Stability Analysis of Underground Storage Cavern Excavation in Singapore

Abstract

The first underground rock caverns for hydrocarbon storage in South-East Asia has recently been completed in Singapore. The storage capacity of 5 twin-caverns is 1.47 million cubic meters with total volume of rock excavation is about 3 million cubic meters. Geologically, it was excavated in low-angled bedded meta-sedimentary rocks, namely Jurong Formation, of late Triassic to early Jurassic age. The dimensions of cavern are 20 m in width, 27 m in height and 340 m in length. Extensive rock tri-axial tests were carried out to determine intact strength properties. Consequently equivalent rock mass properties were calculated based on GSI (Geology Strength Index) and D (Excavation Damage Factor) those were recorded from rock face mapping. In situ horizontal stresses of rock mass were measured before and during rock cavern excavation. The stability analysis was carried out based on those parameters by using 2D FEM program such as Phase2 and UDEC. The calculated vertical displacements of cavern roof and wall by FEM analysis were compared with those values monitored during excavation. The calculated axial forces were far below design capacity of rock bolts. The strong rock mass strength and high horizontal to vertical stress ratio enhanced safe working condition throughout the excavation period. Thus wide span caverns could be successfully excavated in meta-sedimentary rocks.