Experimental and numerical study on tunnel faces reinforced by horizontal bolts in sandy ground

Abstract

In order to provide insight into the reinforcing mechanism of face bolting, in this work a series of experimental tests and numerical simulations on reinforced and unreinforced tunnel faces were conducted in sandy ground. Some aspects of the collapse at the tunnel face were investigated for four different bolt numbers (nb = 0, 26, 47, and 97 bolts), including the face support pressure, face failure process, face failure pattern, soil stresses and the behaviour of face bolts. The results showed the apparent influence of face bolting on face stability, and that this auxiliary technique is an effective method for stabilizing the tunnel face. Specifically, the limit/residual support pressure, the degree of soil stress release ahead of the tunnel face, and the extent of the failure zone were significantly decreased by face bolting; the reinforcing effect increased with an increasing bolt number; and the final configuration of the failure zone of unreinforced and reinforced tunnel faces all showed a “chimney-wedge” pattern. In addition, the reinforcing effect of each bolt can be simplified as a support force applied to the failure zone, which is mainly determined by the bond strength at the soil-bolt interface and the anchorage length. The results in this work provide a reference for establishing and verifying the theoretical model.