Weakness zones in tunnelling

摘要

Weakness zones in competent rock masses are associated with tectonic movements in the history. Shear faulting during the tectonic movements results in that the in-situ stresses in a weakness zone are different from those in the host rock around. In general, the in-situ stresses in a weakness zone are smaller than those in the host rock and their orientations are either sub-parallel with or sub-perpendicular to the tabular plane of the weakness zone. After excavation, stresses change in such a manner that the radial stress on the exposed surfaces drops to zero and the tangential stress is elevated somehow. The stress change would result in rock crushing in the weakness zone. Without taking immediate support measures, cave-in would quickly extend upward. A key measure to prohibit cave-ins is to create a certain amount of confining pressure on the exposed rock surfaces. This could be achieved either forepoling or immediate external support after excavation. The paper gives an overview of the in-situ stress state in weakness zones through the study results collected in the literature. The stress changes after excavation are presented with the help of a modeling study. The mechanics of rock crushing in cave-ins are explained using the Mohr's circle.