholes during the construction phase of a tunneling project. In this study, ground penetrating radar (GPR) was used ahead of the tunnel boring machine (TBM) at the Rondout West-Branch Bypass Tunnel to aid in the characterization of structural features that may indicate a change in ground behavior, such as crushed and/or saturated regions of bedrock. Preliminary look-ahead geophysical tests were conducted in the face of the project starter tunnel, which resulted in high confidence in the ability of GPR borehole reflection and cross-hole tomograph to identify changes in lithology and linear discontinuity features. Following these initial experiments, during the bypass tunnel construction phase, numerous GPR surveys were performed through holes drilled ahead of the TBM. These radar interpretations were successfully correlated with ground-truth borehole images and established structural trends. While no major features with significant implications for construction methods were encountered as part of this study, this research suggests that GPR can be used practically to identify changes in ground behavior ahead of an active TBM.