First segmentally lined TBM tunnel in 10 years for Bangkok’s Metropolitan Electricity Authority (MEA) requires sharp radius curve to accommodate tight alignment constraints.
On Sept. 26, 2016, TERRATEC celebrated the successful factory acceptance test of a new 3.2-m diameter earth pressure balance tunnel boring machine (EPBM) destined for the Phra Khanong Cable Tunnel Project in Bangkok, Thailand. The event was attended by representatives of Bangkok’s Metropolitan Electricity Authority (MEA) and Thai contractor Nawarat Patanakarn PCL.
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 machine will be launched from a 7-m diameter shaft that is currently being constructed beneath the Ram Inthra – At Narong Expressway (toll road) ramp with little working head room.
The first of two drives for the project will see the TBM head 495 m south along Sukhumvit Road, at an upgrade angle of 1.2%, and terminate at a reception shaft located under the BTS Skytrain structure just short of On Nut Station. From there, the machine will be lifted and transported back to the launch shaft under the expressway ramp.
In order to negotiate the foundation piles of the ramp – and complete its second, 293-m long drive northward under the Phra Khanong canal – the TBM will be launched onto a sharp 32-m radius curve on a 2% up-grade trajectory.
To achieve this, the TERRATEC machine was designed with an X-type articulation system that provides a maximum articulation angle of 6.6 degrees to accommodate a minimum radius curve of 30 m. Although this type of extreme TBM articulation is uncommon in the global market, it is popular in Japan. TERRATEC therefore teamed up with Japanese manufacturer JTSC to develop the design with the aim of exporting this technology to other countries.
The TBM’s back-up systems are also tailored to accommodate the tight project requirements, including a mucking system that conveys spoil from the TBM’s screw to a transfer pipe using air pressure.
Geological conditions along the alignment consist of fine sand and stiff clay, with an average overburden of 26 m and a groundwater head of about 2 bars.
The TBM’s soft ground cutterhead features an open spoke design with the addition of knife bits to assist break-in and break-out of the steel fiber reinforced concrete shaft eyes. Traditional tapered precast concrete segments (left/right/straight) will typically be installed as the machine progresses, with shorter steel segments used during the course of the sharp radius curve.
Following the successful factory acceptance test, the machine will now be shipped to Thailand and is expected to arrive by the end of October. Excavation is due to commence in mid-December.
Machine operation will be assisted at all times by TERRATEC’s field service staff, providing after sales support to ensure optimum performance and successful project completion. The tunnel is scheduled to go into operation at the end of 2017.