Damage initiation and propagation in hard rock during tunnelling and the influence of near-face stress rotation

One of the critical design problems involved in deep tunnelling in brittle rock with continuous excavation techniques, such as those utilizing tunnel boring machines or raise-bore equipment, is the creation of surface spall damage and breakouts. The mechanisms involved in this process are described in this paper. The onset and depth of damage associated with this phenomenon can be predicted, as a worst case estimate, using a factored in situ strength value based on the standard uniaxial compressive strength (UCS), of intact test samples. The factor applied to the UCS to obtain the lower bound in situ strength has been shown repeatedly to be in the range of 0.35-0.45 for granitic rocks. This factor varies, however, across different rock classes and must be determined or estimated for each class. Empirical guidance is given for estimating the in situ strength factor based on the UCS for different rock types and for different descriptive parameters. Laboratory testing procedures are outlined for determining both this lower bound strength factor and the upper bound in situ strength. This latter threshold is based on the definition of yield based on crack interaction. These techniques are based, in part, on theoretical principles derived from discrete element micromechanical experimentation and laboratory test results. The mechanisms that lead to in situ strength drop, from the upper bound defined by crack interaction and the lower bound limited by crack initiation, are described. These factors include the influence of tunnel-induced stress rotation on crack propagation, interaction and ultimately coalescence and failure. A case study illustrating the profound impact of near-face stress rotation is presented. (C) 2004 Elsevier Ltd. All rights reserved.