Unusual articulation for Phra Khanong project

The project is one of several upcoming tunnels to accommodate new high voltage cables for the Thai capital. This drive is at an intersection in a busy area of central Bangkok and faces tough alignment constraints.

The TBM will be launched from a 7m-diameter shaft being constructed beneath a toll road ramp and then head 495m at an upwards grade of 1.2 per cent to a reception shaft. It will then be transported back to the launch shaft and launched on a second, 293m-long drive. This second drive requires the TBM to be launched into a 32m-radius curve with a 2 per cent upwards gradient to navigate through the piles of the ramp.

To achieve this, the 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 30m. Although this type of extreme TBM articulation is uncommon in the global market it is popular in Japan. Terratec teamed up with Japanese manufacturer JTSC to develop the design with the aim of exporting this technology to other countries.

A spokesman for Terratec added: “Geological conditions along the alignment consist of fine sand and stiff clay, with an average overburden of 26m 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 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.”

The machine was due to arrive in Thailand in mid-October. Excavation is due to commence in mid-December. The tunnel is due to be commissioned at the end of 2017.