NISHIMATSU IS likely to argue unforeseen ground conditions played a major part in Singapore's Nicoll Highway tunnel collapse if legal proceedings are launched against it, sources close to the tunnel's main contractor said last month.
Last month the Committee of Inquiry (COI) into the disaster, in which four workers were killed, published its fi nal report, in which it recommended Singapore's Public Prosecutor take action against main contractor Nishimatsu and a number of engineers.
Nishimatsu is now waiting for the minister for public works and attorney general to state whether they plan to prosecute it.
Three Nishimatsu engineers and one employed by client the Land Transport Authority face fines and imprisonment. Those registered as professional engineers in Singapore are likely to have their licences revoked.
Both criminal and civil proceedings are possible. All those facing prosecution could be sued by relatives of the workers killed when a section of cut and cover tunnel at Nicoll Highway caved in on 20 April last year (GE May 2005).
The Nicoll Highway excavation, for the Metro's new Circle Line, was one of the deepest ever attempted in Singapore.
It had reached 30m below ground level when the connection between struts and walers supporting the retaining diaphragm walls gave way, triggering progressive failure from the bottom up. The full collapse mechanism was detailed in the summing up of the year-long inquiry in March (GE May).
Last month's final report says critical design and construction errors led to the failure of the earth retaining wall system.
These errors were:
l Use of an inappropriate soil simulation model which overestimated the soil strength at the site and underestimated the forces on the retaining walls in the excavation.
l An error in the design of the strutwaler support system with the connections being underdesigned.
l Omission during construction of props which would have spread load from struts into the walers.
'The net effect of these errors resulted in the strut-waler system being about 50% weaker than it should have been, ' the COI report states.
These failings stemmed from and were compounded by 'human and systemic failures'. 'Technical and administrative factors contributed to the collapse, ' it says.
'From the early stages of the project through to final collapse, there were failures to demonstrate the necessary level of care. Serious human errors were made.
'Warnings of the approaching collapse were present from an early stage but these were not taken seriously. The builder did not adequately deal with insidious warning signs.
'A multiplicity of events led to the position where design, construction, instrumentation, management and organisational systems used by the builder and their sub-builders failed.
'There were failures in the defensive systems. There were no proper and appropriate design reviews.
There were inadequate contingency and remedial measures.' The report says evidence submitted to the COI shows several breaches of Singapore law and recommends prosecution.
Nishimatsu is accused of committing two offences under the Factories Act - failing to ensure that the temporary retaining wall system was of sound construction and properly maintained, and failing to ensure the worksite was safe. The contractor could face two fines of Singapore $200,000 (£67,000).
Nishimatsu project director Shun Sugawara is accused under the Factories Act of neglect by allowing the catalogue of events leading to the collapse to build up.
Nishimatsu project co-ordinator Paul Broome is accused of two offences under the Factories Act.
He is accused of neglect by failing to ensure that the temporary retaining wall system was of sound construction and properly maintained, and of failing to ensure the site was safe.
Sugawara and Broome face fines of £67,000 and/or 12 months' imprisonment.
Broome could also be charged under the Singapore Penal Code for causing the death of the four workers through his negligence. This carries a two-year sentence.
Nishimatsu design manager Kazuo Shimada faces the same charges and penalties as Broome under the Factories Act. He could be charged under the Penal Code for causing the death of the four workers through rashness. He would also face a 24-month sentence.
As the Land Transport Authority's qualifi d person, Ng Seng Yoong was responsible for ensuring the safety of building works, performing checks on construction work. He is accused of breaching the Building Control Act for failing to monitor the excavation with due diligence.
He could be fined £3,300 and jailed for six months.
The inquiry recommends warnings be issued to Nishimatsu deputy project director Tomio Ueno, design engineer Ishibashi Atsushi, assistant geotechnical engineer Arumaithurai Ahilan, and LTA senior design engineer Shirley Jayanthi Sivakumaran.
It says warnings should also be given to soil and diaphragm wall instrumentation contractor L&M Geotechnic, as well as to its supervisor Chakkarapani Balasubramani, and to strut instrumentation contractor Monosys, and its project engineer Eugene Tang.
A Nishimatsu source said the report was still to be fully digested by the contractor's legal and technical advisers, but it appeared information had been misinterpreted and that there were factual inaccuracies. An appeal was likely, he said.