The global stability of underground rock cavern excavated in a Mohr-Coulomb material is investigated by means of Universal Distinct Element Code (UDEC) in this paper. The following stochastic variables are considered: the friction angle, the cohesion, the deformation modulus of the rock mass and the in-situ stress ratio. The overburden thickness, the unit weight of rock materials, the Poisson's ratio and the joint strength are assumed as deterministic. The cavern width and height are also assumed as uncertain variables in order to optimize the shape of the rock cavern. Most cavern shapes are horse-shoe or bullet-head shaped. It is widely argued that the use of a flat-arch cavern would make the best use of cavern space. Thus for this study, the initial cavern width and height are set as 30m and 18 m, respectively. The influences of the flattening process on cavern stability can be investigated through incremental increases in the cavern width and reduction in the cavern height through six design levels to assess the changes of safety factor and probability of failure. For each configuration of the six levels, the probability of failure is determined by Monte Carlo simulation incorporated by neural network results. The configuration satisfying the critical safety factor and the expected performance level with the flattest cavern roof can be termed as the optimal design. It is also suggested that the critical factor of safety and the targeted performance level be used together, as complementary measures of acceptable design.

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