How does the groundwater table level impact the choice between EPB and Slurry TBM?

The groundwater table level is a significant factor in selecting between an EPB (Earth Pressure Balance) and a Slurry TBM. A high groundwater table generally favors the use of a Slurry TBM. Slurry TBMs are designed to operate in fully saturated ground conditions. They use a pressurized slurry to support the tunnel face and control groundwater inflow. An EPB TBM, on the other hand, relies on the excavated soil to provide support. A high groundwater table can make it difficult for an EPB TBM to maintain face stability, as the saturated soil may be too fluid or unstable. If the groundwater table is low or can be effectively lowered through dewatering, an EPB TBM may be a suitable choice. The decision also depends on the soil type. EPB TBMs are generally better suited for cohesive soils, such as clays, while Slurry TBMs are more versatile and can be used in a wider range of soil types, including sands and gravels. In situations where the groundwater table is variable or the soil conditions are mixed, a Mixshield TBM, which combines features of both EPB and Slurry TBMs, may be the best option. The term 'groundwater table' refers to the upper surface of the zone of saturation in the ground. 'EPB TBM' is a Tunnel Boring Machine that uses excavated soil as a support medium by controlling the pressure. 'Slurry TBM' is a Tunnel Boring Machine that uses a slurry to support the excavation face.