Numerical analysis of roof failure mechanisms of cavities in a soft rock

At the University of Napoli Federico II (Italy), a research program was recently started on the behavior of underground openings in soft rocks, with the aim of finding simple tools for evaluating roof safety conditions. As a first step, assuming a simplified geometry of the cavity, parametric numerical analyses were carried out in plane strain with a FD code and a FE code, by varying the depth and the width of the cavity as well as the thickness of the roof beam. The soft rock (tuff) in which cavities are dug underneath Napoli was modeled as a linear elastic-perfectly plastic material. Two critical roof failure mechanisms were considered: one for which cracks progressively open until it is assumed that a block falls from the roof (local collapse), and one for which the whole roof collapses (general collapse). The results allow collapse loci for the two mechanisms to be plotted, which can be used also for different soft rocks. The results show that the local collapse locus basically coincides with the general collapse locus analytically derived from a simplified arch scheme. As a consequence, the use of this arch scheme to establish roof safety conditions with reference to general collapse is conservative. Even though the studied cases are ideally simplified, they improve our confidence in using simple tools for the mitigation of hazard due to shallow cavities in an urban areas.