Singapore deep excavation fatalities

Singapore deep excavation fatalities inquiry puts blame on design and build contractor.

In releasing the final report of the Committee of Inquiry into the retaining wall failure at one of Singapore's MRT Circle Line cut and cover worksites which killed three construction workers and one of the Land Transport Authority's site inspectors, the island's Ministry of Manpower said: "The report lays bare a string of lamentable errors relating to design, construction, monitoring and supervision, regulatory weakness and the lack of defensive systems and emergency planning.

In the Committee of Inquiry's words, 'warnings of the approaching collapse were present from an early stage but these were not taken seriously'.

In its executive summary the committee, chaired by Senior District Judge Richard Magnus with the assistance of two assessors, says that the on-site failure which caused the fatalities and the collapse of the adjoining Nicoll Highway began with two critical design errors.

These were the under-design of both the diaphragm wall and the waler beam connections in the strutting system.

According to the Committee of Inquiry, it was these design errors that resulted in the failure of the ninth level strut-waler connections together with the inability of the overall temporary retaining wall system to resist the redistributed loads as the ninth level strutting failed.

The catastrophic collapse then ensued.

The diaphragm wall and the strutting system were key elements in the temporary works required for a short section of cut and cover tunnel in the first stage of Singapore.s Circle Line underground rail system.

The excavation being undertaken alongside the Nicoll Highway was the deepest recorded in the island's soft clay.

The contract on which the accident happened was C824, a design and build lump sum contract covering part of Stage I, awarded in May 2001 to Nishimatsu Construction-Lum Chang Building Contractors Joint Venture.

It included bored tunnels as well as cut and cover, and construction of the Nicoll Highway and Boulevard Stations.

The other part of Stage 1, C825, similarly a design and build contract, is in the hands of Woh Hup-Shanghai Tunnelling Engineering Co-NCC International AB joint venture.

As the committee said, the collapse at the Nicoll Highway did not develop suddenly.

"From the early stages of the C824 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." Two contributory factors, said the report, were the abuse of the back-analyses [of soil parameters] relating to the Type M3 wall installed by the contractor at the place where the collapse took place and the failure to institute a regular, close and effective monitoring regime.

"The two critical back analyses at Type M3 were geotechnically flawed.

There were repeated breaches of the instrumentation review levels at Type M3.

All the experts agreed that on the basis of the second back analysis for Type M3, work should not have been allowed to proceed in that area.

"The catastrophic collapse was the finale to mounting incidences and warnings in the C824 project of excessive wall deflections, surging inclinometer readings, waler beam buckling, stiffener plates buckling, ground settlement, trespass of water and soils into the excavation through cracks in the diaphragm walls, failure of concrete corbels, wailing waler beams, falling support brackets, plunging strain gauge readings, and the 'thung' sounds of distress over six hours on 20 April 2004 from the heart of the strutting system.

"The collapse" the committee found, "falls squarely on the builder, Nishimatsu-Lum Chang Joint Venture." But it added, "the Nicoll Highway collapse could have been prevented." Structural safety of temporary works What stood out for the committee in the course of its inquiry was the continuing need for safety and a safety culture where deep excavation works are concerned.

For that reason it devotes an entire chapter of its report (52 pages) to safety on the grounds that "the real question is not what safety costs but what it saves." One of most important observations that emerged from the inquiry was that temporary works were not given the same respect as permanent works.

In response to this, the Singapore Government has agreed that the structural safety of temporary works is as important as that of permanent works and they should be designed according to established codes and checked by competent persons.

In addition, said the committee, in deep excavation works, it is useful to evaluate the project on the basis of its risk profile.

It insists: "There must be a strong safety and safety culture in all construction projects.

The Government's response to the CoI's interim report agreed that safety systems and a pervasive culture of safety consciousness that permeates every level from developers down to the least skilled worker must be in place." The Ministry of Manpower is now to introduce the Workplace Safety and Health Act which will address these issues through the life-cycle of a building including design, construction and maintenance.

Some of the key points made by the committee are: Safety policies must be clear and unambiguous: for example it is not sufficient to establish emergency evacuation plans and conduct drills for site personnel.

Clear guidelines are required on what type of situations call for immediate evacuation from the worksite.

There must be effective safety management systems that recognise two types of accidents: those that happen to individuals and those that happen to organisations.

Contracting organisations must have defensive systems in place that adequately deal with hazard identification, risk avoidance and reduction, and the control of residual risk.

Since the committee's interim report was published last September, the Land Transport Authority has set up a risk register for all its sites covering safety, design and construction matters.

Site staff are now required to report immediately instrumentation readings which are above the trigger value to senior and project staff for review and follow-up.

"Unsatisfactory trends" says the final report, "must not only be identified sufficiently early, but doggedly monitored and the subsequent risks appreciated to enable corrective steps to be taken.

A regular supply of accurate and up-to-date monitoring information is essential.

Its correct and timely interpretation, including comparisons between predicted and actual design values and the trend line from the history of the movements of the temporary walls, is critical to safety".

Also, "the integrity of a back analysis is critical to safety, and is dependent on the basic assumptions that it would be done properly, honestly and in good faith.

As soon as the back analysis departs from its basic objective of safety assessment and degenerates into a curve fitting exercise for the purpose of justifying the continuation of work, it would have been transformed from a benign tool to a treacherous contrivance." The assessors were Dr Teh Cee Ing, Associate Professsor at Nanyang Technological University and Er.

Lau Joo Ming, director of the Singapore Housing and Development Board's building technology department.

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