Protecting Slopes to Build Resilient Roads in Nepal

The Narayanghat-Mugling road section in Nepal is a critical 33-kilometer link in the highway corridor connecting Kathmandu with major border trade points such as Birgunj and Bhairahawa. 

Both these border points carry about 90% of the country's trade volume to and from neighboring India and other countries. This highway corridor, part of the Asian Highway Network, sees more than 8000 vehicles daily carrying passengers, goods, and essential supplies.  

This four decade-old road section used to be narrow and is highly prone to landslides each year as it traverses steep hills on one side and the Trishuli River on the other posing safety risks and increasing travel time. 

The World Bank supported the upgradation and widening of the road through the Nepal-India Regional Trade and Transport Project (NIRTTP) which was completed in 2021. The road was upgraded to meet the standards of the Asian Highway Network and widened to 9-11 meters from the previous 5-5.5 meters helping save time and costs for users and improve trade competitiveness. 

The Narayanghat-Muglin road was upgraded to meet the standards of the Asian Highway Network and widened to 9-11 meters from the previous 5-5.5 meters.

The project also showcased resilient road construction by employing fortified slope protection measures, combining bioengineering techniques with plants and grass plantations, small-scale structures, drainage systems, and advanced technology. This was achieved amid a challenging topography and high vulnerability to climate-induced disasters, further exacerbated by the 2015 earthquake that impacted the project’s upgradation works. 

Using bioengineering solutions for slope stabilization

Nepal’s Department of Roads (DoR) implemented the upgradation and widening works. Both sides of the road were widened as feasible, resorting to excavation of the mountain side slope and construction of retaining structures at the valley side. Measures were taken to minimize disturbing the slopes as far as possible and excavation works carried out only in a few sections that were unavoidable. 

Immediately after excavation, the World Bank helped assess and treat the cut and fill slopes with bioengineering solutions. The designs for nature-based solution"s were based on well-established bioengineering techniques described in DoR's Reference Manual and Site Handbook on roadside bio-engineering. The new cut and fill slopes, fortified with proper bioengineering and drainage helped strengthen the slopes to withstand the monsoon rains and improve stability. 

Bio-engineering techniques were used for more than a hundred slopes. Local species of plants were selected best suited for the environment and local people employed to execute the bioengineering techniques, given their knowledge of the local terrain.

Engineer Shiva Khanal from the project said, “It is critical to select bio-engineering measures suitable to the site-specific requirements with effective water management for preventing surface erosion. Also, a timely intervention of bioengineering, that is immediately after slope cuts is very important for its effectiveness. "

Bio-engineering techniques were applied to over a hundred slopes using locally suited plant species and employing local people familiar with the terrain.

Introducing new technology in complex slopes 

A few slopes along the NM stretch were steep, long, and complex in nature, as well as very active causing frequent road closures during the monsoon period. These slopes required advanced engineering solutions. 

For such slopes, the project team conducted detailed drone mapping and geological and geotechnical assessments. Based on the results, 23 slopes were protected using wire mesh netting with anchors/rock bolts together with necessary retaining structures, drainage, and bio-engineering works. 

Sites with river erosion problems towards the valley side were further investigated with Bathymetric Survey. Considering the severity of the site condition and information from the survey, a design introducing Pile-based retaining structure at the riverside with necessary earth anchors, anchored walls, geo-reinforced earth walls, and micro piles were introduced. 

Complex slopes were protected using wire mesh netting with anchors/rock bolts together with necessary retaining structures, drainage, and bio-engineering works. 

According to Rohit Kumar Bisural, Project Manager at DOR, the implementation and success of these treatment works prior to the monsoon period helped significantly reduce road closures during the monsoon and also increased safety.

Lessons for the future 

The methods and techniques adopted for investigation and protection works in more than 100 slopes along the Narayanghat-Mugling road offer valuable lessons for building safe and resilient roads. Based on the experiences gained from the project, the DOR has published a supplementary guide for engineers working on roads needing slope stabilization.

Mr. Tuk Lal Adhikari, Expert Geotechnical Engineer, leading the investigation and design for slopes, expressed that investment in proper investigations, analysis, and designs for slope stabilization certainly pays back through increased resilience and sustainability. 

Key lessons from the slope stabilization works under NIRTTP include: 

•  Engineering measures together with drainages and bioengineering works are highly effective in enhancing the resilience of slopes. 
•  Use of local and mixed species of grass, shrubs, plants, and timely maintenance or replantation showed good results. 
•  For large/complex slope failures, a detail investigation is mandatory to uncover the cause of failure. 
•  Safekeeping the slopes also depends on regulating the upstream and downstream construction work that can destabilize the slopes. 
•  Early warning and traffic control system must be reinforced to prevent the damage of lives and properties in case of extreme weather conditions  

The upgradation works of the Narayanghat-Mugling road has in effect helped reduce travel time from two hours to 50 minutes which has in turn supported Nepal’s trade efficiencies.

The upgradation works of the Narayanghat-Mugling road has in effect helped reduce travel time from two hours to 50 minutes which has in turn supported Nepal’s trade efficiencies.

 As Gopi Parajuli, a truck driver taking this route for years opines, “Earlier, we were scared to drive on this road. It was narrow and dangerous, especially during the monsoons. We would get stuck for hours. Now, the road is much safer, and the travel time shorter.”