As T&TI was going to press, contractor See Sang Karn Yotah [1979] Co. Ltd. had just finished the first of five drives on a 17.52km long water supply tunnel in Bangkok, Thailand.

Five 3.66m diameter Hitachi Zosen EPB shields are being used, one for each drive of contract G-MC-7C, which is part of the Bangkok Metropolitan Waterworks Authority’s 7th Bangkok Water Supply Improvement Project. For this first breakthrough, See Sang Karn Yotah managed to steer the shield through the tunnel eye in the reception shaft just 9mm low and 19mm right of line after 2.9km of driving.

The tunnel route follows the outer ring road and Bang Na Trad highway to Bang Plee at a depth of 20 – 25m below the surface. The five sections vary between 2.9km and 4.1km in length. Ground conditions along the route are characterised as medium to very stiff clay, sandy clay and sand.

A 3.2m i.d. primary lining of a 1.1m wide reinforced concrete bolted ring consisting of six trapezoidal segments is installed as the machine advances. With curves along the tunnel alignment being as tight as 93m radius, T&TI was told the teams still achieved a best shift advance of 20 rings (22m) and a best daily advance of 33 rings (36m). An inner steel lining of 2.8m i.d. with concrete infill of the created annulus will be installed before the tunnels are commissioned.

The engineer on the project is TEC Thai Engineering Consultants Co. Ltd., who also had to oversee the works for the seven shafts built by the See Sang Karn Yotah. The two permanent shafts, 10m diameter and 32 m deep, were sunk as open caissons using 20tonne precast reinforced concrete segments. Four temporary shafts, 9 m diameter and between 26m to 32m deep, were also sunk as open caisson, again using large precast reinforced concrete segments. Another temporary shaft, elliptical in profile, 9m by 5m, was sunk as a caisson to a depth of 32m. These temporary shafts were required for machine launch and retrieval, as well as for the installation of the secondary lining.

Works began on the 26 month contract in April 2004 and are planned for completion by July next year. T&TI was told that to date the contractor has completed 9.2km, just over 50%, of the tunnel.