ABSTRACT
Shield TBM excavation in rock masses prone to face instability results in low advance rates, difficult driving, frequent stops due to high torque demands, overloading of the conveyor system, uneven configuration of the tunnel face similar to mixed-face conditions, excessive cutter wear and damage. Face instability can be discontinuity-related or strength-stress-related including spalling and rock bursting. Frequent variation of face stability conditions and its effect on the machine-geology interface, and thus the TBM performance, generally cannot be defined beforehand. The TBM contractor can do little when dealing with such conditions as systematic face reinforcement is not applicable on TBMs and therefore not a feasible measure for enhancing TBM performance; reducing TBM advance speed, cutter head rotation and the size of cutter head openings are amongst the few driving correction tools that the TBM operator can activate while boring through unstable face conditions.
