Hill roads – placement of side drains on spur and re-entrant curves Along a hill road, on which side should the longitudinal side drain be provided for spur curves and re-entrant curves to effectively intercept hillside runoff?

Introduction / Context:
On mountainous roads, controlling surface water is crucial for slope stability and pavement longevity. Improper drain placement can let hillside runoff flow across the carriageway, causing erosion, rutting, and safety hazards.

Given Data / Assumptions:

• Two plan forms: spur (projecting outward) and re-entrant (valley-facing) curves.
• Objective: intercept hillside runoff before it reaches the carriageway.

Concept / Approach:
The side drain should be located on the hill side (inner side) so that runoff descending from upslope is captured immediately. Whether the alignment traverses a spur or skirts a re-entrant, the inner side is the controlling upslope side for interception, minimizing cross-traffic of water.

Step-by-Step Solution:
Identify the hill side (inner side) for both curve situations.Locate the drain on this inner side to catch hillside water.Provide catch water drains as needed above the road to further intercept flows.Therefore, the correct placement is the inner side for both spur and re-entrant curves.

Verification / Alternative check:
Field manuals emphasize placing longitudinal drains on the hill side together with cross-drainage at regular spacing to lead water safely to the valley side via cross-drains or chutes.

Why Other Options Are Wrong:
Placing drains on the outer (valley) side fails to intercept hillside runoff, allowing water to cross the pavement, which increases damage and accident risk.

Common Pitfalls:
Insufficient cross-drains for conveying the intercepted flow; neglecting protection of outlets against erosion.

Final Answer:
inner side of both (a) and (b)