Use of a look-ahead geophysical method to delineate water-bearing features ahead of a tunnel boring machine, The

Abstract

In the field of underground construction and tunneling, comprehensive geologic investigations are both time-intensive and expensive, and can be logistically challenging. As such, it is common to encounter unexpected ground conditions while tunneling. To reduce the inherent uncertainty in underground construction operations, the use of a geophysical look-ahead system is proposed. This system will aid in the development of a model of ground conditions ahead of the excavation face at the Rondout West-Branch Bypass Tunnel. By performing both reflection and tomographic borehole ground penetrating radar surveys, discrete features such as cracks and voids, as well as bulk features such as fractured and saturated regions can be identified. Identifying potential water bearing features allows for the application of appropriate pre-excavation grouting procedures, reducing the likelihood of tunnel support degradation and annulus grout washout and helping to preserve the project's critical path timeline. In the Fall of 2017, proof-of-concept radar surveys were conducted from the face of the Rondout West-Branch Bypass starter tunnel. Over the Summer of 2018, numerous geophysical look-ahead surveys were conducted through holes drilled ahead of the tunnel boring machine. A possible correlation between radar data as well as probe drill behavior and tunnel boring machine performance is explored, while additional ground truth observations confirm the efficacy of the geophysical method in question.