Double shields for complex geology in Thailand 21 Dec 2017

Challenging geology demands specialist technology and two hard rock TBMs from Australian Terratec have been chosen to excavate the Mae Tang-Mae Ngad water diversion project in Northern Thailand for the Royal Irrigation Department.

Two 4.74m diameter hard rock double shield TBMs to tackle complex geology

The 4.74m diameter double shield machines will be used by Thai contractors Right Tunnelling Co and Siamphan Enterprise on two contracts to excavate 15km of the 25km-long Phase 2 tunnelling works, which traverse the challenging mountainous terrain north of Chiang Mai city.

The TBMs have been designed to overcome the complex and variable geology of the region, which 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 cutterheads each have 17in disc cutters with large bucket openings, anti-wear plates and lubrication systems for the injection of foams and polymers to prevent wear and supress dust. The 1,500kW electric variable frequency main drives (VFD) will allow the cutterheads to cut efficiently in harder rock zones at a maximum speed of 9rpm and deliver 2,896kNm of torque to cope with more fractured zones of the alignment.

The machines are also 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. They also have dewatering pumps with the capacity to manage groundwater inflows of up to 100lt/sec.

Celebrating the arrival of the TBMs for the Mae Tang-Mae Ngad project

“We are delighted to be working with Terratec on this landmark water tunnel,” said Teerapon Wangmontri, President of Siamphan Enterprise PCL. “Due to the complex geology expected on the project, we wanted to choose a tried and tested manufacturer of hard rock machines such as Terratec.”

As the two machines progress along their respective alignments, they will install precast concrete segmental lining rings consisting of four 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.

The Mae Tang-Mae Ngad water diversion tunnel is located about 55km north of Chiang Mai, in northern Thailand, and is designed to convey 28m³ per second of raw water from the Mae Tang River to Mae Ngad and Mae Kuang reservoirs. The project aims to augment the current water supply to the Chiang Mai and Lampun Provinces involving over 50km of tunnel that is being built in two Phases.

When the overall project is complete, in 2021, a saving of more than 160 million m³ of water will be seen each year. The extra water will help support a range of industries and sectors, such as the consumer sector, agriculture and tourism.