| Purpose | Pass water under a road and protect the embankment |
|---|---|
| Types | Pipe culvert, slab culvert, box culvert, arch culvert |
| Main parts | Barrel (waterway), headwalls, wing walls, apron, foundation and cut-off walls |
| Materials | RCC, precast concrete pipes, masonry, steel |
| Design guide | IRC publications for small bridges and culverts |
Every road crossing a stream or low area needs a way for water to pass. A culvert does this, and if it is too small or blocked, water backs up and can overtop or wash away the road. Culverts are part of road construction, and poor drainage is one of the biggest reasons roads fail, as explained in why roads deteriorate.
Types of culverts
| Type | Shape and material | Suits |
|---|---|---|
| Pipe culvert | One or more circular RCC pipes | Small discharges, low fills, quick construction |
| Slab culvert | RCC slab over abutments or walls | Small to medium spans, shallow streams |
| Box culvert | Rectangular RCC box with one or more cells | Larger discharges, soft soils, low headroom |
| Arch culvert | Stone, brick or concrete arch | Higher fills; traditional or heritage works |
Parts of a culvert
• Barrel: the waterway through the embankment.
• Headwalls: walls at the ends that hold back the embankment soil.
• Wing walls: walls that guide the water in and out and retain the sides.
• Apron: a protective floor at the inlet and outlet to prevent scour.
• Cut-off walls: walls below the apron that stop water from flowing under the structure.
Design points
• Estimate the design discharge from the catchment area, rainfall and runoff (for small catchments, the rational method is common).
• Provide enough waterway so that water does not back up above the permitted level.
• Set the culvert level and slope to match the natural stream bed.
• Check the loads from traffic and the fill above the barrel.
• Protect inlet and outlet from scour with aprons, pitching or riprap.
Always check the clause numbers and limits against the latest IS/IRC code and your structural designer before using them on a project.
Why culverts fail
| Problem | Result |
|---|---|
| Blocked barrel (silt, debris) | Water backs up and overtops the road |
| Undersized waterway | Flooding in heavy rain |
| Scour at the outlet | Foundation is undermined |
| Water seeping under the structure | Piping and washout of the embankment |
| Poor compaction around the culvert | Settlement and cracks in the road above |
Culverts should be cleaned before the monsoon and checked after heavy rain. For the road surface above, see flexible pavement layers and design.
What is a culvert?
What is the difference between a culvert and a bridge?
What are the types of culverts?
Why do culverts get blocked?
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