Extracted from New Civil Engineer May 2005 Design and construction failures caused Singapore tunnel INADEQUATE highway was also destroyed. collapse'TEMPORARY The disaster was triggered by works and design the failure of a connection and construction between horizontal struts and waling beams, which between errors led to the them supported the diaphragm fatal collapse of walls, the inquiry heard last Singapore's week. deepest ever cut The explanation was given and cover tunnel, the public as part of a summary of inquiry into the disaster heard evidence submitted to the last week. Committee of Inquiry in The collapse hit a 110m section Singapore. of tunnel being con structed for The general causes of the Singapore Mass Rapid Transit's col lapse were agreed last month new Circle Line, adjacent to the by client the Land Transport six lane Nicoll Highway. Authority (LTA), main con tractor Four workers died when steel Nishimatsu-Lum Chang joint struts supporting the excava- venture (NLC), NLC's designer tion's diaphragm walls failed, Maunsell Asia, NLC project causing the tunnel to cave in on engineer Paul Broome, base slab subcontractor L&M, 20 April last year. Part of the strutting subcontractor Kori, persisted in using it and refused had S$25M (£83M) against it in to change." claims for late delivery against Combined with further errors schedule. Nishimatsu-Lum Chang for undrained soils, Shanmugam in temporary works design, this "If LTA knew that NLC's (NLC) was negligent, stated. led to strut-water connections design had serious defects, and reckless and dishonest "Use of Method A was being under-strength by a factor that NLC was concerned and during design and grossly erroneous. It was a of two, he told the inquiry. uncomfortable about its own construction of the collapsed substantial error. Use of Method LTA claimed that the design, the LTA would probably section of tunnel, client A led to serious underprediction problem was compounded by have stopped work, and in these Singapore's Land Transport of the forces acting on the NLC's substitution of C-channel circumstances NLC would have Authority (LTA) claimed last temporary works, and hence led shaped steel sections for plate had to bear the costs of delay on week. to underdesign in the temporary stiffeners to strengthen the strut- its own." NLC's catalogue of design works in general. wafer connection. Even after struts failed at errors started with the use of "This was an error which NLC cut costs by using two adjacent NLC sites the inappropriate soil analysis during bedevilled the entire design, and "scrap material to replace contractor maintained that its the early stages of temporary consequently the entire system stiffener plates which they had temporary works designs were works design, LTA's counsel had insufficient capacity to cater run out of," Shanmugam stated. satisfactory, and pressured K Shanmugam told the for the loads coming onto it." "NLC ignored its own risk LTA to allow it to resume work, inquiry into the Nicoll NLC's soil analysis meant analysis which stated that a Shanmugam said. Highway collapse. that as excavation got deeper fundamental reassessment NLC breached its legal Finite element analysis of deflections of the diaphragm wall had to be done in light of duties to reveal key information, ground conditions used the increasingly exceeded those stiffener plate buckling." he added. "LTA never had predicted. mechanical properties of drained The situation worried NLC, sufficient material information soil - known as Method A. For Shanmugam insisted that but it sought to suppress from NLC to justify exercising its deep excavation of the Nicoll "NLC knew from the outset in information to prevent LTA contractual powers to stop work." Highway tunnel in the highly 2001 that there would be interfering in its construction plastic marine clays found on potential problems with Method schedule, Shanmugam site, NLC should have used data A, but recklessly and dishonestly claimed. By April 2004 NLC Extracted from New Civil Engineer May 2005 diaphragm walling subcon- diaphragm walls apart during tractor Bachy Soletanche, cut and cover tunnel works. project insurer Aviva, LTA These connected with hori- project directors Ng Seng Yoong zontal walers running along the and Sripathy and Singapore's tunnel walls (see box overleaf). Ministry of Manpower. The catastrophic failure Although the general col lapse started at the ninth level strut- scenario is agreed, the details waler connections, 30m below are being disputed by the ground, and only 3m above contractor and the LTA (see formation level, the Committee boxes). of Inquiry heard. In all, nine out of 10 levels of Yielding of the level nine temporary horizontal struts had connections allowed the diaphragm wall to deform, been installed to hold the Unforseen downward movement relative to king posts had been NLC dismissed the importance in diaphragm walls triggered the observed on other sections of attached by the client LTA to Nicoll Highway tunnel collapse, Singapore's Circle Line cut and the use of drained soil data in main contractor Nishimatsu-Lum cover excavation. But there finite element analysis of Chang jv (NLC) claimed before was no evidence of this at ground conditions on site. the public inquiry last week. Nicoll Highway, said the It said this was appropriate as, A sudden drop in the height contractor. "Collapse was not at shallower depths, it tends to of the wall relative to king posts inevitable. The collapse give conservative results, supporting the temporary struts occurred because the forced although there was no precedent altered the angle at which struts sway mechanism dramatically for such deep excavation using connected with the walers. This reduced the time in which it Method A in Singapore. During construction NLC caused them to deform rapidly, developed," claimed NLC During the early stages of discovered a buried channel leading to "sway failure", the counsel Philip Jeyaretnam. construction, NLC claimed LTA in the old alluvium base contractor said. Although the struts were accepted that deflections strata into which the Sway failure happens when close to the limit at which they measured on site were close to diaphragm walls were toed. the junctions between the flanges could fail, there was no evidence those anticipated using Method A This was filled with deep, and web of an I-beam act as that loads exceeded the ultimate finite element analysis, soft, organic clay. There was hinges, causing the beam to capacity of the temporary works, Jeyaretnam said. also a bowl of soft estuarine flatten under heavy loading. he added. "There was no NLC's independent clay below the marine clay NLC coined the term evidence of a build up of loads engineer Dr Andy Pickles of which covers most of the "forced sway failure" to before 20 April. consultant CPG had briefed site. Neither was detected describe the violent forcing of "If the loads exceeded actual LTA on the soil analysis in during site investigation. this mode of collapse. capacity the connections would May 2002 and LTA's The temporary works had not Movement of diaphragm walls have failed as excavation supervising engineer had been designed for these beneath each strut reached the described alternative conditions, and had to be 10th level, or as loads built up analysis models as over- adapted, said NLC counsel with time after completion of cautious, he added. Philip Jeyaretnam. excavation.. Extracted from New Civil Engineer May 2005 overloading struts in levels above, causing them to buckle. This triggered a progressive collapse of the tunnel walls. The failure was rapid. Only an hour elapsed between failure of the first strut-waler connection and total collapse of the excavation. Overloading of the temporary works coincided with the excavation of a sacrificial prop installed using jet grouting methods just below the ninth level struts. NLC admitted to the inquiry that failure of the temporary works resulted from underdesign and inappropriate detailing of the strut-waler connections. NLC's design engineers misinterpreted building code BS5950, said counsel for NLC Philip Jeyarentnam. This resulted in the adoption of smaller than required steel sections for the struts, reducing redundancy in the design. The collapse occurred east of installation of reinforced the planned Nicoll Highway concrete diaphragm walls station in a section of cut and and two deep level jet- cover tunnel adjoining a grouted slabs before large diameter reinforced excavation got under concrete shaft. way. This had been built in The lower slab formed the preparation for the tunnel base while the upper launch and extraction of one was sacrificial, located a tunnel boring machine, 6m above the base slab. which was to drive a It was installed to section of tunnel under provide temporary the Kallang river. support to the The collapsed cut and cover diaphragm walls. section was on a curve in an The sacrificial slab was to area of reclaimed land, consisting of 6m of made ground over 27m of soft marine clay and 5m of estuarine clay. It was unusually deep, with a base slab at 33m below ground level. This was to accommodate the LTA's plans to build a future road tunnel above the rail tunnel. Construction involved the Extracted from New Civil Engineer May 2005 But this design deficiency NLC replaced waler web plate was made drastically worse by stiffeners with a C-channel omission of load spreading stiffening detail - a "C" shaped splays at the ends of the struts piece of steel commonly used during construction (see in Nishimatsu's home country diagram). Japan. The contractor's failure to All parties agreed that the include these crucial sudden failure of this detail on structural components in the the level nine strut-waler con- strut-waler connection was not nections caused the collapse.