PROBLEMS WITH retaining wall excavations close to Nicoll Highway are well known, according to engineers GE spoke to.
Nicoll Highway was built in the 1950s on land reclaimed from beyond Beach Road, the original shoreline (see map). Fill material would have been dumped over the soft marine clays, which are unusually deep in the area, extending to between 50m and 60m.
Excavations for the Concourse tower and the Gateway complex which lie between Nicoll Highway and Beach Road were problematic. Those undertaken inland of Beach Road have been less of an issue.
Contractors experienced serious problems with a 9m deep basement during construction of the Gateway complex in the late 1980s. At one point sheet piles started yielding.
A few years earlier, excavation for the 7m deep basement for the adjacent Concourse building ran into similar problems, suggesting the soft marine clay was overstressed at depths only a quarter of those attempted by Nishimatsu-Lum Chang on the Circle Line.
William Powrie, professor of geotechnical engineering at Southampton University, said with retaining walls in soft clays, base stability can be more critical than lateral, which is often the main focus of the design.
If the clay is overstressed it starts behaving like a toothpaste and will flow, he said. Overstressing of the clay would put tremendous pressure on the slab, which is designed to behave like a plug.
Richard Davies, head of geotechnics at consultant Benaim, has more than 30 years' experience of basement construction in Singapore. He described building a 30m cut and cover excavation in this particular location as 'a very brave thing to do'. PW