The Land Transport Authority on Tuesday (Dec 5) released its investigation report into the Oct 7 MRT tunnel flooding that left a stretch of the North-South Line without train services for more than 14 hours. This is the report in full:

On 7 Oct 2017 at 5.14pm, a train captain reported seeing water in the southbound tunnel between Bishan and Braddell MRT stations on SMRT’s North-South Line.

At 5.27pm, the duty Braddell station manager reported the same in the northbound tunnel, and that waters had risen above the train tracks.

Within three minutes, the SMRT Operations Control Centre (OCC) suspended train services on both bounds between Ang Mo Kio and Toa Payoh MRT stations.

By 5.38pm, the water at the deepest section of the tunnels had risen to almost meet the electrified third rail. A decision was immediately made to de-train all passengers at station platforms, so that no commuters would be left on board trains in the affected tunnels.

As a further safety precaution, SMRT OCC also cut off all traction power to the affected stretch.

By the evening of the incident, flood waters at the deepest point in the tunnels had reached up to 1 metre in depth and extended for 100 metres between Bishan and Braddell stations. As such, train services between Ang Mo Kio and Newton MRT stations were suspended from 5.30pm until the end of operating hours on 7 Oct 2017.

With the assistance of PUB and SCDF officers who worked through the night with SMRT and LTA, flood waters were cleared by 11.06am the following morning. Thereafter, normal train services resumed at 1.36pm on 8 Oct 2017, following the completion of safety and operational checks of the train tracks.

This flooding incident resulted in a cumulative service disruption of 14.5 hours – 7 hours on 7 Oct 2017 (Saturday), and 7.5 hours on 8 Oct 2017 (Sunday). During the service disruption, free boarding of public buses and free bridging buses were provided by SMRT and SBS Transit along the affected stretches.

Announcements on the status of MRT train services and alternative transport options were made network-wide in MRT stations and on-board trains, as well as on SMRT’s and LTA’s websites, and through mainstream and social media channels.

SMRT has accepted full responsibility and apologised for the incident which affected some 231,000 commuters.

SMRT’s internal investigations and disciplinary inquiry found 13 staff responsible for lapses in maintenance and/or supervision. This includes the work team who had falsified pump maintenance records, their supervisors, as well as other management executives who had failed to exercise sufficient supervision, due care and diligence in relation to the maintenance of the pumps.

Eight of the staff have since been dismissed, two have resigned, and three others have been disciplined.

KEY FINDINGS

Adequate Design of Flood Protection Measures

The Bishan stormwater sump pit is designed with a capacity of 5,044 m3, and is equipped with three high capacity pumps (each with a flow rate of 85ℓ/sec) that extract water to an external surface drainage system.

The flood protection system at Bishan, which has safeguarded the tunnels well for the last 30 years, has a huge capacity buffer.

The 640 m3 of rainfall that was cleared from the tunnels between Bishan and Braddell MRT stations amounted to only 13 per cent of the capacity of the stormwater sump pit. Had the pump system and stormwater sump pit been properly maintained, our assessment is that the flooding incident would have been preventable as the pit should have been empty prior to rainfall on 7 Oct 2017.

Float Switches, Pumps and the Pump Motor Control Panel were Functional

Independent post-incident tests by Singapore Test Services (STS) found that all five float switches and the pump motor control panel at Bishan were functional.

The three pumps in the Bishan stormwater sump pit were also found to be functional, as SMRT staff were able to manually activate them to clear the accumulated water in the Bishan stormwater sump pit from about 9.00pm on 7 Oct 2017.

As part of the LTA investigation, subsequent tests by STS have focused on why these pumps did not operate as they should on 7 Oct 2017.

Water Likely Accumulated in Storm Water Sump Pit Starting Early July 2017

The pump event logs kept at SMRT revealed that all three pumps in the stormwater sump pit did not operate from 14 Jul 2017 until they were manually activated at about 9.00pm on the day of the incident.

Rainfall data over the period of pump inactivity suggests that accumulated rainwater could have filled at least 87 per cent of its 5,044 m3 capacity by 7 Oct 2017.

Lack of Maintenance of Bishan Storm Water Sump Pit

Post-incident inspection of the Bishan stormwater sump pit found debris, plant detritus and silt/sludge, in areas, of up to about 400mm in depth – indicating long-term neglect or improper upkeep of the sump pit.

The build-up of materials not only reduced the holding capacity of the sump pit, but could have also impeded the proper functioning of the float switches, and hampered the efficiency of the pumps, one-way non-return valves, and the piping system along which the extracted water would flow to the surface drainage.

The gratings above the pumps in the Bishan stormwater sump pit also easily enabled maintenance staff to assess the water level inside without the need to enter the sump pit during engineering hours.

No Preventive Maintenance on the Bishan Storm Water Sump Pit and Pump System for at least 9 months

SMRT’s preventive maintenance regime requires the pumps and float switches to be maintained every three months. This is supplemented by a more comprehensive annual inspection.

However, SMRT’s internal investigations showed that there had been no record of track access on the purported maintenance dates, pointing to the fact that the maintenance team for the Bishan pump system not only failed to carry out preventive maintenance on the purported maintenance dates, but also falsified maintenance records.

POSSIBLE FAILURE SCENARIOS

Following the flooding incident, STS was engaged to conduct independent laboratory tests of components of the Bishan stormwater pump system.

Representatives from LTA, SMRT and the national water agency PUB, participated in the tests and reviewed the results.

As highlighted earlier, STS found that all five float switches and the control panel that were in use at time of the flooding incident were functional.

The three pumps in the Bishan stormwater sump pit were also functional, as they could be manually activated after the incident to clear the accumulated water in the Bishan stormwater sump pit.

Based on further laboratory tests by STS, there are three possible failure scenarios for why the Bishan stormwater sump pump system, while functional, failed to work as intended.

These are:

(i)            Lowest Float Switch Likely Impeded by Accumulated Silt/Sludge. The “stop” float switch is the lowest and could have been stuck in the sludge which accumulated in the Bishan storm water sump pit.

STS established that a float switch with more than 25% of its surface submerged in sludge (using actual sludge removed from the Bishan storm water sump pit) could be impeded from floating and functioning properly.

To close the electric circuit, the float switch must float with its rounded-end up.
 

(ii)           Failure to normalise pump controls back to “Automatic” mode after corrective maintenance on abnormalities with the functionality of the pump control panel for the different operating modes, i.e. “Automatic”/ “Manual” / “Off” mode for the pumps and “Normal” / “Bypass” mode for the float switches.

When set to “Manual” and “Bypass” mode for the pumps and float switches respectively, the pump control panel would be able to override the triggering of the pumps by the float switches, as intended.

Based on SMRT’s records, the maintenance team responsible for the Bishan pump system was recorded as having carried out corrective maintenance works on 13 Jul 2017 in response to reports of frequent pump trips.

To do this, the team would have had to access the pump control panel to toggle the pumps between “Automatic” and “Manual” modes, in order to test their functionality. These activities were recorded in the pump event logs as multiple pump activations between 3.21am and 3.32am that night.

As the pump event logs indicated no further pump activations after that night, until about 9.00pm on 7 Oct 2017 when they were manually activated to respond to the flooding incident, this could indicate that the pump controls were not put back to “Automatic” mode after corrective maintenance was done.
 

(iii)          Floating debris could also impede the float switches from operating normally. STS tests revealed that accumulated floating debris and other flotsam in the storm water sump pit (as shown in Figure 2 above) could have impeded normal functioning of the float switches, in particular the “alarm” float switch which sends an alert to the SMRT OCC when high water levels are detected. STS investigations also ascertained that the “alarm” float switch triggered no such alarm to the SMRT OCC.

In relation to failure scenario (ii) above, since the “alarm” float switch is wired independently from the other pump float switches, it should have activated even if the pump control panel was left in “Manual” mode after the corrective maintenance on 13 Jul 2017.

Precisely which of these three possible failure scenarios, or a combination of them, led to the events of 7 October 2017, cannot be established definitively, as the exact conditions cannot be recreated following immediate remedial works to the Bishan stormwater sump pit and pump system following the incident.

This includes clearing of silt, sludge and debris from the sump pit as well as the one-way valves, the replacement of the float switches, the installation of new pumps, and the moving of the control panel away from trackside followed by its replacement. These remedial works are necessary and cannot be deferred given the monsoon season.

IMMEDIATE REMEDIAL MEASURES

Following the incident, SMRT and LTA swiftly implemented a number of enhancements to augment flood protection infrastructure at the Bishan stormwater sump pit and to further mitigate the risk of a recurrence.

Firstly, SMRT has replaced all existing float switches at the Bishan stormwater sump pit.

Secondly, LTA has replaced the pumps at Bishan with heavier-duty ones capable of handling water with sediments to overcome any possible gaps in maintenance. It has also installed additional parallel float switches.

Thirdly, a new radar-based sensor system has been added to monitor water levels in the stormwater sump pit independently.

Fourthly, a new sump pump control panel has been installed, in addition to relocating the sump pump control panel away from the tracks to facilitate quicker access to pump controls during operating hours.

Finally, the accumulated sludge, silt and debris in the sump pit has been removed. SMRT has also replaced and repaired all non-serviceable pumps discovered at other tunnel portals within the North-South and East-West Line network, with the float switches also replaced as an added precaution.

CONCLUSION

From the available evidence and the tests conducted by STS, LTA’s investigations concluded that the flooding incident which occurred in tunnels on both bounds between Bishan and Braddell MRT stations from 7 to 8 Oct 2017 was preventable if SMRT’s maintenance team diligently maintained the Bishan stormwater sump pit and pump system, and kept them in good and efficient working condition.

If regular preventive maintenance had been carried out by SMRT’s maintenance team, as stipulated under the maintenance protocol, the abnormalities with the pump system could have been detected and rectified before the flooding occurred.