Hill road drainage structures

Drainage of a hill road must remove (a) rain falling on the carriageway and hillside above it and (b) groundwater seeping into the slope. Poor drainage is the main cause of landslides, slips and pavement failure in the hills. Structures follow IRC:SP:48 (Hill Road Manual) and Nepal Road Standard (NRS 2070) / DoR hill-road guidelines.

Surface drainage structures

1. Catch water drain - cut along the hill side, about 5-10 m above the top of the cut slope, parallel to the road. It intercepts runoff from the upper catchment. Lined (stone masonry/concrete) in erodible soil; bed slope generally 1-6%.
2. Side (longitudinal) drain - on the hill side of the carriageway; V-shaped or trapezoidal, lined with masonry/concrete; carries water from the road surface and cut face to the nearest cross-drain.
3. Cross drainage works - culverts (pipe, slab, box), causeways/Irish bridges and small bridges, placed at every natural stream and, on long sections, at spacing of about 50-100 m so that side-drain flow does not build up.
4. Chutes / cascades and drop structures - stepped masonry channels that carry water safely down steep valley-side slopes from culvert outlets.
5. Camber/crossfall - 2.5-3% on the bituminous surface (3-4% on gravel), or inward slope on the hill side, to shed water quickly.
6. Berms, kerbs and spouts on the valley side to direct water away from fill slopes.

Sub-surface drainage structures

1. Cut-off (intercepting) drains - trenches filled with graded filter material and a perforated pipe, placed across the seepage flow to lower the water table.
2. French drains - gravel-filled trenches (with filter fabric or graded sand) for seepage from the cut slope.
3. Horizontal (weep) drains - 50-100 mm perforated pipes driven 10-30 m into the slope with 5-10% upward inclination.
4. Sand/gravel blanket under the subgrade and weep holes (75-100 mm at 2-3 m c/c) in breast and retaining walls.

Layout

   Hill slope (uphill)
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   \ catch water drain  (5-10 m above cut)
    \_____ 
       \  cut slope
        \ breast wall
   ======\=side drain=====  <- road
   ____camber____ 
        shoulder  \  fill slope
                    \ retaining wall
   culvert  ====>  chute -> natural stream

Water collected in the catch and side drains is led to a cross-drain, passed under the road and discharged by chute or masonry channel into a natural watercourse, never onto the fill slope.