Main contractor NishimatsuLum Chang (NLC) was negligent, reckless and dishonest during design and construction of the collapsed section of tunnel, client the Land Transport Authority (LTA) claimed.
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 said.
Finite element analysis of ground conditions used the mechanical properties of drained soil - known as Method A. NLC should have been using data for undrained soils for deep excavation in the site's highly plastic marine clays, he 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 on to 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 contributed to strut-waler connections being understrength by a factor of two.
LTA claimed the problem was compounded by NLC's substitution of C-channel stiffeners for plate stiffeners at the strut-waler connection.
NLC sought to save 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 the light of stiffener plate buckling.' NLC was worried by the situation, but 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 to bear the costs of delay on its own.' Even after struts failed at two adjacent NLC sites, the contractor maintained 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 sufficient material information from NLC to justify exercising its contractual powers to stop work.' LTA refuted NLC's claim that an unforeseen trigger would have been required to set off the sequence of collapse.