Special considerations in hill road design

Hill road construction is more challenging than in the plains because of steep terrain, weak geology, heavy rainfall, and limited access. The following special considerations apply (IRC SP:48 and NRS 2070).

1. Alignment and survey: study the geology and drainage, and avoid unstable slopes, landslide zones and slip-prone geology. Choose the alignment on stable (rock) slopes and ridges where possible, with the minimum number of hairpin bends and cross-drainage crossings. Follow the contour with a balanced grade.
2. Gradient: the ruling gradient is about 5 to 6% (limiting 7%, exceptional 8%) in the mountains, with grade compensation on curves. Gradients are kept as steady as possible, and on long grades steeper than 5% a stretch of easier gradient (not steeper than 2.5%, at least 100 m long) is provided after every 2 km of rise.
3. Geometric design: reduced design speeds (20 to 40 km/h) and narrow widths; hairpin bends with a minimum inner radius of about 14 m, easy approach gradients and a minimum of 60 m between successive bends; extra widening on curves; sight distance with setbacks; and parapets or guard stones for safety.
4. Cross-section: balanced cut-and-fill sections with retaining and breast walls; the road is given an inward slope towards the hill (about 4 to 5%) for drainage.
5. Drainage: more extensive than in the plains, with catch drains, frequent cross drains and mitre drains, lined side drains, energy dissipation, and no concentrated discharge onto slopes.
6. Slope stability and protection: correct cut slopes for each soil or rock, benches, retaining walls, gabions, bio-engineering (vegetation, "green road" approach), and controlled spoil disposal at proper tipping sites.
7. Construction methods: controlled blasting, labour-based methods and phased works, planned to avoid the monsoon.
8. Pavement: light, flexible pavements with good sub-surface drainage; stage construction where necessary.
9. Hazards: allow for landslides, debris flows, seismic effects and flash floods; include protective works, warning signs and maintenance plans.
10. Environmental and social issues: minimise forest clearance and slope disturbance, and follow DoR's environmental guidelines.

These considerations make hill roads more costly and slower to build, and they need sound geotechnical judgement and regular maintenance.