3-D global–local finite element analysis of shallow underground caverns in soft sedimentary rock

We present a realistic approach for a 3-D finite element analysis of shallow underground openings excavated in soft and discontinuous rock mass. We address the stability analysis on two different scales, local-discontinuous and global-continuous, and promote the use of computationally effective linear analysis and multiple loading scenarios as a trade-off for elaborate non-linear analysis. The research was performed using the well-documented case of the ancient caverns of the Bet-Guvrin National Park in Israel. Well preserved damage zones indicate that local instabilities are associated with exposure of pre-excavation joints and development of tensile stresses exceeding the tensile strength of the rock. Global stability is compromised when supporting pillars within the caverns are diminished.Results of the local-discontinuous analysis were compared with a well-documented local failure, and were found to be in agreement with the ground truth. Following, we performed a comprehensive global stability analysis using a linear analysis of a large-scale global model (6.3×106 DOF). We modeled different scenarios of supporting pillar integrity, thus gaining comprehensive understanding of possible stress fields otherwise requiring excessive computational resources. We conclude this article with simple guidelines for performing local-discontinuous and global-continuous analysis.

► Stability of the Bet Guvrin caverns, excavated in soft discontinuous rock, is studied.
► FE analysis of the caverns is performed on two scales: global continuous and local discontinuous.
► Local discontinuous FE analysis of failure conforms with ground truth.
► Global stability scenarios are analyzed realistically and efficiently by linear FE.
► The advantages of linear FE analysis, over non-linear analysis, are presented.