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Overbreak of rock beyond the desgined periphery of a tunnel is a structural risk which more than occasionally results in filing of a claim by the contractor. It is difficult to estimate the overbreak risk at the time of tender with sufficient degree of accuracy because of the uncertainty associated with the geology, construction technique, and the interaction of these two domains. The key variables that influence the extent of overbreak, and their interactions, are illustrated through the proposed interaction matrix. The assessment of overbreak for a tunnel excavated using the conventional, drill-and-blast technique, is performed using an approach that integrates simulation and sensitivity analysis. The stability of wedges formed by two joint sets is examined using a two-dimensional limit.equilibrium analysis incorporating the negative influence of blasting. The results of the analysis can be used to define overbreak thresholds at a given level of confidence. |
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