Mitigating Groundwater Risks with a Microtunnel Boring Machine
The Deer Creek Intake Project involves the construction of a new intake tunnel using a Microtunnel Boring Machine (MTBM) wet recovery method. A major construction risk during microtunneling is groundwater seepage flowing through higher permeability zones in the rock into the sloping sides of the left abutment near the launch portal, particularly within 30 feet from the launch portal where rock cover is lowest. One mitigation measure is to form a grouted rock mass collar, 8-10 feet thick, around the circumference of the proposed tunnel alignment, beginning from the launch portal and extending 30 feet along the alignment.
Malcolm performed the Pre-Excavation grouting at the launch portal by drilling and grouting 9 primary, 9 secondary, and 3 verification holes at a hole length of 30 feet. Both the balanced and stable Type III cement and microfine cement grout were used based on the prior water testing results to address various fracture zones in the rock. They completely sealed all water leakage around the tunnel portal by injecting over 1,800 gallons of grout.
