Discussion of the paper “The interaction between yielding supports and squeezing ground” by L. Cantieni and G. Anagnostou [Tunnelling and Underground Space Technology 24 (2009) 309–322]

Abstract

Ground response curve (GRC) is a concept that describes the relationship between the ground and the support system. It is usually to consider a circular tunnel excavated in a rock mass with a hydrostatic in-situ stress state when the concept of GRC is discussed. Assume that a support pressure equal to the in-situ stress is applied to the periphery of the tunnel immediately after excavation. In this case, the stress state on the periphery is exactly the same as before excavation so that no deformation occurs. Now we start to release the support pressure stepwise. Radial displacement occurs as soon as the support pressure starts to drop. For any new support pressure there is a corresponding radial displacement. The final radial displacement, u1, is reached when the support pressure is reduced to zero (namely, no support). The trajectory of this process is the so-called ground response curve, Fig. 1. Here we have only taken into account the instant displacement immediately after excavation. In soft and weak rocks a certain amount of creeping displacement always occurs. It means that the radial displacement increases with time. The dashed line in Fig. 1 represents the GRC of a creeping rock mass after a certain time of excavation. Fig. 2 illustrates the radial displacement profile of a tunnel along its length. The radial displacement already occurs in the rock ahead of the face. The displacement is not fully developed behind the face until the distance to the face is long enough. The tunnel is not engineering supported within a certain distance behind the face, but the radial displacement is still not fully developed there. The reason for this is the face rock provides a support action to the tunnel roof close to the face. The face-supporting effect is equivalent to a distribution of fictitious supporting pressure acting on the excavation boundary, as sketched in Fig. 2. The radial displacement reaches its final value, u1, when the distance to the face is long enough where the face-supporting pressure completely disappears.