PROBLEMS WITH retaining wall excavations close to the 1950s constructed Nicoll Highway are well known, engineers said.
The ground upon which the Nicoll Highway is constructed was reclaimed out from Beach Road - the original shoreline - in the 1950s (see map page 5).
Fill material was dumped over the soft marine clays, which happen to be unusually deep in the area, extending to between 50m and 60m.
Excavations for the Concourse tower and Gateway complex between Nicoll Highway and Beach Road were problematical.
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 University of Southampton said that with retaining walls in soft clays, base stability can be more critical than lateral stability, 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. And overstressing of the clay would put a tremendous pressure on the slab which is designed to behave like a plug.
Richard Davies, head of geotechnics at consultant Benaim, has over 30 years experience of basement construction in Singapore.
He described construction of a 30m cut and cover excavation in this particular location as 'a very brave thing to do'.