Factors controlling hill road alignment

The alignment of a hill road is controlled by the following factors (IRC:SP:48, NRS 2070).

Factors controlling alignment

1. Topography - an alignment on a gentle slope with a minimum of cut and fill is chosen; use spurs and benches; avoid crossing deep gullies.
2. Geology and soil - stable rock or soil, no slip zones, favourable dip of strata.
3. Drainage - a minimum number of stream crossings, at good bridge sites; avoid water-logged and seepage areas.
4. Hydrology/rainfall and climate (see below).
5. Traffic and design speed, which fix the maximum gradient and the geometric standards.
6. Gradient and curves - keep within the ruling gradient; use hairpins only where height must be gained.
7. Economy and construction - cut-fill balance, access for equipment, availability of materials.
8. Environment and settlements - villages served, forests, farmland and cultural sites.

Effect of temperature, rainfall, pressure and geology

• Temperature: in cold regions, sun-facing (southern) slopes are preferred so that snow and ice melt early and frost action and icing are reduced. Frost heave needs a good sub-base and drainage. On warm, sunny slopes the road is drier and more stable.
• Rainfall: heavy rainfall demands good drainage and stable slopes. Choose slopes that do not receive large runoff and avoid slopes prone to rainfall-triggered landslides; roads on the leeward (less rainy) side are less affected. Keep the road away from gullies and streams, and provide catch drains.
• Pressure (wind/atmospheric): at high altitude the air pressure is low, so engine power drops and vehicles climb poorly; a flatter ruling gradient is therefore used (reduced above 3000 m altitude). Strong wind and snow drifting favour a sheltered, lee-side alignment.
• Geology: choose hard, sound, well-bedded rock with joints dipping into the hill; avoid fractured rock, faults, loose scree, old landslides and clay beds that dip towards the valley. Stability governs the cut slope angle and the number of retaining structures.