ABSTRACT
Due to the outstanding performance in ground movement control and internal clearance from being strut free, circular shafts are predominantly preferred as the earth retaining system for very deep excavation works. The Deep Tunnel Sewerage System Contract T08 (DTSS T08) consists of more than 10 circular shafts with diameter range from 8m to 25m and average depth of excavation for 53m. This paper examines the construction and design methodologies for DTSS Contract T08, focusing on the comparative performance of Circular Diaphragm Wall Shafts versus Circular Caisson Ring Shafts. The performance of a circular shaft is best illustrated in complex geological conditions arising from Singapore Marine Clay and fractured and undulating Singapore Jurong Formation Rock. This paper will also present the readings captured from actual instrumentation to substantiate the ERSS performance. The monitoring and instrumentation provide an opportunity to calibrate construction and design and serves to provide insight of the large excavation works in Singapore adjacent to surrounding sensitive structures. Additionally, Arup innovatively proposed a peanut shaft consisting of two circular overlapping shafts with central cross wall to form an integrated solution for hydraulic, geotechnical, and structural engineering.
