Unforseen downward movement in diaphragm walls triggered the Nicoll Highway tunnel collapse, main contractor Nishimatsu-Lum Chang jv (NLC) claimed before the public inquiry last week.
A sudden drop in the height of the wall relative to king posts supporting the temporary struts altered the angle at which struts connected with the walers. This caused them to deform rapidly, leading to 'sway failure', the contractor said.
Sway failure happens when the junctions between the flanges and web of an I-beam act as hinges, causing the beam to flatten under heavy loading.
NLC coined the term 'forced sway failure' to describe the violent forcing of this mode of collapse.
Movement of diaphragm walls relative to king posts had been observed on other sections of Singapore's Circle Line cut and cover excavation. But there was no evidence of this at Nicoll Highway, said the contractor.
'Collapse was not inevitable.
The collapse occurred because the forced sway mechanism dramatically reduced the time in which it developed, ' claimed NLC counsel Philip Jeyaretnam.
Although the struts were close to the limit at which they could fail, there was no evidence that loads exceeded the ultimate capacity of the temporary works, he added.
'There was no evidence of a build up of loads before 20 April.
'If the loads exceeded actual capacity the connections would have failed as excavation beneath each strut reached the 10th level, or as loads built up with time after completion of excavation.
NLC dismissed the importance attached by the client LTA to the use of drained soil data in finite element analysis of ground conditions on site.
It said this was appropriate as, at shallower depths, it tends to give conservative results, although there was no precedent for such deep excavation using Method A in Singapore.
During the early stages of construction, NLC claimed LTA accepted that deflections measured on site were close to those anticipated using Method A finite element analysis, Jeyaretnam said.
NLC's independent engineer Dr Andy Pickles of consultant CPG had briefed LTA on the soil analysis in May 2002 and LTA's supervising engineer had described alternative analysis models as over-cautious, he added.
During construction NLC discovered a buried channel in the old alluvium base strata into which the diaphragm walls were toed.
This was filled with deep, soft, organic clay. There was also a bowl of soft estuarine clay below the marine clay which covers most of the site. Neither was detected during site investigation.
The temporary works had not been designed for these conditions, and had to be adapted, said NLC counsel Philip Jeyaretnam.