AN INTERIM report from the Committee of Inquiry set up to investigate the Nicholl Highway cut and cover tunnel collapse in Singapore (GE May) identifies 'glaring shortcomings in the execution of the Circle Line Project' The committee also believes, according to the report published last month, that these shortcomings 'are common to the construction industry' It calls for immediate action to prevent a repeat incident.
The catastrophic collapse on 20 April of diaphragm walls, which were supporting the 30m deep cut and cover tunnel excavation, was unprecedented in scale and led to the death of four construction workers.
The report's lead recommendation is that 'there should be a comprehensive engineering review of the design and safety measures of all ongoing projects, particularly where deep excavations are being carried out' In a covering letter, the committee, chaired by Richard Magnus, told Singapore's Minister for Manpower Dr Ng En Hen that it had taken the unexpected step of producing an interim report because it was so concerned that corrective measures should not have to wait until the committee has reached its full conclusions.
The 16-page report makes 30 recommendations that address shortcomings in the temporary and permanent works, the culture of safety, professionalism and competence of subcontractors and the complicated contractual relationships among the different parties and the impact this had on 'ownership and resolution of design and site problems' The report highlights the need for a more rigorous review and inspection regime when unfamiliar technologies are used, or when a known technology is extended beyond the normal range of application. In particular, it says, 'such a review is necessary for the floating cofferdam system proposed for the Nicholl Highway Station- and the installation of jet grouted pile slabs in deep excavations needs to be similarly reviewed.'
The committee also observes that 'safety is an attitude of mind and those responsible for the safe operation of any construction works should not only be conversant with the relevant legislation but should be actively committed to a safe approach in any operation.'
With regard to instrumentation and monitoring the committee concludes that a consistent supply and collation of up to date and accurate monitoring is essential.
Furthermore skilled personnel, with the minimum knowledge, qualification and experience, must be engaged for the interpretation of the instrumentation data. And these senior managers must be experienced enough to make the right judgement call either to suspend or to stop the work when instrumentation data exceed the predetermined risk levels.
The committee devotes a whole section of the report to the professionalism and competence of sub-contractors. While it says subcontractors are essential partners in the team and deserve greater attention and encouragement, the report highlights sub-standard workmanship and quality on the Circle Line contract.
It maintains: 'Specialists did not have the basic knowledge and experience' and in particular recommends that all jet grouting piled slab works should be 'reviewed in terms of their field trials, control and supervision of installation, and the quality assurance process' The report also recommends that specialist subcontractors 'must go beyond mere contractual compliance and alert the employer of any deficiency in design, drawing and methods of construction which impacts safety'