Planning for monitoring of probe hole drilling and pre-excavation grouting with measurement while drilling (MWD), vibration monitoring, and ground penetrating radar (GPR)

Abstract

Probe hole drilling and pre-excavation grouting are methods commonly used in rock tunneling to mitigate the significant risk caused by high groundwater pressures and potentially high groundwater inflows. The current state of the practice involves measuring groundwater inflow rates through the probe holes and comparing the measured inflows to pre-determined, project specific baseline flowrates. Grouting is done when the measured inflows are greater than the baseline flow rates. The success of grouting is then determined by drilling verification probe holes and again measuring the inflows. This thesis presents a preliminary analysis of three methods to advance the current state of the practice, including: measurement while drilling, vibration monitoring, and borehole ground penetrating radar, with the goal of determining the locations of water bearing features ahead of the TBM and monitoring if grouting has sealed off or filled these features. Drill rig vibrations while drilling with a down-the-hole water hammer were used to determine the locations of fractures that were drilled through. A procedure for determining the vibrations at the drill bit by monitoring the vibrations at the drill rig and applying a transfer function was developed, with the aim of advancing vibration monitoring to a seismic while drilling approach. A borehole ground penetrating radar survey was attempted, and although results from this survey were inconclusive due to equipment complications, an extensive literature review suggests that this method shows promise to determine locations of water filled features and to monitor grout take into the rock mass.