1. Landslide Investigations on Phuentsholling – Thimpu Road, Bhutan

August 2004 to July 2005
(i) Central Road Research Institute, New Delhi (R)
(ii) Border Roads Organization (S)


Present Status and Progress
Findings/ Conclusions
Field investigations were carried out regarding four major landslides located at Kharbandi, Sorchin Bypass, Sorchin and Jumbja, which usually blocked the highway for a considerable period every year. The investigations and the survey of the area indicated that the drainage has important role in all these slides. All the slopes were found to be critical and factor of safety was less than one. The stability analysis has been carried out using GEO 4 software and found that the factor of safety varies from 0.37 to 0.68. The analysis indicated that the slopes were unstable and the failures were likely to continue. Considering the adverse conditions responsible for slope stability, remedial measures were proposed to stabilize the slope.

• Scaling of boulders from left flank i.e. the eastern side of the slide, which is presently affected by rackfall problem from the highly fractured and fragmented rock slope

• The debris mass above road level has acquired angle of repose, however, the non cohesive material in the soil-boulder matrix is expected to be eroded away due to surface runoff. Flexible boulder fencing, benching of slope in reinforced gabion walls and provision of rock collection trench between the slope and the fence must be made as per the scheme mentioned in the report.

• The boulders resting on the surface of slope in the eastern side of the slide may be removed, however, no querrying should be allowed from this zone as the same would accelerate the surface erosion/debris slide.

• The downhill slope on the extreme left side has a large natural drain which has been experiencing channel erosion leading to damages on the road stretch. The rocks on the side of the channel comprises of transported boulders or debris mass. The channel should be provided check dams at a few locations. Ballie check dam construction with horizontal reinforcement and pre anchoring would be of help as there is hardly any location in the gulley to support the foundation of check dams.

• The rock slopes adjoining the road stretch may be protected using wire mesh and the shortcrete as suggested in the report.

• Considering the state of land slide and other conditions, it is suggested to explore the possibility of construction of a tunnel to by pass the slide area.