Float switches at Bishan MRT likely faulty before Oct 7 flooding, bottom of water storage pit lined with sludge

SINGAPORE — In a sign of how poorly SMRT maintained its anti-flooding system, investigations revealed the rainfall on Oct 7 would have filled only 13 per cent of the stormwater pit under the entrance to the Bishan MRT tunnel.

Instead, the water flooded the tunnel leading to the underground Braddell station, causing a disruption to a stretch of the North South line for about 20 hours.

It was likely the 5,044 cubic metre stormwater pit was already close to full on Oct 7, indicating the float switches to pump water out of the pit were faulty before the incident, said the Land Transport Authority (LTA) on Monday (Oct 16). Most of the 640 cubic metres of rainwater collected went into the tunnel, it said.

Checks indicated a fault with the switch to pause all pumps when the water level is near the bottom of the stormwater pit, said the authority. This is an important safety feature to prevent pumps from running in the absence of water, which would damage parts such as the shaft and impeller and cause the pump to overheat.

Separately, the “high water level” switch also failed to send an alarm to SMRT’s operations command centre when the water rose beyond the critical level, said the LTA.

SMRT said one empty train was stalled in water between Bishan and Braddell stations, with commuters already safely on the station platforms during the incident.

The operator cleared water from the tunnel, then replaced and tested trackside equipment before resuming service. Its engineers manually ran the three pumps at the Bishan pit and cleared the water in it. But water in the Braddell tunnels remained trapped and it had to seek help from the Singapore Civil Defence Force and national water agency PUB to deploy emergency pumps and hoses.

SMRT said the Bishan portal pumps and float switches were last replaced in 2011 and had their most-recent quarterly checks on June 18. The next servicing (annual and quarterly checks) were planned on Oct 12 – five days after the incident occurred.

Defects were also found – and rectified – when SMRT conducted checks on other portal sump pump systems at four other locations: Lavender-Kallang, Kembangan-Bedok; Redhill-Tiong Bahru and Expo-Changi Airport. It did not specify how many defects were found.

The operator will beef up measures to address its failure to prevent the incident. It will replace all portal pumps as a precautionary measure, have radar sensors as “redundancies” for pump activation and notification of the operations command centre, among other measures. It will also introduce emergency equipment to enhance flood-control measures.