By David Crouthamel, GE, and Dan Van Roosendaal, PE

This two-part Just Answers looks at essential environmental risk factors to consider when choosing an excavation method: high groundwater flow (Part 1) and adverse ground conditions (Part 2), and how each can affect project schedule and costs. A tunnel boring machine’s (TBM) ability to provide protection from groundwater inflows and adverse ground conditions during excavation often makes it a favorable selection. However, TBMs are not one-size-fits-all. For projects where these risks are prevalent—drill-and-blast might be the better solution.

Groundwater flows into tunnel excavation can result in substantial environmental impacts. High flows can cause groundwater depletion, which can result in loss of groundwater resources at the surface, affect groundwater wells, disperse contaminated groundwater, and cause surface or structural settlement. Extreme groundwater inflows can impact tunneling operations and advance rates, increasing safety concerns and negatively affecting the final lining. The choice of excavation method should therefore be based on the sensitivity of the groundwater regime, magnitude of inflow potential, and impact inflows on the tunneling operations.

The use of properly selected TBM technology can improve management of groundwater inflows into tunnels, particularly shielded TBMs and the erection of bolted and gasketed segments. In some cases, pressurized hard rock TBMs have been used to limit water inflow. However, if the TBM is not fully compatible with groundwater conditions, pretreatment ahead of the TBM, typically through the tunnel face, is necessary.

This water management can be difficult with TBM excavation. Depending upon the machine’s configuration, face access for pre-excavation probing and treatment of the ground can be limited or difficult. Thus, TBM compatibility with groundwater inflows and pressures at and near the face must be done at the time of TBM selection. If the machine is incompatible, significant pretreatment must be done ahead of the face. The greater the incompatibility, the greater the negative impact on overall TBM production and project economics.

Drill-and-blast methods have a higher level of accessibility at the face and a greater ability to thoroughly probe, identify, and pretreat the ground ahead of the face. They allow the immediate risk of high inflows to be better identified and managed. However, the overall water inflow into the remaining tunnel is more difficult to manage. This inflow may impact installation of the final lining or cause long-term issues such as groundwater depletion. For drill-and-blast, the costs associated with the remaining steady-state inflows should be assessed relative to management costs and impacts to installation of the final lining.

Part 2 looks at some of the issues associated with identifying and interpreting adverse ground conditions, and provides some general guidelines in choosing an excavation method.

Dave is a principal working out of the San Francisco office, and Dan is a principal working out of the New York office and is the East Coast Regional Manager.