Highway Research Record

III. GEOTECHNICAL ENGINEERING
ON GOING PROJECTS
Landslide Investigations on Phuentsholling-Thimpu Road, Bhutan

Date of Start: August 2004
Central Road Research Institute, New Delhi (R)

Present Status and Progress
The Phuentsholling-Thimpu Road in Bhutan has been experiencing landslides and rockfall problems at a few locations. The investigations for a few of these unstable slopes have been carried out by CRRI. Report on the investigations and remedial measures to stabilize the slope has been submitted to the Border Roads authorities.

Findings / Conclusions
The hill slopes in Bhutan are generally termed as water charged hill slopes. There are severe erosions on the slopes at Kharbandi landslide leading to gulley formations. The buildings in the check post area have been experiencing damages due to movements in the slope. The toe walls constructed in the slide are to prevent the progress of gulley formation have been very effective and would certainly help in slope stabilization. Sealing of cracks in check post area, promotion of vegetation on damaged slopes and construction of toe walls in gullies would further stabilize the slope. There is evidence of debris flows and subsidences in road stretch at kilometer 12. Provision of suitable surface drainage measures would help in stabilizing the slope at this location. Sorchin slide is a mega landslide affecting over 1000m of road stretch. The slide has a large catchment area draining into the main body of slide. The severe damage has been experienced in the toe portion of the slide. A contour drain in the crown portion, chute drains between the upper and lower road stretch, guiding the surface drainage up to the toe and sealing of cracks on the lower stretch are some of the measures recommended to stabilize the slope.

There is severe problem of rockfall at Zumbja slide area. The rocks at this location are highly disintegrated. The volume of surface runoff is very high and has the capacity to carry heavy loads including big boulders of size 73 m. Plugging of the drainage channel on the downhill slope and rockfall protection measures for the uphill slope are expected to mitigate the problem at this location. The percolation of water into the slopes, training of drains, protection from toe erosion and subsidence etc need to be implemented. Some of the failure problems are site specific and need site-specific solutions.

Limitations
Detailed mapping of the slide areas and exploration for getting the realistic slope profile is needed for design of remedial measures. It is difficult to access some of the slide locations for detailed mapping/survey and also the subsoil exploration using bore log to know the subsoil strata.

Recommendations

At least one of the slides must be investigated in an exhaustive manner gathering all relevant information and mapping. The slide should be taken as model to demonstrate the efficacy of remedial measures.