The Purple Line electric train project, from Tao Poon to Rat Burana (Kanchanaphisek Ring), is closely followed by the Thai public awaiting its completion and opening. The route crosses densely populated old city areas, passes several important historical sites, and includes a tunnel beneath the Chao Phraya River. Once operational, the line will alleviate traffic congestion, shorten travel times for commuters, and reduce pollution emissions.
Although the Purple Line South project has encountered issues such as the subsidence on Samsen Road near the construction site at Wachiraphayaban Station (PP19), the Mass Rapid Transit Authority of Thailand (MRTA), the project's owner, continues to push forward to complete the project on schedule. Construction adheres to stricter safety standards, while repair work in the affected areas proceeds continuously under stringent safety supervision to build maximum confidence among users.
MRTA emphasizes the highest safety standards in construction.
Mr. Kitti Aekwallop, Assistant Governor of MRTA and Acting Deputy Governor (Finance), who is also Director of the Purple Line South Project from Tao Poon to Rat Burana (Kanchanaphisek Ring), stated that MRTA executives regularly visit construction sites to monitor strict compliance with maximum safety measures. They inspect the quality of machinery, equipment, and materials to ensure they meet international standards and rigorously follow all relevant safety protocols. Modern technology and innovations are employed at every stage of the operation.
As of the end of April 2026, overall construction progress stands at 72.69%. Contract 1, Tao Poon to National Library, is 85.14% complete; Contract 2, National Library to Phan Fa, 82.06%; Contract 3, Phan Fa to Phut Bridge, 66.61%; Contract 4, Phut Bridge to Dao Khanong, 71.74%; Contract 5, Dao Khanong to Khlong Noi, 52.24%; and Contract 6, design and construction of the rail system along the route, 63.25% complete.
Plans unveiled for constructing the "Chao Phraya River Underpass Tunnel."
Mr. Kitti added that a key challenge of the Purple Line South project is constructing the tunnel section beneath the Chao Phraya River in Contract 3, between Phan Fa and Phut Bridge. The project must contend with riverbed soil conditions, high underground water pressure, and limited space while ensuring no adverse impact on the river, infrastructure, or surrounding communities.
To meet international standards and ensure safety, MRTA has carefully planned and prepared, employing suitable technology such as the Earth Pressure Balance (EPB) tunnel boring machine. This internationally recognized technology is used in many countries and major cities worldwide. The EPB system controls the pressure at the tunnel face to balance soil and underground water pressure by coordinating the cutter head, soil chamber, screw conveyor, and an automated sensor system monitoring real-time parameters.
Additionally, the tunnel boring machine technology selected comes from Japan, proven effective in multiple projects in Thailand. This choice enhances safety standards, precision, and construction quality, assuring the public that the project is both safe and community-friendly.
Importantly, the tunnel design features a robust structure meeting international safety standards. The tunnel depth is optimized to ensure sufficient soil coverage, and structural strength includes safety margins as per design standards. Parameters such as cutter head pressure, rotation speed, soil conveyance rate, and others are calculated specifically for conditions ranging from soft to hard clay soils in Bangkok, enabling precise operation along the entire tunneling route.
Detailed geological surveys have been conducted, including soil drilling and sampling for laboratory testing, alongside hydrological surveys using 3D scanning technology. This provides information on riverbed depth and condition along the tunnel path and records water level data of the Chao Phraya River throughout the construction period.
Readiness checks for personnel and technology throughout operations.
The project places great importance on thorough preparation before work begins, including inspecting the tunnel boring machine to ensure continuous operation without maintenance breaks. The workforce is organized for 24-hour operation. During tunneling, real-time drilling control measurements are taken continuously, alongside monitoring of structural movement and deformation inside the tunnel.
Simultaneously, soil and building movements on both riverbanks are monitored to detect any potential impacts. The movement of the nearby Phra Pok Klao Bridge is also measured to guarantee construction safety throughout the project duration.
Mr. Kitti further reported that the tunnel boring under the Chao Phraya River is divided into two parts: the Northbound tunnel and the Southbound tunnel. The Northbound tunnel, approximately 220 meters wide and about 10 meters below the riverbed (river depth around 26 meters), began in early January and has tunneled beneath the riverbed up to Ventilation Shaft Building No. 04 (IVS04). It is currently entering Sam Yot Station to connect with the MRT Blue Line (Chaloem Ratchamongkhon line). The Southbound tunnel is expected to reach Ventilation Shaft Building No. 04 around June 2026.
"Risk management at every stage has been thoroughly analyzed, including close coordination between MRTA, project consultants, and contractors, ensuring safe and highly efficient operations. We want the public to have full confidence in the safety of MRTA’s electric train construction projects," he said.
Mr. Katphachon Udomthamphakdi, Governor of MRTA, stated that construction of the Purple Line South (Tao Poon to Rat Burana, Kanchanaphisek Ring) continues steadily under full safety standards. Within one to two months, MRTA will submit the project feasibility study under the Public-Private Partnership Act B.E. 2562 (2019) to the MRTA board. The study finds that continuing operations as a single route under a PPP Gross Cost model—contracting a private operator similar to the Purple Line North (Tao Poon to Bang Yai)—is the most suitable approach. This will provide convenience for the public by seamlessly connecting both Purple Line North and South. Pending board approval, negotiations with the Purple Line North contractor will proceed. The goal is cabinet approval by late 2026 or early 2027, aiming for service commencement in 2030.