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The strength reduction method (SRM) can be used to calculate a stability factor of a proposed excavation through the use of well-calibrated numerical models. The method models the excavation and support system while the rock mass strength is reduced in stages until failure is indicated. The stability factor is expressed as the ratio of the expected rock mass strength to the rock mass strength that results in failure. Using the stability factor, various support alternatives can be compared and the impact of specific changes to the support system can be evaluated. Strength reduction analyses were conducted using calibrated models of coal mine entries in various geological settings. The calculated stability factors were validated against the empirically derived Coal Mine Roof Rating (CMRR) and the Analysis of Roof Bolt Systems (ARBS), which is based on observation and statistical analysis of roofbolt systems in US coal mines. The results showed that the stability factors estimated through the SRM capture the essence of the CMRR, indicating improved stability at higher CMRR values and also replicating details such as the effect of a strong bed in the roof and the effect of a weak overlying bed. A linear relationship was found between the stability factors calculated by the strength reduction method and the ARBS. The calculated stability factors for instrumented field sites fell within the expected range of values based on observed entry stability. It is concluded that the stability factors calculated by the strength reduction method provide a meaningful interpretation of overall excavation stability. The numerical model outputs can be used to evaluate the contribution of support elements, allowing optimisation of the support design.
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