Culverts: Types, Function and Design of Culverts

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A culvert is a closed conduit placed under the embankment to carry water across the roadway. Culverts are preferred to minor bridges since culverts are always beneath the carriageway, whereas bridge surfaces form a part of the carriageway. Culverts are designed to flow ultimately under m conditions, and bridges are designed to pass floating debris or vessels.



  1. Functions of Culvert  

The function of culverts is as follows:

1. Collecting and leading the water across the road so as not to cause damage to the road bank or the stream bed by scour.
2. Allowing sufficient waterway to prevent heading up of water above the road surface.



  2. Design of Culverts  

Culvert design requires a broad knowledge of hydrology, hydraulics, and hydraulic characteristics mechanics. Culverts may have different shapes and hydraulic characteristics (pressure or non-pressure—flow., construction material. The selection of culvert type and material is the subject of engineering based on comparative cost, suitability for a particular location, availability of materials, labor, etc.

Culverts mainly consist of three parts: culvert barely, inlet structure, and outlet structure. Depending on the soil foundation, Culvert bedding may be of sand, PCC, or RCC.

The alignment of the culvert should conform closely to that of the natural waterway in respect of plan and profile. Therefore the culvert may be skewed and right-angled concerning highway alignment. But the culvert slope should confirm the natural bed slope of the waterways.





  3. Types of Culvert  

The selection of culvert types depends on the nature of the waterway, cost, the importance of roads, availability of construction technology, soil foundation etc.

1. Pipe Culvert

2. Box Culvert

3. Slab Culvert

4. Arch Culvert


1. Pipe Culvert

When a stream carries low discharge and alignment has a high embankment, pipe culverts are considered more suitable. The pipe is laid slightly inclined. There should be at least a 50cm soil cover so that the traffic load transmitted on the pipe is of minor intensity and without vibrations.

Single or double-barrel precast concrete pipe culverts are commonly used for small openings up to 2 m2. The minimum diameter of the pipe culvert is limited to 600 mm to facilitate cleaning and avoid blocking. The standard length of the RCC pipe is fixed at 2.5 m, jointed by collar or tongue and groove. Pipes may be made of stoneware, concrete, R.C.C., etc. The standard size of pipe culverts is 0.5m, 0.75m, lm, 1.25m, and 2.0m in diameter.


Pipe Culvert


2. Box Culvert

These culverts are constructed where the nature of the soil below the foundation is not suitable for individual footing under piers an(l abutments. The size of the rectangular passage should not be less than 60cm x 60cm for easy cleaning of debris.

RCC rigid frame box for square or rectangular opening span up to 3.0m is used as box culvert whose height rarely exceeds 3m. These culverts have superior durability for the worst environmental conditions, excellent resistance to debris damage, greater hydraulic efficiency, and more significant life spans.


Box Culvert


3. Slab Culvert

These culverts have masonry abutments with stone slabs over them. In localities where the stone is readily available, these culverts are mostly U. Where stone patties are not available, R.C.C. slabs are used, RCC slabs are designed as supported slabs, and the span of R.C.C. may be about 3m.

Where the water opening is less than 15 m2, and the road curves the waterway on a relatively high barrier, slab culverts are Multiple slab culverts that may be used for the large waterway. These should be located where the road crosses a valley, where the stream and watercourse should not move. A minimum freeboard of 0.5m is required in this type of trench.


Slab Culvert


4. Arch Culvert

This type of culvert is preferred under conditions where high fillings are involved and heavier loadings on the culvert. Arches may be built from brick or stone masonry or plain cement concrete. The span of each arch should be kept less than 3m.

The selection of the culvert to be used is made based on the availability of construction materials and economic considerations.


Arch Culvert



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