论完整岩体的现场强度

目前国际上普遍认为完整岩体的现场强度近似等于(0.4?0.1)?c,其中,?c为室内岩石单轴抗压强度。此外,也有学者建议原位岩体的破坏强度,即地下工程围岩的启裂强度,可等价于室内单轴压缩试验或现场微震监测确定的岩石裂隙初始的应力;其原理主要以基于Kirsch解析解或简化的数值模拟(光滑的开挖边界)来近似表达隧道开挖面上的最大切向应力?max。然而,这些方法均忽略了开挖边界的几何非规则性对计算结果的影响。经论证表明,若考虑开挖面的几何非规则性因素,完整岩体的现场破坏强度并不等于(0.4?0.1)?c,其破坏强度可高达(0.8?0.05)?c。以加拿大地下实验室Mine-by试验隧道为例,并以该隧道的实际断面形状为几何边界条件,采用有限元软件Phase 2模拟隧道围岩的渐进破坏过程。研究结果表明,当原位岩体强度为0.8?c时,模拟结果与实际观测结果具有很好的一致性。因此,忽略开挖边界的几何非规则性而解读的原位岩体强度(0.4?0.1)?c仅是"等价"强度值,其低估了岩体的实际强度。

ON FIELD STRENGTH OF MASSIVE ROCKS

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.