Evaluation of Bearing Capacity for TBM Tunneling in Deep Soft Soils

This paper reviews two analytical methods for calculating the bearing capacity of tunnels, based on conventional equivalent footing approach and slip-line theory for partially embedded pipes. Two-dimensional finite element modeling was performed to investigate the accuracy of these two methods for application to TBM tunneling in deep soft soil conditions. It was found that both analytical methods tend to overpredict TBM bearing capacity for fully embedded tunnels. Numerical modeling suggests that failure zones were more localized for deep tunnels than assumed for the analytical models. An alternate method for evaluating the bearing capacity of TBM tunneling in soft soils was developed based on the numerical modeling results. The ultimate bearing capacity of a TBM can be calculated in terms of an equivalent bearing capacity factor N′c and the undrained shear strength at the tunnel spring line. The derived N′c factor varied from 3.9 for a tunnel depth of 1D up to a limiting value of approximately 9.0 for tunnels deeper than 10D. An improved definition of factor of safety was introduced for evaluation of TBM bearing safety margin based on self-weight of the TBM and soil removed.