Tunnel road survey defies last snag

An aerial survey report of the geology identifies Nabisa in Wangdue, where the tunnel ends, as the weak zone. A Danish company Sky carried out an aerial survey using a helicopter last April. Another survey was carried out on site.

An expert team from the Norwegian Geotechnical Institute (NGI) and the geology and mines department under Bhutan's economic affairs ministry will soon visit Nabisa to locate sites for boreholes to investigate the underground earth strata to determine the weak zone.

The department's head of seismology and geophysics division, Dowchu Drukpa, said the report is as good as final.

"They wanted to incorporate the geological mapping that we did with NGI earlier this year to the survey results," he said.

"While the underground structure is suitable for tunnel construction in most parts of the alignment, the only concern is that there is a weak band near the portal at Nabisa."

Two options were identified after the preliminary survey. The first, a 10km tunnel from Yusipang to Nabisa, although shorter, is steep, with 7% gradient towards Thimphu.

"The second option, a 15km tunnel between Semtokha and Nabisa, has a gradient of less than 3%," Dowchu Drukpa said, adding that the elevation could also be lower at the portal or tunnel end at Nabisa.

"This reduces the distance between the tunnel and the highway or other places."

Dowchu Drukpa said if the rocks are competent, then the tunnel does not need much support system. "This could bring down the cost of constructing the tunnel," he said.

The tunnel survey is a joint project between NGI and the geology and mines department carried out with a fund of US$300,000 from Norwegian Agency for Development (NORAD).

The geology and mines department has sought additional funding of Nu 5 million from NORAD to conduct the borehole investigations. The investigations include surface geophysics and geological mapping. It would be decided, officials said, during the annual meeting between the two parties in Thimphu this month.

"If the budget comes through, the study will be complete by the end of this year," Dowchu said.

Besides that, the team will also have to do thorough geological mapping.

"This is to be able to relate incidences to the geological profile of the area in the future," he said.

"We'd already made some assessments at the portals with NGI experts, and studied the joint settings of the rocks that's important to know before the construction of the tunnel."