The Theun Hinboun Expansion Project is a hydroelectric project that requires a 5.5 km (3.4 mi) headrace tunnel bored by a Robbins Single Shield TBM – the first instance of TBM-driven tunneling in the country of Laos. Located on the banks of the Nam Theun River, the project consists of a new station, dam, and headrace tunnel which will add electricity supplies to Laos and neighboring Thailand by 2012. The USD $270 million project will address the power needs of the two countries by adding an additional 280 MW annual generating capacity. The original plant, built by Recchi-CMC JV between 1995 and 1998, already produces 220 MW annually. Power will be shared, with approximately 220 MW going to Thailand and 60 MW to the Laotian national power company, Electricite du Laos (EDL). The project also promises to improve the supply of electricity in Laos by extending the 115 kV transmission grid to the project area and increasing power supply to the existing grid.
On December 22, 2008, CMC di Ravenna signed a contract with Robbins to provide a 7.6 m (15.1 ft) diameter Single Shield TBM. The TBM was assembled at Robbins’ Solon manufacturing facility in Ohio and shipped to the jobsite along the Nam Theun River. CMC di Ravenna chose the Robbins Single Shield for its short shield length, based on the geology and the need for continuous tunnel lining.
The Robbins TBM was designed to accommodate moderate squeezing ground conditions. Ground along the tunnel alignment consisted of alternating strata of sandstone, siltstone, and mudstone. An articulating cutterhead with overcutters allowed the machine to excavate 100 mm (4 in) beyond the nominal tunnel diameter. To support the ground and provide final lining, 280 mm (11 in) thick, pre-cast concrete segments were used in a 5+1 arrangement, making a finished tunnel diameter of 6.9 m (22.6 ft).
During excavation, the Robbins Single Shield TBM averaged 19 m (62 ft) per day, with a peak advance rate of 37 m (121 ft) in one day. Ground conditions consisted of fair to good rock for 95% of the tunnel length, with some sections of poorer rock quality.
Challenging conditions included an anticipated 15 m (50 ft) wide fault zone at about the 4,700 m (2.9 mi) mark with flowing water. The crew was able to drill a borehole and use expanding foam to consolidate the ground, allowing boring to continue. The machine broke through on schedule, on November 21, 2010.