On Sept. 22, 2017, a TERRATEC Tight Radius Shield successfully completed a technically challenging tunnel drive for the Bangkok Metropolitan Electricity Authority’s (MEA) Phra Khanong Cable Tunnel Project, in Bangkok, Thailand. The 3.20-m diameter TERRATEC S48 Earth Pressure Balance Tunnel Boring Machine (EPBM), which is being operated by Thai contractor Nawarat Patanakarn PCL, was required to excavate a sharp 32-m radius curve as it exited the project’s launch shaft in order to negotiate the piles of an adjacent expressway ramp.
To achieve this challenging curved alignment, the TERRATEC machine was manufactured with an extreme X-type articulation system that provides a maximum articulation angle of 6.6 degrees. “The TBM was specifically designed to cope with small radius curves,” said Pakpong Phongsaisri, Project Engineer for Nawarat Patanakarn. “It is a highly efficient machine and has done an excellent job in difficult conditions.”
Designed to accommodate a new high-voltage cable system, the Phra Khanong Cable Tunnel Project is the first of a series of planned tunneling projects by the MEA that are being built to meet increased power demands in the Thai capital.
Located at the intersection of the Phra Khanong canal and Sukhumvit Road, the project is situated in one of the busiest areas of downtown Bangkok and is subject to tight alignment constraints. The TBM was launched on its first 495-m long drive in early February from a 7.0-m diameter shaft located beneath an elevated expressway ramp with limited working head room. Due to the presence of the expressway piles, the TBM was required to embark on a sharp 32m-radius curve immediately upon exiting the shaft.
Tunneling operations were closely observed throughout the maneuver via a settlement monitoring program that demonstrated minimal impact on the existing above ground structures. As the machine progressed through the curve it installed a tunnel lining of short 300-mm wide by 125-mm thick steel sets, before transitioning to 1.1-m wide by 225-mm thick traditional tapered precast concrete segments (left/right/straight).
Featuring a classic soft-ground, open-spoke cutterhead design, with knife bits to assist break-in and break-out of the steel fiber reinforced concrete shaft eyes, the EPB machine has coped extremely well with the geological conditions along the alignment, which consist of fine sands and stiff clays with a groundwater head of about 2 bars.
Prior to the TBM’s breakthrough, the machine completed a second, 30-m long, 99-m radius curve on its approach to the reception shaft, which was handled with ease.
Since then, the TBM has been retrieved and is currently being prepared for its second, 293-m long drive from the central launch shaft, which will commence in mid-November. Once again, the TBM will launch into a tight curve, this time with a 43.7-m radus on a 2% up-grade trajectory, passing under the Phra Khanong canal and heading northwards to a second retrieval shaft.
Throughout the duration of the project, machine operation is being assisted at all times by TERRATEC’s highly-experienced Field Service team. “We are extremely pleased with the service and support offered by the TERRATEC team and are looking forward to successful completion of the tunneling works in the coming months,” says Phongsaisri.