Route location in hill road design

Route location in hill roads is the process of choosing a stable, economical alignment between the fixed end points over difficult, steep and unstable terrain.

Steps

1. Identify control points - the end points, mountain passes, saddles, bridge sites on rivers, settlements and market centres, and points to avoid (landslide zones, unstable slopes, forests, religious sites).
2. Study of maps and photographs - topographical maps, aerial photographs, satellite imagery and geological maps give possible routes.
3. Reconnaissance - walk the corridor to assess slope, geology, drainage, water sources and construction material.
4. Selection of route - the choice between a ridge, a mid-hill or a valley (river) route according to the following.

Main principles of hill road location

• Prefer sunny, stable and dry slopes; avoid shaded, wet slopes.
• Gradient near the ruling gradient (5%; limiting 6%); use hairpins only to gain height (spaced well, on stable ground).
• Keep the cross-drainage crossings as few as possible and at the best sites (right angle to the flow).
• Keep cut and fill balanced and the cutting height lower than the stability limit; avoid deep cuts and high fills.
• Avoid landslides, scree, rock falls, springs and unstable geology.
• Take minimum land and environmental damage, and keep the cost low for construction and maintenance.
• Provide for future widening and stage construction (NRS 2070 recommends the engineering survey followed by a detailed geotechnical study).