Short note: hairpin bends

A hairpin bend is a sharp turn of about 180 degrees (deflection angle larger than about 120 degrees) that is introduced on a hill road to gain height when the natural ground slope is too steep for the road to climb within the ruling gradient. The road doubles back on itself in a "U" shape on a spur or gentle spot.

Types

• Circular type - one circular arc of radius R with straight approaches.
• Compound/Transition type - a central circular arc with two transition curves; used for larger radii and where the road is wide.
• Symmetrical and unsymmetrical hairpins, depending on the layout around the apex.

Design and location (IRC:SP:48; NRS 2070 follows similar limits)

• Locate on a stable, gentle (preferably < 30 degrees) and well-drained hill slope, on a spur or bench, never on a landslide-prone or unstable slope; avoid cutting a deep slice.
• Minimum radius of the inner curve about 14 m (IRC). Extra widening is provided on the inner side so that vehicles can turn.
• Gradient on the curve: not steeper than 1 in 40 (2.5%) on the hairpin (and not flatter than 1 in 200 for drainage); approaches within the ruling gradient (about 5-6%).
• Superelevation up to 10% (1 in 10) maximum; the full superelevation is achieved by rotating about the inner edge.
• A minimum intermediate distance of 60 m between consecutive hairpins (and between a hairpin and other curves) where possible.
• Provide good drainage, retaining/breast walls and parapets, adequate sight distance, and an adequate roadway width at the bend (IRC: about 9.0 m for single-lane and 11.5 m for double-lane roads, including widening).
• Avoid more than a few hairpins in a series; if they are needed, space them well apart.

Hairpins increase the road length, construction cost and vehicle running cost, so they should be used only when developing length by switchbacks cannot be avoided.