What are the Special Considerations for Hill Road Alignment?

What is Hill Road?

According to Nepal Rural Road Standards (2055), 2nd Revision 2071, the terrain is classified as Terai & Hills based on the topography of the country. The Terai covers the plain & rolling terrain having a cross slope of 0 to 25%. Hill covers the mountainous & steep terrain having a cross slope of 25 to 60% and more. The road passing through the hilly terrain with a cross slope of 25% or more is generally termed as Hill Road. A hill road usually consists of either a river route or a ridge route.

What are the Factors Affecting the Alignment of Hill Roads?

There are various factors affecting road alignment. Moreover, there are some special considerations to be followed while selecting a hill road alignment. The major factors to be considered while deciding the alignment of hill roads are as follows:

1. Geological Stability

The road alignment should pass through a stable hill slope. The area should not be prone to erosion, landslides, rockfall, etc.

2. Availability of Construction Materials

Availability of construction materials near the construction site will reduce the transportation cost of materials thus making the project economical.

3. Cross Drainage Structures

Due to numerous watercourses present in the hilly regions, there may be the necessity of intense cross drainage works. The alignment should be so selected in such a way that the number of cross-drainage structures required becomes minimum.

4. Geological Structures

Excessive cutting of hard rock will be expensive. So, such areas should be avoided as much as possible from the road alignment.

5. Geometric Design

The alignment should be chosen to minimize the ineffective rise & fall, steep gradients, number of hairpin bends, etc. Also, the geometric design parameters should comply with the design guidelines & standards for hilly regions.

6. Altitude of the Road

• Rainfall (or Snowfall) ∝ Altitude
• Atmospheric Pressure ∝ $\frac{1}{Altitude}$
• As the altitude decreases, the number of cross drainage works required increases.