Design in Hill Roads

Design and Construction of Hill roads are more complex than in plain terrain due to factors summarized below: 

a) Highly broken relief with vastly differing elevations and steep slopes, deep gorges etc. which increases road length.

b) The geological condition varies from place to place.

c) Variation in hydro-geological conditions. 

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

e) High-speed runoff due to the presence of steep cross slopes. 

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

g) Need of design of hairpin bends to attain heights.

h) Need to save Commercial and Residential establishments close to the road.

i) Need to save the ecology of the hills. 

Special Consideration in Hill Road Design

(i) Alignment of Hill Roads 

The designer should attempt to choose a short, easy, economical and safe comforting route. 

(ii) General considerations 

When designing hill roads the route is located along valleys, hillsides and if required over mountain passes. 

Due to complex topography, the length of the route is more. In locating the alignment special consideration should be made in respect to the variations in: 

• Temperature 

• Rainfall 

• Atmospheric pressure and winds 

• Geological conditions 

• Resettlement and Rehabilitation considerations 

• Environment Considerations

(iii) Temperature 

a) Air temperature in the hills is lower than in the valley. The temperature dropped approximately 0.5° per 100 m. 

b) On slopes facing south and southwest snow disappears rapidly and rain water evaporates quickly while on slopes facing north and northeast rain water or snow may remain for a longer time. 

c) Unequal warming of slopes, sharp temperature variations and erosion by water are the causes of slope failure facing south and southwest.

(iv) Rainfall 

a) Rainfall generally increases with increase in height from sea level. 

b) The maximum rainfall is in the zone of intensive cloud formation at 1500-2500 m above sea level. Generally, the increase of rainfall for every 100 m of elevation averages 40 to 60 mm. 

c) In summer very heavy storms/cloud bursts may occur in the hills and about 15 to 25% of the annual rainfall may occur in a single rainfall. The effects of these types of rainfall are serious and should be considered in design.

(v) Atmospheric pressure and winds 

Atmospheric pressure decreases with increase in elevation.  At high altitudes, the wind velocities may reach up to 25-30 m/s and depth of frost penetration is also 1.5 to 2 m.  Intensive weathering of rocks because of sharp temperature variations.  

(vi) Geological conditions  

  • The inclination of folds may vary from horizontal to vertical stratification of rock. These folds often have faults. Limestone or sandstone folds may be interleaved with layers of clay which when wetted may cause fracturing along their surface. This may result in shear or slip fold.  

  • The degree of stability of hill slopes depends on types of rock, degree of strata inclination or dip, occurrence of clay seams, the hardness of the rocks and presence of groundwater. 

  • When locating the route an engineer must study the details of geological conditions of that area and follow stable hill slopes where no ground water, landslides, and unstable folds occur. 

(vii) Resettlement and Rehabilitation 

Due to limited availability of flat areas and connectivity issues, most of the residential and commercial activity happens very close to the road leading to large scale R&R and becomes a challenge in alignment design. 

(viii) Environment  

Hills are ecologically sensitive areas relatively untouched by human activity. The alignment design must attempt to minimize tree cutting and large scale earth filling/cutting to minimize damage. 

(ix ) Route Selection 

Hill road alignment may follow alignment at Valley bottom or on a ridge depending on the feasibility of the road. The first is called River route and the second is called Ridge route. 

a) River route  

  • Most frequent case of hill alignment as there is a great advantage of running a road at a gentle gradient. Runs through lesser horizontal curvature.  

  • Requirements for the construction of bridges over tributaries.  

  • Construction of special retaining structures and protection walls on hillside for safeguarding the road against avalanches in high altitude areas.  

  • Benefit of low construction cost and operation cost.

Ridge route  

  • Characterized by the very steep gradient. Large number of sharp curves occur on the road with hair pin bends. Extensive earthwork is required.  

  • The requirement for the construction of special structures. High construction and operation cost.