Face Pressure Evaluation in Serviceability Limit State

In urban areas, tunneling is accompanied by ground surface settlement. To reduce the risks of the operation, it is always required to have an assessment of ground surface settlement and face pressure. In the evaluation of the face pressure, there are two major sets of methods based on the ultimate limit state (ULS) (e.g. tunnel face failure), and serviceability limit state (SLS) (e.g. unacceptable settlement or heave). In a serviceability limit state, volume loss and surface settlement are used for the analysis of the face pressure. The methods developed for the analysis of SLS face pressure are based on the results of small-scale centrifuge tests or case studies with the unpressurized face. Hence, realistic face pressure cannot be evaluated with these methods. This paper summarizes the major strategies for the evaluation of face pressure in an undrained condition. The outlined methods are utilized to evaluate tunnel face pressure in the Tehran metro line 6, south extension (ML6-SE) project. Results of the analyses showed that predicted face pressures with the SLS methods are far above the required face pressure. To make realistic SLS face pressure evaluations in this project (considering a maximum allowable surface settlement of 10 mm), using ground characteristics and TBM operational parameters, a new formula was introduced with a coefficient of determination of 84%. The results of this paper can be very helpful for both the design phase of a tunneling project and the construction period to adjust the face pressure based on the requirement of the allowable surface settlement.