Short note: slab culvert

A slab culvert is a cross-drainage structure in which a reinforced concrete slab (deck) rests on two abutments, with an open rectangular waterway below. It is used for spans of up to about 6 m when the foundation is firm and the stream bed is stable, and it needs little height between the bed and the road level. In Nepal it is widely used on hill and Terai roads.

Elements

1. Deck slab: RCC slab designed for IRC wheel loads (Class A / 70R, as adopted in DoR practice). Bearing on abutments through a bearing strip (a simple bitumen/felt layer).
2. Abutments: stone masonry or concrete walls that support the slab and retain the road fill.
3. Wing walls and return walls: guide the flow, retain the embankment, and protect from scour.
4. Foundation: strip or raft footing on firm soil or rock, taken below the scour depth.
5. Bed protection (floor/apron, curtain walls and cut-off walls): upstream and downstream to prevent undermining.
6. Parapet / kerbs and wearing coat: provide safety to traffic and a riding surface.
7. Drainage: weep holes in the abutments.

 parapet |-- wearing course --| parapet
         |=== RCC deck slab ===|
  wing   | abutment   abutment | wing
  wall   |_____waterway_______| wall
          apron + cut-off walls

Advantages: the open waterway is easy to inspect and clean, and the structure is quick to build. A disadvantage is that it needs abutments on good foundation, so it is not suited to soft soil (where a box culvert is better).