Wongwian Yai road sinks 20cm as Purple Line tunnel leak checks widen

A follow-up inspection at the southern Purple Line construction site near Wongwian Yai has found the road surface has subsided by a further 20 centimetres, while nearby buildings are being checked for slight tilting and possible new cracks.

Deputy Transport Minister Siripong Angsakulkiat visited the site on Thursday to monitor the water-leak situation inside the tunnel under construction for the MRT Purple Line southern extension, Tao Poon–Rat Burana section. The inspection came after local residents reported large cracks in nearby buildings, raising public concern over safety.

The latest update adds a more cautious note to earlier assessments by the Mass Rapid Transit Authority of Thailand (MRTA), which said the tunnel seepage was likely caused by a faulty joint in a sump pit beneath the tunnel and that pumping and ground-improvement work were under way to control the situation.

Engineers weigh pressure-control plan

Siripong said the water level inside the tunnel had fallen on Thursday morning, but the road surface in the area had sunk by around another 20cm, prompting engineers to hold urgent talks on the safest repair method.

One option under consideration is to seal both ends of the tunnel and add water into the system to balance internal and external pressure. The aim would be to prevent soil and sand from flowing into the tunnel.

Engineers are also considering injecting fast-setting chemically mixed concrete from outside the tunnel to strengthen the structure. Siripong said any repair plan must ensure the area becomes fully safe before construction can continue.

Nearby buildings under inspection

Officials have detected slight tilting in some nearby buildings, particularly those fitted with monitoring sensors since the previous day.

When assessed alongside the condition of building extensions, officials said some cracks could be new. However, one building of concern is currently unoccupied.

All surrounding buildings are now being checked. If damage is confirmed, the construction contractor will be required to compensate and assist affected people.

No evacuation yet, but warnings planned

Siripong said the situation had not yet reached the stage where residents must be evacuated, but officials would wait for the engineering team’s full assessment.

For commercial buildings that remain in use, authorities are preparing warnings advising operators and occupants to avoid staying inside while risk remains.

He stressed that the subsidence affects the road surface, not the buildings themselves, and that the problem has so far been found only on the left side of the tunnel alignment, where the water leak occurred.

Leak followed heavy rain in Bangkok

The tunnel leak was first reported after heavy rain in Bangkok on July 8, when water entered an underground construction area near Wongwian Yai. Prime Minister Anutin Charnvirakul later inspected the site with senior transport officials and ordered close monitoring to prevent a repeat of previous tunnel-related incidents.

The affected area is part of Contract 4 of the southern Purple Line project, covering the Saphan Phut–Dao Khanong section. The contract, worth 14.982 billion baht, is being carried out by Unique Engineering and Construction Public Company Limited, or UNIQ.

MRTA says seepage likely from sump pit joint

MRTA’s latest technical assessment suggested the seepage likely came from a joint of the sump pit beneath the tunnel. The contractor has been carrying out ground-improvement work to harden the soil layer and reduce the water flow, while pumping water from the tunnel.

MRTA also said continuous measurements had been carried out at the pedestrian overpass and nearby buildings, with safety monitoring being stepped up under instructions from the prime minister and transport authorities.

Safety checks tightened across underground works

MRTA chairman Montri Dechasakulsom has instructed construction supervisors and contractors across all MRTA projects to strictly comply with safety measures, especially for underground works.

The agency has also been told to prepare emergency response plans with support from the Department of Disaster Prevention and Mitigation and police, while maintaining communication with workers and the public before work resumes.