The contribution of horizontal arching to tunnel face stability

The article revisits the classic problem of tunnel face stability with special emphasis on the effect of horizontal stresses. These are important for shear resistance and thus also for the equilibrium of the potentially unstable body in front of the tunnel face, but they also present the difficulty of static indeterminacy. Starting from the computational model of Anagnostou and Kovári, an alternative model is presented, which is based on the so-called method of slices, and is consistent with silo theory, but does not need an a priori assumption as to the distribution of horizontal stress. In addition, a simple design equation for estimating support pressure under this model is presented and the results of comparative analyses concerning the average stresses in the wedge and the effects of shear resistance at the lateral slip surfaces are shown. The analytical results obtained by the method of slices agree very well with published results of numerical analyses and physical tests.