The first tunnel ever driven by TBM in Laos has been completed. Italy-based contractor CMC di Ravenna excavated the 5.5km headrace tunnel on the Then Hinboun Expansion project.

A 7.6m single shield TBM was equipped with an articulated cutterhead fitted with overcutters to enable the machine to excavate up to 100mm beyond the normal tunnel diameter. This was to combat possible squeezing caused by the soft rock present. The 17-inch disc cutters were back loading.

A probe drill was used to check ground conditions up to 60m ahead of the machine. Some 95 per cent of the rock encountered along the drive was rated at good to fair with small sections of poorer rock quality.

“We encountered an anticipated 15m wide fault zone at about the 4,700m mark with flowing water. 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,” said Luca Barbara, far east manager for CMC di Ravenna.

To support the ground and provide final lining, 280mm thick, pre-cast concrete segments are being used in a 5+1 arrangement, making a finished tunnel diameter of 6.9m.

The TBM will now be disassembled and moved to other upcoming projects in the region. The last 100m of the works will be completed by drill and blast once the tunnel flood, caused by the rainy season, recedes below the tunnel invert.