Methods of locating hill road alignment

Hill road alignment is located by the following methods (IRC:SP:48; NRS 2070 also recommends an engineering survey procedure).

1. Reconnaissance and desk study

Topographic maps, aerial photographs and satellite images are studied to find the controlling points (passes, river crossings, settlements) and possible routes. A walk-over reconnaissance then examines the geology, slope, drainage, existing trails and soil conditions.

2. Preliminary survey - "contour/grade line" method

A gradient (grade) line is set on the ground using an Abney level/clinometer and ranging rods so that the road stays at the chosen gradient (usually 5-6%) with minimum cut and fill. The method gives a trial line along the contours:

• A ruling-gradient line is set from the starting point; the length of road between successive contours equals the contour interval divided by the gradient (for example, a 5% gradient with a 10 m contour interval gives 200 m of road per contour).
• Where the natural slope is gentler than the ruling gradient, the line is free; where it is steeper, the length is developed using hairpin bends.

3. Method of ridge/valley (route classification)

• Ridge line (spur) alignment - follows the ridge; stable and well drained, but long.
• Valley / river route - follows a river; low gradient, but flood and bank problems.
• Mid-slope alignment - along the hillside; most common; requires side-hill cutting and retaining walls.

4. Final location survey

The chosen route is surveyed by traverse and levelling; the centre line is marked on the ground with pegs at 20 m (or closer) intervals, curves are set out, and the final alignment is checked against the geometric standards.

5. Modern methods

Photogrammetry, GPS, Total Station and GIS/DEM-based route selection can be used to study and optimise routes.