A method to estimate the pressure arch formation above underground excavation in rock mass

Arching effect has been attributed as a factor of roof stability of underground rock excavations, which affects excavation geometry and rock support. In this paper, a series of numerical investigations were performed using the 2D finite element method (FEM) to study the formation and features of pressure arches. Based on horizontal and vertical stress distributions, three characteristic lines of the arching area were defined, i.e., the outer boundary line, the inner boundary line and the centroid line. It is found that the height of the inner boundary of the arching area is an indicator of the roof stability of underground excavations. Furthermore, detailed parametric studies including the Geological Strength Index (GSI from 20 to 80), the overburden depth (H from 40 m to 420 m), in-situ stress ratio (k(0) from 0.8 to 3) and excavation roof rise-to-span ratio (h/B ratio from 0.1 to 0.5) were conducted to discuss their influences on the arching area. The results show that the initial stress state (H and k(0)) has more significant effects on the roof stability of underground rock caverns than the GSI and cavern geometry (h/B). A semi-circular roof is optimal design scheme for underground excavation in surrounding mass if only considering the cavern roof stability.