Bound for the Mae Tang-Mae Ngad water diversion project, the machines will contend with complex and variable geology that ranges from granite, gneiss, quartzite and schist, to sandstone, siltstone, shale, slate, mudstone and claystone, and varies from massive to highly fractured and weathered rock masses (with a UCS range of 8MPa to 220MPa), including numerous fault and fracture zones and the potential for high ground water inflows.

The TBM manufacturer Terratec supplied the machines to contractors Right Tunnelling and Siamphan Enterprise on two contracts. The TBMs will bore a total of 15km of tunnel in the mountains north of Chiang Mai city.

As the two machines progress along their respective alignments, they will install precast concrete segmental lining rings consisting of four x 1400mm wide honeycomb style segments, with an inner diameter of 4m, which will be backfilled with pea-gravel and grouted in place. Excavated muck and supplies will be conveyed via rail-bound systems.

A spokesperson for the manufacturer said: “The cutterheads have each been equipped with heavy-duty 17” disc cutters and feature large bucket openings, as well as anti-wear plates and lubrication systems for the injection of foams and polymers to assist with wear prevention and dust suppression. The 1,500kW Electric Variable Frequency Main Drives (VFD) will allow the cutterheads to cut in harder rock zones at a maximum speed of 9rpm and deliver 2,896kNm of torque of to cope with more fractured zones of the alignment.

“In addition, the machines are fitted with probe drilling systems, located behind ring gear style segment erectors, that provide 360-degree coverage for probing and grouting of the ground ahead of the TBMs; as well as dewatering pumps that have the capacity to manage groundwater inflows of up to 100lt/sec.”