(1. Bharti School of Engineering，Laurentian University，Sudbury P3E 2C6，Canada；2. Beijing Research Institute of Uranium Geology，Beijing 100029，China；3. Centre for Excellence in Mining Innovation，Sudbury P3E 2C6，Canada)
Abstract：It is widely accepted that the field or in-situ strength of massive rocks is approximately (0.4?0.1)?c，where ?c is the uniaxial compressive strength obtained from unconfined laboratory tests. In addition，it has been suggested that the in-situ rock spalling strength，i.e. the strength of the wall of an excavation when spalling initiates，can be set to the crack initiation stress determined from laboratory test or field microseismic monitoring. These findings were based on either Kirsch?s solution or simplified numerical stress modeling(with smooth tunnel wall boundary) to approximate the maximum tangential stress ?max at the excavation boundary. In this article，it is suggested that these approaches ignore one of the most important factors，the irregularity of the excavation boundary. It is demonstrated that the“actual”in-situ spalling strength of massive rocks is not equal to (0.4?0.1)?c，but can be as high as (0.8?0.05)?c when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic“as-built”excavation boundary condition is honored，the “actual”in-situ rock mass strength，given by 0.8 ?c，can be applied to simulate progressive brittle rock failure process satisfactorily. We conclude that the interpreted，reduced in-situ rock mass strength of (0.4?0.1)?c without considering geometry irregularity is therefore only an“apparent”rock mass strength.
READ R S，CHANDLER N A，DZIK E J. In-situ strength criteria for tunnel design in highly-stressed rock masses[J]. International Journal of Rock Mechanics and Mining Sciences，1998，35(3)：261-278.
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