Repairs stepped up at Purple Line tunnel site

Engineers using sensitive equipment to monitor subsidence around the clock

The Bangkok Metropolitan Administration (BMA) is stepping up efforts to address water seepage inside the under-construction Purple Line extension tunnel and road subsidence on Prajadhipok Road, while expanding safety measures for nearby residents with round-the-clock monitoring.

Bangkok Governor Chadchart Sittipunt, accompanied by civil engineering expert Thanes Weerasiri, visited the site in the Wong Wian Yai area of Khlong San district on Saturday to monitor efforts to stabilise the area following reports of water leakage inside the tunnel and settlement of the road surface.

The seepage was first reported on Wednesday night after heavy rain affected the tunnel under construction for the Southern Purple Line MRT project (Tao Poon-Rat Burana section) near the Wong Wian Yai station. City Hall evacuated people from nearby buildings and closed the road near the site.

Mr Chadchart, who began his second term as governor this week, ordered the removal of the pedestrian bridge across the construction site after subsidence was detected at its midpoint.

The move will also facilitate the installation of four grouting machines to inject cement-based materials into the ground to strengthen soil conditions.

Workers have begun drilling to a depth of 37 metres, below the tunnel base level of 31 metres, in an effort to reinforce underground soil, prevent further water ingress and stabilise the surrounding area.

Mr Thanes said monitoring equipment installed in cooperation with Kasetsart University showed no significant movement in surrounding buildings.

Engineers were using tiltmeters to detect structural leaning and inclinometers to measure horizontal and vertical ground movement. Data from the highly sensitive instruments is being transmitted via a wireless network to the BMA control centre, allowing officials to monitor conditions around the clock.

Despite rainfall overnight, groundwater levels remained stable and did not worsen the situation, said the engineering expert.

The tunnel incident differed from last year’s sinkhole incident at Vajira Hospital because there was no opening at the surface allowing water to flow downward. The seepage appeared to be originating from below ground, said Mr Thanes.

However, minor cracks have been observed on the road surface, prompting authorities to continue close monitoring and maintain heightened caution.

Mr Chadchart reported that the water seepage had affected 141 residents of two apartment buildings nearby, with some of them reported to have left the area. Fifty-two residents reported being temporarily evacuated, while 34 remained in place.

The governor said the BMA had set up a complaint centre in the area to assist residents and explain the close monitoring measures in effect in the area.

Prajadhipok Road remains closed from the King Taksin the Great Monument to the Yaek Baan Kaek intersection. Drivers are advised to use Issaraphap, Lat Ya and Somdet Chao Phraya roads instead while repairs take place, said Mr Chadchart.

The civil works for the Purple Line extension have been split into six different contracts, with the MRTA awarding them to five major Thai construction companies.

A joint venture between Italian-Thai Development Plc and Nawarat Patanakarn Plc is responsible for the 3.1-kilometre underground section that passes beneath the Chao Phraya River.