Stability analysis of tunnel keyblock: a case study

Abstract

The probabilistic block theory which was suggested by Hatzor and Goodman (1992) was applied to an telecommunication tunnel by taking into consideration the individual joints obtained from face mapping during tunnel construction. Using the actual unrolled joint trace that was developed in the Banpo telecommunication tunnel during tunnelling, a statistical analysis of observed discontinuity data was performed. The deterministic analysis and probabilistic keyblock concept were applied and their results were compared with the observed failure modes. The result obtained from the deterministic analysis resulted in a large difference compared with the observed failure pattern; the analysis method which considers the joint combination probability gave more similar results to the observed data than the deterministic analysis. Furthemore, individual keyblock analysis was performed by means of the newly developed program. While the positions of the keyblocks were predicted properly by the individual keyblock analysis, the predicted sizes of the keyblocks were found to be different from the observed data. It was acknowledged from the individual keyblock analysis that most keyblocks are composed of steeply inclined joints in the crown or in the side wall fo a tunnel. But, in the fault-zone, any method based upon the block theory could not predict the failure pattern properly. It can be concluded that the continuous careful investigations of geological conditions were very important in tunnelling through a rock mass that experiences mainly structure-induced failures.