Techniques for Mountain Road Construction in Nepal

Mountainous road building is challenging and often environmentally destructive.  However, this need not be so.  The mountainous regions in China provide a good template for understanding how road infrastructure can be developed in both a cheaper and environmentally friendly way.

Conventional road construction can create extensive surface disturbance, with correspondingly serious impacts on the surrounding environment, especially where the soil is highly susceptible to erosion, where the vegetation cover is poor, and where the ecosystem is fragile (Xu et al., 2006). In addition, the construction can have significant impacts far from the construction site; for example, traditional road construction requires large volumes of fill materials, particularly in mountainous areas, and the granular materials required for this purpose must often be obtained from borrow pits located outside the mountains. To reduce ecosystem damage in cliff areas, an ancient Chinese cliff path technique has recently been resurrected. The same technique that is used for canopies and balconies, namely cantilevering, has been used to construct slab roads on the sides of cliffs. At some sites, the slab road is combined with bridge support structures where additional support is needed (Cao, Ye, & Zhan, 2010).

“To improve communication between regions in mountainous areas with many cliffs, engineers traditionally (as early as 300 B.C) built cliff paths by cutting holes into the cliff face and driving wooden beams into the holes” which would then jut out from the side of the mountain (Cao, Ye, & Zhan, 2010). Planks were then laid on the beams to form a path, 2–3 m wide, for people to walk on. Canopies were built in some places to protect travelers from falling rocks and other debris (Cao, Ye, & Zhan, 2010).  “The bridge technique, in which additional support structures were installed below the path, was used in other areas, particularly to cross gullies. Although this approach was originally devised to reduce construction costs, it also improved environmental conservation as a result of decreased disturbance of the original landscape. To promote environmental protection as well as infrastructural stability, this ancient technique is now being resurrected for use in mountainous parts of China, with the planks replaced by modern construction materials” (Cao, Ye, & Zhan, 2010).

Conventional road building is often too focused on “stabilizing slopes to improve road safety and on the engineering aspects of side-slope design” to control erosion (this often means using dynamite to blow up the slope to then build a road) rather than the environmental consequences on the surrounding vegetation of the construction (Cao, Ye, & Zhan, 2010).  Disturbance and edge effects mainly result from damage caused by infrastructure construction and use. Physical disturbance appears during construction activities, when soils and the landscape relief are modified most severely, thereby altering the vegetation even at relatively long distances from the road.  One particular concern in mountainous terrain is that sediments eroded from construction sites can be delivered into streams and rivers (Lane and Sheridan, 2002). Compared with the conventional construction technique, the new techniques described disturb much less of the construction site and its vegetation, and this should reduce the risks of erosion and of sedimentation of streams and rivers near the construction site.  The results of the study support this as there was a 96% reduction in the area of vegetation damaged using the cantilevering technique versus convention road-building means (Cao, Ye, & Zhan, 2010).

The choice of construction technique at a given site typically depends on the construction cost, ease of construction, and design goals for the road (Cao, Ye, & Zhan, 2010).  For instance, highways constructed in the mountains of Nepal have to be able to withstand the monsoon season.  As it exists right now, the Karnali Highway (which is more a dirt path than an actual highway) has to be closed four months a year because of rain; however, a more effective design could mitigate this.  South Africa faced a similar problem.  In August 2006, the Garden Route, a major tourist attraction and national highway in the southern part of South Africa, experienced “abnormally high rainfall” which the slope the road was built on to slip and severe damage to the road was caused (Kruger, Smuts, & Newmark, 2009).

To reconstruct the area, the following procedure was used which utilized the cantilevering technique previously discussed in the Chinese case: “the existing roadway was fitted with a temporary soil nail structure to ensure accommodation of two-way traffic during construction. This technique would allow space for a working platform to be created to drive oscillator piles to support a quarter roadway-width cantilever structure. It was envisaged that the horizontal stability of the piles would be secured by sloped rock anchors in this instance. The main advantage of this concept was the minimization of the risk as a result of the stabilizing effect of multiple anchored pile shafts in the rock formation” (Kruger, Smuts, & Newmark, 2009).

After eight months, the revamped road looked like this:

These considerations may lead planners to add bridge supports to a cantilever road or to choose conventional road construction instead of the cantilever technique. However, improvements to highway infrastructure have both systemic and local impacts that must be accounted for in future road designs (Kanaroglou et al., 1998). The Chinese cliffside road construction practices described  “demonstrate that improving the technology used in road construction can reduce both construction costs and environmental damage, thereby balancing the need to improve communication between regions with the need for ecological conservation” (Cao, Ye, & Zhan, 2010) This is particularly important for cliff ecosystems, like the ones in Nepal, that are extremely fragile.

The picture below of the Karnali Highway reflects where the state of road construction in the mountains is now.  The picture below of a mountain highway in Western China reflects a desirable outcome for road construction using the cantilevering technique.  The South African case demonstrated that cantilevering can effectively protect a road from slope slippage that can prove fatal to the highway.  In Nepal, which receives torrential amounts of rain during the monsoon season, this security is crucial.  Further, the Chinese case clearly demonstrated that road building does not have to be environmentally destructive and can keep Nepal’s mountain ecosystems intact while connecting Karnali to the rest of the country.