SMRT said it accepted full responsibility for the incident where flood waters 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, affecting 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 functional, as were three storm water sump pumps at Bishan station that could be manually activated following the incident.
LTA in its report listed three possible reasons why the system had failed to prevent the flooding:
1) Accumulated silt and sludge in the storm water pit could have impeded the lowest ‘stop’ float switch from activating and prevented the storm water sump pumps from being turned on.
2) Floating debris in the pit could also have prevented the highest ‘alarm’ float switch – which is wired independently from the other four float switches – from activating and alerting the SMRT Operations Control Centre to rising water levels in the pit.
3) SMRT staff who conducted corrective maintenance on July 13 could also have failed to switch the pump controls back to automatic mode after completing their work.
LTA said it could not definitively say which of these possible scenarios had happened and that any of these or a combination of them could have led to the incident.
“Nonetheless, all three possible failure scenarios could only arise as a result of a lack of proper maintenance, audits and supervision,” it said. Following the incident, silt, sludge and debris had to be cleared from the sump pit, it added.
SMRT sacked eight employees and disciplined three others, following investigations into the falsification of maintenance records for the flood prevention system. Two others, who were also responsible, resigned shortly 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 mitigate the risk of a recurrence.
These include replacing the pumps at Bishan station with heavier-duty ones capable of handling water with sediments to overcome any possible gaps in maintenance.
An additional set of parallel float switches has also been installed, as well as a radar-based sensor system to monitor water levels in the storm water sump pit independently.
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