Multi-approach stability analyses of large caverns excavated in low-angled bedded sedimentary rock masses in Singapore

Abstract

The first commercial underground rock caverns for hydrocarbon storage in South East Asia, the Jurong Rock Caverns Project (JRC Project), have recently been completed in Singapore. The storage capacity of 5 caverns is 1.47 million cubic meters and 3.5 million cubic meters of rocks were excavated in this project. Geologically, it was excavated in low-angled bedded meta-sedimentary mudstone, pyroclastic rocks and sandstone. Although traditional arch-roofed style excavations are typical for tunnel and cavern for stability condition, flat-roofed profiles tend to form in low-angle stratified sedimentary rock mass. In this study, the stability of such flat-roofed tunnel and cavern with structurally controlled stability was analyzed using geotechnical classification systems including the stability graph method, analytical Voussoir techniques and numerical models. Accordingly, and once presented the main features of the project, characterization of the rock masses is presented. Some classification system approaches providing a convenient but probably over-estimated support and reinforcement recommendation are illustrated. Then, basic wedge stability analysis is briefly illustrated. Voussoir analogue techniques are applied to unsupported cavern profiles showing instability, but clear stability for the supported case. Discontinuous models of the final designed cavities are presented, showing a good agreement with indicative measurements on place, and stress distributions associated with the excavation sections are analyzed using Examine2D to show that abutment relaxation was unlikely to occur in JRC Project. The study shows that classification systems are useful tools but application of analytical techniques and numerical models contribute to a better understanding of the excavation and reinforcement process.