Unstable Regions Around Underground Openings: A Model Based on Block Theory, Reliability, Stress Analysis and Linear Programming

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CMS-9503855 Matthew Mauldon and Karen Chou, The University of Tennessee (Knoxville) "Unstable Regions Around Underground Openings: A Model Based on Block Theory, Reliability, Stress Analysis and Linear Programming" Knowledge of unstable regions is crucial to the safe design of underground structures and facilities. At low to moderate in situ stress, the unstable zone location and dimensions are strongly influences by excavation geometry, in situ effective stresses, and rock mass inhomogeneities and discontinuities. A rational approach to excavation design, including design of support systems, should be based on an understanding of the combined effects of these major influences. This research addresses the problem of determining the size and shape of the unstable zone around underground openings in cases where the block theory model is appropriate, i.e., hard, jointed rock at low to moderate stress levels. Unstable keyblocks, the dominant failure mechanism, are limited in size by two main factors in addition to the excavation size: the sizes and shapes of the bounding discontinuities, and the stabilizing effect of in situ stress. This research will be completed with 1) development of an analytical model for keyblock size distribution, 2) development of characteristic probability density functions for keyblock size, 3) combining block theory with stress analysis to determine maximum size of unstable keyblocks, and 4) combining 2) and 3) to develop a reliability analysis for support systems. The results of this research will be applicable to tunneling, mining, and the development of underground space, as well as to infrastructure rehabilitation and safety assessments. The methods developed can be extended to rock slopes and foundations, with significant cost savings possible from designs based on objective, consistent and rational procedures.