Demonstration of concave deformation of arc-shaped rock slabs in deep circular tunnels

Slabbing is a failure phenomenon that the surrounding rock mass is separated into parallel slabs because of splitting effect in underground excavation. It always occurs in intact or relatively intact, hard, brittle and deep buried surrounding rock mass. The slabs are always arc-shaped in circular tunnels. When an arc-shaped slab experiences a bending deformation, which directs to the external of a tunnel, decreases the radius of the slab and increases its curvature, this kind of bending deformation can be named as concave deformation. In the case of the drainage tunnel excavated by TBM in Jinping Ⅱ Hydropower Station, the existence of arc-shaped slabs owing to splitting in the circular tunnel is definitely pointed out according to the in-situ characteristics of the surrounding rock mass, and the possibility of the concave deformation of the arc-shaped slab is put forward as well. Then the possibility of the concave deformation is demonstrated by means of mechanical analyses and numerical tests respectively. The demonstration reveals that the occurrence of the concave deformation is dominated by the stress difference between vertical and horizontal geostresses which press the circular tunnel into an approximatively elliptical one. The demonstration of the concave deformation for the arc-shaped slab will play an important role in intensive studies on mechanism of dynamic failure of deep and intact rock mass in circular tunnels.