A 6.39m Terratec EPBM was chosen for the drive, which forms part of the 23km east section of the Orange Line project. Contract E3, which was awarded to ITD in May 2017, totals over 6km of TBM driven tunnel and three underground stations. The machine began mining westwards from the Khlong Ban Ma station box on 12 January 2019. By late-March, the TBM was achieving progress rates of up to 18 rings per day and had already undertaken its first intermediate breakthrough. July 2019 marked the TBM’s best month of progress, where it achieved a total advance of 663.4m and the best day of 40.6m.

Geology comprised the typically variable soft ground geology of Bangkok, which ranges from soft and medium to stiff and very stiff clays with lenses of dense sand and the potential for high pressure groundwater inflows – as well as the need to mine through numerous diaphragm wall shafts and, potentially, concrete piles.

“Due to extensive project planning we were well prepared for the 3.3km long drive and any challenges had pretty much been solved on paper before the TBM commenced.” says Bunjerd Emnuch, tunnel manager for the manufacturer. “The only remaining challenge was obstruction cutting. The TBM had to cut through 10 diaphragm wall panels during break ins and break outs with only two cutterhead interventions.”

In order to handle these challenging conditions, the TBM’s soft ground cutterhead features a spoke style and the addition of back-loading knife bits to assist break-in and break-out of the shafts. In addition, the machine is fitted with an active bentonite face support injection system and double gated screw, to ensure face stability and mitigate settlement during excavation in areas of flowing sands and high groundwater pressure.

As the machine progresses along the alignment, it has been installing a precast concrete segmental lining consisting of 1,200mm-wide universal segments in a five plus key configuration, for an internal diameter of 5.7m.