The flooding of the tunnel near Bishan MRT station on Oct 7 was "entirely preventable" had SMRT carried out proper maintenance, the Land Transport Authority (LTA) said yesterday when it unveiled the findings of its investigation.
Rail operator SMRT said it accepted full responsibility for the incident in which floodwaters reached up to a metre in depth in train tunnels and led to a section of the North-South Line being shut for 14½ hours.
The shutdown affected 231,000 commuters.
Independent laboratory tests by Singapore Test Services (STS) found that the individual components of the Bishan storm water pump system - part of a system to prevent tracks from flooding - were not defective. All five float switches and the pump motor control panel were working.
Three storm water sump pumps - each capable of removing 85 litres of water per second - at Bishan station were also found to be in working condition, as they could still be activated manually following the incident.
The LTA said that the flood protection measures were "more than adequate" to handle the Oct 7 rainfall.
"The storm water sump pit has a capacity of 5,044 cubic metres, which is much more than the estimated 640 cubic metres of rainfall that was cleared from the tunnels between Bishan and Braddell MRT stations," an LTA spokesman said, adding that the flood protection measures have a "huge buffer".
Further STS laboratory tests found three possible reasons the system failed to prevent the flooding.
One was that the lowest float switch, which would have activated the storm water sump pumps, could have been obstructed by accumulated silt and sludge in the pit.
Another was that floating debris in the pit could have impeded the normal functioning of the float switches - in particular, the highest float switch, which alerts the SMRT Operations Control Centre to high water levels.
A third possible reason was that an SMRT maintenance team - which repaired the pump system on July 13 after reports of frequent pump trips - could have left the pump controls in manual mode.
LTA said it could not say definitively which of the three reasons was behind the system's failure, and that any combination of these could have led to the flooding of the tunnel.
"Nonetheless, all three possible failure scenarios could only arise as a result of a lack of proper maintenance, audits and supervision," it added, noting that silt, sludge and debris had to be cleared from the pit after the incident.
LTA added that SMRT had also made "numerous engineering enhancements to improve the robustness and resilience of flood protection measures" at Bishan station and other tunnel portals to further reduce the risk of a recurrence.
These include replacing the pumps at Bishan station with more durable ones that can handle water with sediment, and installing an extra set of parallel float switches and a radar-based sensor system to monitor water levels .
A new pump control panel - accessible to maintenance staff even during passenger train services - has also been installed, said SMRT corporate communications vice-president Patrick Nathan.
He added that all non-serviceable pumps on the North-South and East-West lines had been replaced following checks, with float switches also replaced as a precaution.
While extra measures may help, improving the culture of maintenance is more important than adding "safeguard after safeguard", said Mr Chong Kee Sen, former president of the Institution of Engineers, Singapore.
He added that in any organisation, it is important for top management to engage with maintenance teams to boost their morale and ensure they understand the importance of their work.