Nishimatsu-Lum Chang (NLC) was negligent, reckless and dishonest during design and construction of the collapsed section of tunnel, client Singapore's Land Transport Authority (LTA) claimed last week.
NLC's catalogue of design errors started with the use of inappropriate soil analysis during the early stages of temporary works design, LTA's counsel K Shanmugam told the inquiry into the Nicoll Highway collapse.
Finite element analysis of ground conditions used the mechanical properties of drained soil - known as Method A. For deep excavation of the Nicoll Highway tunnel in the highly plastic marine clays found on site, NLC should have used data for undrained soils, Shanmugam stated.
'Use of Method A was grossly erroneous. It was a substantial error. Use of Method A led to serious underprediction of the forces acting on the temporary works, and hence led to underdesign in the temporary works in general.
'This was an error which bedevilled the entire design, and consequently the entire system had insufficient capacity to cater for the loads coming onto it.' NLC's soil analysis meant that as excavation got deeper deflections of the diaphragm wall increasingly exceeded those predicted.
Shanmugam insisted that 'NLC knew from the outset in 2001 that there would be potential problems with Method A, but recklessly and dishonestly persisted in using it and refused to change.' Combined with further errors in temporary works design, this led to strut-waler connections being under-strength by a factor of two, he told the inquiry.
LTA claimed that the problem was compounded by NLC's substitution of C-channel shaped steel sections for plate stiffeners to strengthen the strut-waler connection.
NLC cut costs by using 'scrap material to replace stiffener plates which they had run out of, ' Shanmugam stated.
'NLC ignored its own risk analysis which stated that a fundamental reassessment had to be done in light of stiffener plate buckling.' The situation worried NLC, but it sought to suppress information to prevent LTA interfering in its construction schedule, Shanmugam claimed.
By April 2004 NLC had S$25M (£8.3M) against it in claims for late delivery against schedule.
'If LTA knew that NLC's design had serious defects, and that NLC was concerned and uncomfortable about its own design, the LTA would probably have stopped work, and in these circumstances NLC would have had to bear the costs of delay on its own.' Even after struts failed at two adjacent NLC sites the contractor maintained that its temporary works designs were satisfactory, and pressured LTA to allow it to resume work, Shanmugam said.
NLC breached its legal duties to reveal key information, he added. 'LTA never had suffi cient material information from NLC to justify exercising its contractual powers to stop work.'