How does the plasticity index of soil influence the selection of a TBM?

The plasticity index (PI) of soil is a key parameter in determining the appropriate TBM type because it indicates the soil's cohesive properties and its tendency to become sticky or plastic when wet. The plasticity index is the numerical difference between the liquid limit (LL) and the plastic limit (PL) of a soil. A high PI suggests that the soil is highly plastic and will exhibit significant cohesion, meaning it tends to stick together. Soils with high PI values, such as clays, are prone to clogging the cutterhead and screw conveyor of a TBM, hindering excavation and material transport. Therefore, for soils with high PI, an Earth Pressure Balance (EPB) TBM is often preferred. EPB TBMs can control the earth pressure at the tunnel face, preventing instability, and are often used with soil conditioning techniques (like polymer injection) to improve the soil's workability. Conversely, soils with a low PI, such as sands and gravels, are less cohesive and easier to excavate. However, they may be unstable and prone to collapse. For soils with low PI, a Slurry TBM might be a better choice, as it uses a slurry to support the tunnel face and prevent collapse. The slurry penetrates the soil, forming a filter cake that stabilizes the face. In some cases, a Mixshield TBM, which combines features of both EPB and slurry TBMs, might be suitable for soils with intermediate PI values or variable soil conditions. Therefore, the plasticity index helps engineers choose a TBM that can effectively excavate the soil while maintaining face stability and minimizing operational challenges. The 'liquid limit (LL)' is the water content at which a soil transitions from a plastic to a liquid state. The 'plastic limit (PL)' is the water content at which a soil transitions from a semi-solid to a plastic state.