Case study of the excavation adopting peanut-shaped double cell cofferdam for the construction of basement structure

Due to land scarcity in Singapore, there has been an increasing need to build undergroundstructures and facilities. A 95 m diameter peanut-shaped double-cell cofferdam using diaphragm walls was adopted as the earth retaining system to support the 23 m deep excavation. The strutless and tieback-free earth retaining scheme facilitated the construction of basement structure with construction speed as key advantage.Three levels of concrete ring beams were incorporated to enhance the robustness and stability of the cofferdam during the temporary stage. Toe fissure and diaphragm panel-joint grouting were implemented as a temporary water seepage control measure considering the risk of potential flow paths through the highly fractured Singapore Jurong Formation rock. The joint grouting was carried out to supplement the water-stop to control seepage between diaphragm wall panels. This case study focuses on the design considerations of the double cell cofferdam and its performance during excavation. Comparison is made between the design stagepredictions and the field measurement data.