Sharp 32m-radius curve on the tunnel alignment

Tight radius EPBM lowered into working shaft

After completing its first technically challenging drive, Thai contractor Nawarat Patanakarn is preparing for the second drive on the Bangkok Metropolitan Electricity Authority (MEA) Phra Khanong cable tunnel project in Thailand.

The Terratec tight radius EPBM shield was launched on its first 495m drive in February 2017 from a 7m diameter shaft located beneath an elevated expressway ramp. Due to the limited working head room and the piles of the expressway, the EPBM was required to embark on a sharp 32m-radius curve immediately upon exiting the shaft, a feat which it achieved due to the extreme X-type articulation system providing a maximum articulation angle of 6.6 degrees.

Tunnelling operations were closely observed throughout the manoeuvre via a settlement monitoring programme that demonstrated minimal impact on the existing above ground structures. As the machine progressed through the curve it installed a tunnel lining of short 300mm wide x 125mm thick steel sets, before transitioning to 1.1m wide x 225mm thick traditional tapered precast concrete segments.

With its classic soft ground open spoke cutterhead design, fitted with knife bits to assist break-in and break-out of the steel fibre reinforced concrete shaft eyes, the EPBM coped well with the geological conditions, which consisted of fine sands and stiff clays with a groundwater head of about 2 bar.

Fig 1.Routes of the two drives

Before breakthrough in September 2017, the EPBM completed a second, 30m long 99m-radius curve on its approach to the reception shaft. The machine has since been retrieved and is being prepared for launch in mid-November on its second 293m-long drive from the central launch shaft.

Once again, the EPBM will launch into a tight curve, this time with a 43.7m-radius on a 2% up-grade trajectory, passing under the Phra Khanong canal and heading northwards to a second retrieval shaft. Throughout the project, machine operation is being assisted at all times by Terratec field service staff. “We are pleased with the service and support offered by the Terratec team and are looking forward to successful completion of the tunnelling works in the coming months,” said Pakpong Phongsaisri, Project Engineer for Nawarat Patanakarn.

Terratec celebrates the successful factory acceptance test of a new-design 3.2m diameter EPBM destined for the tight radius curve demands of the Phra Khanong Cable Tunnel Project in Bangkok, Thailand.

The event, held on 26 September (2016), was attended by representatives of Bangkok’s Metropolitan Electricity Authority (MEA) and its Thai contractor Nawarat Patanakarn PCL.

3.2m diameter EPBM for Bangkok cable tunnel

Designed to accommodate a new high-voltage cable system, the Phra Khanong Cable Tunnel Project is the first of a series of planned tunnelling projects by the MEA that are being built to meet increased power demands in the Thai capital.

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 7m diameter shaft that is currently being constructed beneath the Ram Inthra – at Narong Expressway toll road ramp, and with little working headroom.

The first of two drives for the project will see the EPBM head 495m south at an upgrade of 1.2%, terminating 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, 293m-long, drive northwards under the Phra Khanong canal – the EPBM will be launched onto a sharp 32m radius curve on a 2% upgrade.

Terratec/JTSC collaborate for exteme-articulation TBM

To achieve this, the Terratec machine is designed with an X-type articulation system that provides a maximum articulation angle of 6.6-degrees to accommodate a minimum radius curve of 30m. Although this type of extreme 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 EPBM’s back-up systems are also tailored to accommodate the tight project requirements, including a mucking system that conveys spoil from the screw conveyor to a transfer pipe using air pressure.

Geological conditions along the alignment consist of fine sand and stiff clay, with an average overburden of 26m and a groundwater head of approximately 2 bar.

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 fibre reinforced concrete shaft eyes. Traditional tapered precast concrete segments (left/right/straight) will typically be installed as the machine progresses, with shorter steel segments utilised 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 highly-experienced field service staff, providing quality after-sales support to ensure optimum performance and successful project completion. The tunnel is scheduled to go into operation at the end of 2017.