Regression models for estimating ultimate and serviceability limit states of underground rock caverns

Performances of underground rock caverns can be unsatisfactory as a result of either instability (collapse) or excessive movements. Collapse refers to ultimate limit state failure, in which the stresses exceed the strength of the rock masses. Failure of the serviceability limit state refers to excessive deformations resulting in difficulties during excavation such as lining placement and reinforcement installation. Both distinct limit states may need to be considered in design. This study used numerical modeling to assess both the ultimate and the serviceability limit states of underground single and twin rock caverns. The global factor of safety is used as the criterion for the ultimate limit state and the calculated percent strain around the cavern opening is adopted as the serviceability limit state criterion. Based on the numerical results, simple regression models were developed for estimating the global factor of safety and the induced percent strain of the single and twin caverns, respectively. Charts for assessing cavern stability for preliminary design use were developed. In addition, the use of the critical strain concept and the elastic and design line methodology to limit the induced strains was discussed.