A hill road may be defined as the one which passes through a terrain with a cross slope of 25% or more. There may be sections along hill roads with the cross slope less than 25%, especially when the road follows a river route. Even then these sections are also referred to as hill roads. Hence, to establish a hill road overall terrain must be taken into account.
The hilly regions generally have extremes of climatic conditions, difficult and hazardous terrains, topography and vast high altitude areas. The region is sparsely populated and basic infrastructural facilities available in plain terrain are absent. Hence, a strong stable and feasible road must be present in hilly areas for overall development of other sectors as well.
Design and Construction Problems
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:
- A hilly or mountainous area is characterized by highly broken relief with vastly differing elevations and steep slopes, deep gorges etc. which may unnecessarily increase road length.
- The geological condition varies from place to place.
- Hill slopes stable before construction may not be as stable due to increased human activities.
- There may be variation in hydro-geological conditions which may easily be overlooked during design and construction
- Due to highly broken relief construction of special structures should be done at different places. This increases the cost of the construction.
- Variation in the climatic condition such as the change in temperature due to altitude difference, pressure variation, precipitation increases at greater height etc.
- High-speed runoff occurs due to the presence of high cross slopes.
- Filling may overload the weak soil underneath which may trigger new slides.
- The need of design of hairpin bends to attain heights.
Special Consideration in Hill Road Design
Alignment of Hill Roads
Selecting an alignment in the hilly region is a complex task. The designer should attempt to choose a short, easy, economical and safe comforting route.
General considerations
When designing hill roads the route is located along valleys, hill sides and if required over mountain passes. Due to complex topography, the length of the route is automatically increased. Due to harsh geological conditions, special structures also have to be provided.
Apart from the highly broken relief which has a fixed role in determining the alignment and location of special structures, climatic and geological conditions are also important. In locating the alignment special consideration should be made in respect to the variations in:
- Temperature
- Rainfall
- Atmospheric pressure and winds
- Geological conditions
Temperature
- Air temperature is in the hills is lower than in the valley. The temperature drop being approximately 0.5° per 100 m of rising.
- 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 the longer time.
- Unequal warming of slopes, sharp temperature variations and erosion by water are the causes of slope facing south and southwest.
Rainfall
- Rainfall increases with increase in sea level.
- 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.
- In summer very heavy storms may occur in the hills and about 15 to 25% of the annual may occur in a single rainfall. The effects of these types of rainfall are serious and should be considered well.
Atmospheric pressure and winds
- It 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 which cause high winds.
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 ground water.
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.
Hill roads may follow different path according to the feasibility of the road. However, a hill road alignment varies for the sections along the valley bottom and along the mountain pass. The first is called river route and the second is called ridge route.
River route
The location of a route along a river valley is the most frequent case of hill alignment as there is a great advantage of running a road at a gentle gradient. Also, there is a benefit of low construction cost and operation cost.
However, a river valley may run through numerous horizontal curves. Requirements for the construction of large bridges over tributaries also may occur. It may also be necessary to construct special retaining structures and protection walls on hill side for safe guarding the road against avalanches.
Some important considerations
- Road bed should be located sufficiently above and away from the maximum water level.
- When the road bed is near to the waste water course embankment slope should be well protected and stabilized.
- More care should be given to geological and hydrological structures.
- Best alternatives should be selected for crossing water sources.
Ridge route
- It is characterized by the very steep gradient.
- A large number of sharp curves occurs on the road with hair pin bends.
- Extensive earthwork is required.
- The requirement for the construction of special structures.
- The necessity of long length away from the air route.
Gradient
In hill roads, a heavy amount of earthwork is required. So to reduce the earthwork for reducing construction cost the gradients selected are close to maximum. Although steep gradients help in reducing earthwork and length of road, it also causes increased fuel consumption and reduction in operating speed as the vehicles will be on low gears which will use more energy. So both these factors must be taken into account and a suitable solution should be chosen.
The cumulative rise or fall in elevation should not exceed 100 m in mountainous terrain and 120 m in steep terrains. Vertical curves are designed as the square parabola. The curves should be provided at all grade change exceeding those indicated in the table below:
| Design speed | Maximum grade change not requiring a vertical curve | Minimum length of vertical curve |
| Up to 35kmph | 1.5% | 15m |
| 40kmph | 1.2% | 20m |
| 50kmph | 1.0% | 30m |