Assessment of the arching effect and the role of rock bolting for underground excavations in rock masses—a new numerical approach

In very jointed rock masses, the ground arch developed around the underground opening is an arch-shaped structure to maintain the excavation stability, and the rock bolting has been attributed as one of the dominant approaches to enhance the arching effect. Through 2D finite element method (FEM), numerical studies were performed to investigate the arching effect and the importance of rock bolting. Based on the distribution of the mean stress σm, the boundary of the ground arch is determined without several limitations existed in other methods and the artificial arching effect of rock bolting could be assessed directly. Impacts of the cover depth Ht, the tunnel span D, the in situ stress ratio k, and the rock mass strength (frictional strength ϕ) were first revealed. The enhancement provided by the rock bolting were then directly evaluated through the artificial ground arch. The formation of the artificial arch is carefully studied considering the bolt spacing and the bolt length. Results show that σm could be used as a reliable indicator to assess the arching effect. Rock bolting reinforces the surrounding rock mass and contributes to the formation of the sub-arch therein. With enough reinforcement, the sub-arch overlaps each other and a global artificial arch could form to enhance the arching effect.