TBM design philosophy for uncertain ground conditions

Experience shows that with proper initial geological investigation and careful planning of the Tunnel Boring Machine (TBM) layout and of the total excavation process the risks of failure can be kept within acceptable limits and that tunnelling by TBM can be successful in poor ground as well as in good ground. Unfortunately, the past also reveals that sufficient, truly exact and reliable geological information is seldom available before the start of a TBM project. Less favorable than expected ground conditions have often caused a considerable loss of time -and money -and sometimes even necessitated abandoning mechanical excavation altogether and falling back on conventional methods of drilling and blasting to be able to finish the project. The basic philosophy behind successful TBM. design should therefore be to create a machine that will remain capable of operating at acceptable rates of advance and without an exorbitant rise in costs in spite of deteriorating ground conditions. This paper illustrates the approach Atlas Copco Robbins takes to safeguard the users of its TBMs against "unpleasant geological surprises" and discusses how the negative influence of poor ground on machine performance can be kept within tolerable limits through "worst case" assumptions already in the design stage.