Face stability assessment for the Lake Mead Intake No 3 Tunnel

Abstract:

The 4.7 km long, 7.2 m diameter Intake No 3 Tunnel will cross tertiary sedimentary rocks underneath the bed of the Lake Mead reservoir. The maximum depth beneath the current lake level is 143 m. The tunnel will be excavated using a slurry shield TBM, which is designed to cope with water pressures of up to 17 bar (the highest pressures ever in closed shield tunneling worldwide). Due to the very poor quality of the ground locally and to the high pore water pressures (in several cases combined with limited rock cover) in the subaqueous tunnel section, a careful investigation of the face stability conditions is of paramount importance. This is particularly true with respect to the execution of inspection and maintenance works in the working chamber, as the high water pressures make hyperbaric interventions extremely demanding. The paper discusses in quantitative terms the effect of the main factors governing the stability of the tunnel face (with special emphasis on the effect of ground permeability, TBM advance rate and the duration of interventions in the working chamber), analyzes possible countermeasures and illustrates the possible options and trade-offs concerning TBM operational modes. More specifically, the paper shows that advance drainage of the ground ahead of the face is a promising alternative to closed-mode operation for long portions of the alignment, as it improves stability considerably even if the given pattern of the drillholes through the cutter head and the limited time available for the drainage process do not allow for complete pore pressure relief.

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