Ground response curves for rock excavations supported by ungrouted tensioned rockbolts

A numerical analysis has been developed for the design of ungrouted tensioned rockbolts as support of excavations under axisymmetric conditions. The bolts dimensions (length, cross-section, longitudinal and circumferential spacings), their stiffness, their pre-tension load and the delay of installation are taken into account. Moreover, the method effects three main improvements in the usual theory, taking into consideration: 1. the reaction force transferred to the rock mass in the bolts anchoring zone, 2. the elastic recompression of the carrying ring surrounding the excavation due to the bolts preload, and 3. the relative displacement of the bolts ends which has a repercussion on their tension. Since the usual rock-support interaction analysis is only available when the rock mass and the support behave independently, an alternative solution has been explored for the bolting system (since it cannot be considered as an internal support). It consists to include the effect of the rockbolts into the ground reaction curve. In this paper, the principles of the analysis are explained and a numerical application is taken.