Terratec has commenced excavation work on Contract 3 of Bangkok’s MRT Purple Line underground extension, following the successful launch of its Earth Pressure Balance Tunnel Boring Machine (EPBM), identified as S114, on 4 August 2025.
The EPBM S114 is assigned to excavate the northbound tunnel between Purple Line station 24 and station 22.
The company stated that another machine, S113, will be deployed to construct the parallel southbound tunnel roughly one month later.
In total, Contract 3 involves boring 3.12km of twin tunnels, each with an internal diameter of 5.7m, two new underground stations, and three ventilation shafts.
The company said the route runs through a densely populated area of Bangkok, near the Grand Palace and several historic temples.
This setting presents “unique logistical and cultural challenges,” particularly as the alignment includes a section beneath the Chao Phraya River at depths exceeding 40m.
Here, both tunnel boring machines (TBMs) will pass through a full-face aquifer sand layer, which requires careful pressure management to ensure ground stability.
Another technically demanding section is at Purple Line station 23, where the new tunnel must pass beneath the existing MRT Blue Line.
Terratec noted that its earlier TBM S17 was used to construct the Blue Line.
Terratec said the S114 was designed specifically for these conditions. The TBM has a 6.39m-diameter shield, a cutterhead with a 43% open ratio and back-loading cutter bits, and can deliver up to 5,305 kN-m of torque at a maximum speed of 2rpm.
The machine’s thrust and steering systems allow it to navigate curves as tight as a 160m radius, which is required for Bangkok’s constrained urban corridors.
Other technical features include a three-way cutterhead seal protection system to ensure reliability under high groundwater pressure, and a comprehensive ground-conditioning setup with multiple lines for foam, additives, grout, clay, and bentonite.
The machine is also equipped with a ring erector system designed for efficient tunnel lining segment installation, with an average assembly time of 30 minutes per ring, the company said.
Terratec highlighted that S114 incorporates lessons and technology from previous tunnelling projects in Bangkok to help ensure “safe and efficient” progress in sensitive areas.
The second TBM, S113, is expected to launch roughly a month after S114, contributing to efforts to expand Bangkok’s underground public transport network while protecting the city’s historic and cultural assets.
