Rallying finish for robust TBM in LaosJan 2011
Desiree Willis, Technical Writer, The Robbins Company
It has been a fine experience for the first TBM to be used in the southeast Asian country of Laos. After launch on its 5.5km (3.4 mile) long journey in February 2010, the 7.6m (25ft) diameter Robbins single shield TBM finish the new hydro headrace tunnel just 10 months later in November 2010. Excavated by Italian contractor CMC di Ravenna, the new headrace tunnel for expansion of the Theun Hinboun hydro scheme on the country's Nam Theun River will support high power generation capacity for much needed increased power supply to both Laos and neighboring Thailand.
First ever TBM breakthrough in Laos
Rates reached up to 37m/day
As dignitaries of the Lao Government with CMC management and local media representatives witnessed breakthrough the machine into an exit shaft close to the tunnel's river intake structure, Luca Barbàra, Far East Manager for CMC di Ravenna, said: "Since this is the country's first TBM project, it has awakened people's curiosity and interest. Our teams included experienced Italian and Chinese crewmembers, but we also trained many Lao workers during the course of the project. This was something they had not seen before."
The machine is now being disassembled and recovered from the shaft by CMC which hopes to use it on upcoming projects in the region. Once retrieved, the headrace works will be finished. Due to the current rainy season and relatively low level of the tunnel, the last 100m of the drive to the intake structure will be completed using drill+blast after the river flows recede below the tunnel invert level.
During its drive, the Robbins single shield TBM averaged advance rates of about 19m (62ft) per day, with a peak of 37m (121ft) in one day. Ground conditions consisted of fair to good rock for 95% of the tunnel length, with some small sections of poorer rock quality. "We encountered an anticipated 15m wide fault zone with water inflow at about the 4,700m mark," said Barbàra. "We were able to drill a borehole and use expanding foam to consolidate the ground, allowing us to resolve the problem and continue boring in about one week."
Ready for the Theun Hinboun hydro drive
The TBM completed a 5.5km drive
Much of the geology for the new headrace was assessed during earlier excavation of the existing nearby headrace tunnel. Ground on the latest tunnel consisted of fairly soft, layered mudstone, siltstone and sandstone, with cover ranging from 26m to 380m (85ft to 1,200ft).
To combat possible squeezing conditions in soft rock, the machine utilized an articulating cutterhead with over-cutters that allowed an excavated tunnel diameter of up to 100mm beyond the nominal TBM diameter. The TBM's 17-inch disc cutters were back-loading for improved worker safety and project efficiency. A probe drill was used to check ground conditions up to 60m (200ft) ahead of the machine. As the shield machine advanced, it erected a 280mm thick, precast concrete segmental lining of five segments and a key to provide immediate ground support and a finished finally lined tunnel diameter of 6.9m (22.6ft).
CMC di Ravenna constructed the tunnel for project owner Theun Hinboun Power Company, Ltd, which is a joint venture of Electricite du Laos (EDL), Norwegian contractor Statkraft, and GMS Lao. Due to go online in 2012, the Theun Hinboun Expansion Project will address increased power demands by adding an additional 280 MW annual generating capacity to the output of an existing power station. Electricity will be shared, with approximately 220 MW going to Thailand and 60 MW to the Laotian national power company, EDL.
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