Unit:2 Hill Roads Note  (Highway Engineering -II

 2.1  Definition, importance of hill roads in Nepal   

Definition:-  

A road which passes through a terrain with a cross slope 25% or more is known as Hill Road. A hilly area is characterized by a highly broken relief with widely differing elevations, steep slopes, deep gorges and great numbers of water courses. The engineer's objective is to establish the shortest, most economical and safe route between obligatory points.  

In hill roads special care has to be given for: 

1. Stability   2. Drainage   3. Geometric standard    4. Resisting length 

        

• Importance of hill roads in Nepal 

• For economical development of country. 
• To meet need of strategic consideration of country in hilly area of the country. 
• Development of Tourism 
• To establish other project such as hydropower, hill irrigation etc. 

     In mountainous terrain, navigation and rail traffic are not possible which automatically demands hill road.  

      Nepal has a total area of 147181 sq.km of which about 66% are covered with hills, mountains and Himalayans with thinly population. More than 90% of the population living in hills and mountains depend upon agro-products of the region. These products are not sufficient for the people living in this region. The terai (34%) belt of country has surplus agricultural as well as industrial products. Goods from surplus regions of the terai area should be served in the deficits area of hills and mountains. Due to highly broken relief, waterways and railways are not feasible in mountain regions. Development of airways is difficult due to high cost of construction required for providing landings strip and high cost of operation

. 2.2 Design and construction problems in hill roads         

Design and construction of roads in hills and mountain are more complex than in plain terrain. It is due to several factors associated in the region. They are:- 

1.    A hilly or mountainous area is characterized by a high broken relief with widely differing elevations, steep slopes, deep gorges and a great number of water courses. This may unnecessarily increase the road length. 

2.    The geological condition varies from place to place. 

3.    Hill slopes stable before construction may not be as stable due to increased human activities. 

4.    There may be variation in hydrological conditions which may easily be overlooked during design and construction. 

5.    Due to highly broken relief construction of special structures should be done at different places. This increases the cost of the construction. 

6.  Variation in the climatic condition such as the change in temperature due to altitude difference, pressure variation, precipitation increases at greater height etc. 

7.    High-speed runoff occurs due to the presence of high cross slopes. 

8.    Filling may overload the weak soil underneath which may trigger new slides. 

9.    The need of design of hairpin bends to attain heights. 

2.3 Special Consideration of Hill Road geometric Design

Alignment through hilly areas is slightly different from aligning through a flat terrain. For the purpose of efficient and safe operation of vehicles through a hilly terrain special care should be taken while aligning the highway. Some of the special considerations for highway alignment through a hilly terrain is discussed below. 

•       Stability of the slopes:- For hilly areas, the road should be aligned through the side of the hills that is stable. The common problem with areas is that of landslides, excessive cutting and filling for road constructions give way to steepening of slopes which in turn will affect the stability.  

•       Hill side drainage: Adequate drainage facilities should be provide across the road. Attempts should be made to align the roads in such a way where the number of cross drainage structures required are minimum. This will reduce the construction cost. 

•       Special geometric standards:  Geometric standards followed in hilly areas are different from those in flat terrain. The alignment chosen should enable the ruling gradient to attain in minimum of the length, minimizing steep gradient ,hairpin bends, and needless rise and fall. 

•       Ineffective rise and fall:- Effort should be made to keep the ineffective rise and excessive fall.

         2.4 Typical cross sections of hill roads 

                         

2.5 Special structures in hill roads like retaining walls, revetment walls, tow wall,slope protection works

  →Construction of hill roads involves many special structures. Those Structures which are not usually required in plain terrain roads are referred as special Structures in hill roads. 

The following types of special structures are mostly used in the hill road in Nepal are:-  

1.  Retaining walls:

-Retaining Walls are constructed to provide adequate Stability to the roadway & to the slope. Dry stone masonry is preferred to masonry in mortar as the former permits easy drainage of seeping water. 

-  A retaining structure is usually a wall constructed for the purpose of Supporting or retaining a vertical or nearly vertical earth bank which is turn may support vertical roads.

-  It is constructed on the valley side of road-way and also on the cut -hill side to provide sliding towards the road-way.

                           

2.  Revetment walls:- 

The embankment slopes are normally protected with rough stone pitching about 30cm thick in order to avoid erosion due to flow of water. If the stopping length is too long, it is preferable to construct a toe wall to support the embankment & depending upon the slope available where the cutting slope is steep & contains loose or scour able soils, Slips are likely to occur. In such locations, revetment walls of dry stone masonry are constructed to retain the soil on the cutting side to prevent occurrence of any such slips. 

 3.  Slope protection works:- 

The soil on the surface of the slopes is subjected to the direct action of sun, rain, winds with the disturbance of the moisture content equilibrium, the formation slopes become prone to failure. The formation slopes are selected based on the type of embankment or cut soil. Flat slopes are provided for loose soil embankment & cutting. In stable soils, steeper Slope may be chosen.