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[1] In this study, the subcritical crack growth parameters for Coconino sandstone under modes II and III loading were determined experimentally by using the constant stress rate test. We extend the constant stress rate test technique to modes II and III subcritical crack growth in rocks. The experimental results of the modes II and III tests, combined with mode I results published elsewhere, show that the values of the modes I, II, and III subcritical crack growth parameters are very similar to each other regardless of the loading configuration and the specimen geometries. The main reason for this is thought to be that subcritical crack growth is environmentally induced crack growth rather than the mechanical rupture of bonds. The effect of confining stress, specimen size, and water saturation on subcritical crack growth under mode II loading has also been investigated. The parameter n linearly increases with increasing confining stress and the parameter A exponentially decreases with increasing confining stress. Increasing the specimen size results in a linear increase in the subcritical crack growth parameter A. But, the parameter n is independent of the specimen size. The parameter n is almost constant regardless of water saturation, and the parameter A is found to increase slightly when the specimen is fully saturated. These results suggest that the subcritical crack growth parameter n can be considered a material constant for a given rock type.

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The abrasiveness of rocks significantly influences the wear endured by excavation tools in tunnel boring machines (TBMs). In slurry shield TBMs, wear occurs not only in excavation tools but also in discharge pipes and pumps of slurry transport systems. To explore this phenomenon, this study investigated the effect of rock abrasiveness on the wear of both disc cutters and slurry discharge pipes in slurry shield TBMs. The radial wear of disc cutters and the thickness of slurry discharge pipes were measured and monitored at a TBM site in Singapore. The relationship between the weathering grade of Bukit Timah granite and the wear coefficient of disc cutters was analysed, and a correlation between the Cerchar Abrasivity Index values and the wear coefficient of 19 inch disc cutters was suggested. Additionally, the average wear rates of slurry discharge pipes used for Bukit Timah granite were determined based on the pipe thickness measurements. The wear rates of weathering grades G(I) to G(IV) of Bukit Timah granite were observed to be highly similar to each other, whereas the wear rates of G(V) grade and mixed ground were 1.55 times higher than those of the G(I) to G(IV) grades. The correlation between the slurry discharge velocity and the wear rate in slurry discharge pipes was thus derived. The accurate assessment of the effect of rock abrasiveness on both the wear of disc cutters and the wear rate in slurry discharge pipes, investigated in this paper, will highlight the need for efficient maintenance of the excavation tools in TBMs and encourage the development of various equipment wear prevention measures in order to minimize overall construction costs and time.