Excavation under Merdeka Bridge for construction of Crawford Street underpass

摘要

About 7m deep excavation under the Merdeka Bridge at the Kallang River bank was carried out for the construction of the proposed underpass at the extension of Crawford Street in Singapore. The Merdeka Bridge, which is a part of the Nicoll Highway, was opened to traffic with uninterrupted flow throughout the construction of the proposed underpass. Since the construction was carried out under a very important structure and in the poor ground conditions with thick soft clay layers of upto 40m deep, a carefully planned and designed retaining system with staged excavation was adopted. An extensive and sound instrumentation scheme had been proposed and used for the monitoring of the surrounding ground and Merdeka Bridge during the construction works to ensure the stability of the bridge and to verify the design. Due to constraint of the working space under the bridge, excavation using retaining systems such as sheet pile wall or diaphragm wall were not feasible. A slope excavation of about 2.5m was carried out to provide working space under the bridge. The remaining excavation works were carried out by using horizontally installed steel pipe retaining walls between the bridge piers. The steel pipes were installed after excavations of the sump pits at both sides of the bridge were completed, by jacking from one sump pit to the other. The ground below and in the excavation area under the bridge was improved by jet grout piles. This paper presents the details of the retaining system and the results of the instrumentation. The effects of the construction on Merdeka Bridge and performance of the retaining system during excavation are also discussed.