On April 20, 2004, six lanes of the Nicoll Highway in Singapore disappeared into a 100-foot hole when a tunnel being constructed underneath the highway collapsed.

Incredibly, no one was driving on the usually congested highway. The tunnel was part of the underground Circle MRT (Mass Rapid Transit) Line, and the supporting structure for the excavation work had failed. Four workers were killed in the incident and three others were injured.

Rescuers hunted in vain for survivors for three days before efforts were called off; by this stage, it was clear that the danger to the rescue teams far outweighed the likelihood of finding any survivors. The other concern was the need to stabilize the ground around the collapse in order to ensure that no further collapses would occur.19 The immediate reason for the collapse was that the retaining wall holding up the evacuation work was insufficient to hold up the tunnel. Two construction cranes had fallen into the hole, and there was evidence of twisted steel support beams.

Initial Response Authorities in Singapore instituted an inquiry into the incident. They also suspended 20 other ongoing excavation projects at great expense and delay while new best practice codes were put in place. Meanwhile, experts from across the world were brought in to investigate the accident and determine exactly what had happened. It became clear that a number of factors had contributed to the collapse.

Most significant was the fact that geological findings had been misinterpreted. The structure of the tunnel had been under-designed as the engineers had assumed that the soil’s shear strength was greater than it actually was.

There were also issues with the structural bracing system being used. The collapse had taken place at a point where the bracing was overloaded, and the system lacked the capacity to redistribute the load between the other supports if this part of the bracing failed. The engineers had underestimated the strut loads.

Apportioning Blame An inquiry committee in May 2005 was told that the disaster was caused by a failure of a connection between horizontal struts and waling beams, which supported the diaphragm walls. The general causes of the collapse had already been agreed upon by the Land Transport Authority (LTA) and Nishimatsu-Lum Chang (NLC), who were the main contractors in the joint venture; the NLC lead designer Maunsell Asia; the project engineer Paul Broome; and L&M and Kori (subcontractors).

The LTA’s View According to the LTA, NLC was negligent, reckless, and dishonest during the design and construction. The LTA stated that the design errors had begun with the soil analysis at the earliest stages of the project. According to K. Shanmugam, the LTA’s counsel, analysis of the ground conditions had been based on the use of “Method A,”

which looked at the mechanical properties of drained soil.

The soil encountered in the deep excavation was, in fact, highly plastic marine clay, and NLC should have used data for undrained soils. By using the wrong method, NLC under-predicted the forces that would act on the work being carried out. This led to an under-design in the temporary supports. As a result of this, the system being installed did not have the capacity required. The incorrect soil analysis also meant that, as the excavations got deeper, the errors and potential failures became more acute. The strut connections were under-strength by a factor of two.

NLC also substituted C channel shaped steel sections for plate stiffeners in an attempt to strengthen the connections. However, NLC had tried to cut costs by using scrap material to replace the stiffener plates when they had run out of supplies. The LTA went on to claim that NLC had ignored its own risk assessments. Some of the stiffener plates were already buckling, but NLC had hidden this from the LTA in order to keep them from insisting on additional work. NLC was already behind schedule in April 2004 and had incurred late penalties of some $25 million. The LTA would have undoubtedly ordered NLC to cease work had it known about the problems, and NLC would have had to bear the cost of the extra delays to the work schedule. Similar struts had failed on two other NLC sites, but it had insisted that the problem was in hand and pressured the LTA to allow work to continue. It was therefore the LTA’s contention that NLC had failed to reveal enough information for the LTA to make an informed judgment.

NLC’s View NLC was certain that the reason for the collapse was unforeseen downward movement of the diaphragm walls. According to NLC:

• There was a sudden drop in the height of the wall relative to the posts that were supporting the temporary struts.

• This changed the angle at which the struts were connected.

• In turn, this caused the walls to deform and fail.

NLC blamed sway failure, which occurs when violent forces act on diaphragm walls. The phenomenon had been seen in other parts of the Circle Line, but not at the Nicoll Highway. NLC maintained that the collapse of the tunnel was not inevitable but was probably caused because the forced sway accelerated the failure. NLC admitted that the struts were close to the limit of their performance, but maintained that the loads did not exceed the capacity of the temporary work they had carried out. Furthermore, NLC expressly countered the LTA’s suggestion that the soil analysis was the root cause of the disaster, contending that

• It was appropriate to use drained soil data, as it provides a more conservative analysis.

• Although Method A had not been used for deeper excavations in Singapore before, the LTA was aware of the method being used and generally agreed with the findings.

• The LTA had been specifically briefed about the soil analysis as early as May 2002.

• At that time, the LTA’s own engineer had stated that other soil analysis types were too conservative.

NLC’s Admissions The failure took place at the ninth-level strut connections, which was around 30 meters below the ground and just 3 meters above formation level. As the connections failed, the diaphragm wall deformed. This overloaded the struts, which caused them to buckle. In turn, this triggered a gradual and progressive collapse of the tunnel walls. Just one hour elapsed between the failure of the first strut connection and the complete collapse, which triggered devastating damage to the highway.

NLC admitted that there had been a failure in the temporary works. They put this down to under-design and inappropriate detail of the connections. They also admitted that their own engineers had misinterpreted the relevant building code, which had resulted in the use of smaller steel beams than were actually needed for the struts.

The greatest engineering disaster of the decade?

Although the general causes for the collapse of the highway were agreed upon between all parties in 2005, by May 2012, just one person had been prosecuted for their part in the disaster. Nonetheless, many lessons had been learned. Work on the MRT commenced once more, but this time, far more heavy-duty temporary supports were demanded. Additional robustness in design is now demanded of contractors.

From the outset, braced excavations were used for transport construction projects in Singapore, and each of the MRT stations were constructed using the “bottomup”

method. This meant excavating and propping up the excavations with steel struts. These were supported at their mid-span by king posts and beams across the face of the diaphragm walls. The struts were placed at 3-meter intervals. Today, the “top-down” method is preferred.

This involves constructing the station with a permanent reinforced concrete roof slab. The slab operates rather like a huge strut, which means that struts are not necessary. At a stroke, the time-consuming installation of struts has been eliminated, and the safety risks are far lower.

Government Response In the immediate aftermath of the inquiry, the Singapore government accepted the findings in full and was quick to announce a series of new legislation that would improve safety standards in the construction industry.

The Joint Review Committee, made up of members from various government departments, announced that there would be far stiffer penalties for professionals who had shown dereliction of duty and care. A new licensing scheme was introduced for specialist contractors.

Additional training and the development of a code of practice for deep excavation work were all part of a raft of new legislation. The Singapore government was convinced that the failures that led to the collapse of the highway and the deaths had been entirely avoidable and were unacceptable. From this point, all major construction projects would be audited in terms of their safety. The government was determined to point out that small, incremental improvements would not bring about the level of safety that is expected of such projects.

As a result, it was necessary for several government departments to prioritize safety issues and bring in sweeping reforms.

With the reforms in place, the pace of work to extend the MRT system was as rapid as before, but far safer. For the Downtown Line, seen as the key to the development of the Marina Bay area, excavations were planned at 24 meters. In order to counter the weak soil, thick diaphragm walls were used, and these were stiffened by cross walls and slabs. Even the cut and cover tunnels (just like the one that caused the collapse at Nicholl Highway) had up to seven levels of strutting to support them. Lessons were learned, and legislation was enacted to back them up.

Questions

1. In what ways were the project’s planning and scope management appropriate? When did the planners begin to knowingly take unnecessary risks? Discuss the issue of project constraints and other unique aspects of the tunnel in the risk management process. Were these issues taken into consideration?

Why or why not?

2. Conduct either a qualitative or quantitative risk assessment of this project. Identify the risk factors that you consider most important for underground tunnel construction. How would you assess the riskiness of the project? Why?

3. What forms of risk mitigation would you consider appropriate for this project?