Thrust force requirements in fault zones with squeezing ground

Abstract

Shield jamming constitutes a critical hazard in mechanized tunnelling under squeezing conditions. These are often encountered within narrow fault zones interspersed between competent rock layers, where the competent rock exhibits much smaller deformations, thus mobilising shear stresses at the interfaces. This so-called “wall-effect” results in substantially reduced deformations and rock pressures in the fault zone and is thus favourable with respect to shield jamming. Building on existing design nomograms for evaluating the required thrust force in homogeneous ground, this paper proposes two novel, simple and practical thrust force estimation methods that additionally consider the favourable influence of the wall-effect in narrow fault zones. The first method performs an a priori modification of the nomogram input via an equivalent improved ground stiffness that depends on the fault zone length. The second method performs an a posteriori correction of the nomogram output via an analytically determined coefficient, established via multiparametric regression that considers the results of 3450 numerical analyses. Both methods allow for a fast evaluation of the required thrust force in fault zones with reasonable accuracy, thereby providing a valuable tool for design.