culverts civil engineering

1. CULVERTS
A culvert is a small bridge having a total length of 6 m or less than 6 m between the faces of abutments.
It is a permanent drainage structure mainly constructed to carry roadway or railway track over small streams or
channels.
A culvert may act as a bridge for traffic to pass on it. They are typically found in a natural flow of water and serves
the purpose of a bridge or a current flow controller.

2. COMPONENTS OF A CULVERT
Following are some of the significant parts of culverts:
• Sidewalk.
• Road embankment.
• Headwall.
• Wing wall.
• Apron.
• Crown.
• Culvert pipe.
• Culvert inlet.
• Culvert outlet.
• Calvert Foundation.

3. TYPES OF CULVERTS
1. Pipe Culverts:
The pipe culverts are used for very small drainage works, passing through the embankment of road or railway. It consists
of one or more pipes placed side by side each other. The pipes are generally made of steel, R.C.C., cast iron etc.
To retain the pipes and embankment at their proper position against the action of spring water etc., they are laid with
masonry work in the form of arches at both the ends.
Sometimes, it is also performed by stone pitching. In the case, when height of embankment is more than 6 m over the
pipe then an extra care is needed.
In this condition, there should be given a proper cushion, all-round. The cushion should not be less than half of the
diameter of the pipe with a maximum 45 cm at the top.

4. 2. Box Culverts:
These culverts are preferred for construction, especially in loose soil condition, and for a larger span.
These can be used for a single span of 3 m or for a double span of 6 m width.
Practically, it has been found that the box culverts are not economical, when span width exceeds 6 in.
The thickness of R.C.C slab should be kept within the range of 1.25 to 2.5 m. These culverts are constructed in the form
of one or more rectangular or square openings. The box culverts usually made of precast R.C.C. materials.
The design of this type of culvert is based on the continuous beam theory. It requires a good foundation. The sinking of
any portion causes an effective change in the bending moment as well as shearing force in the section.

5. 3. Arch Culverts:
These culverts involve the construction of super structure in the form of arches.
They are similar to the Masonry bridges. The arch culverts are not provided with the piers and batters to the sides of
abutment.
4. Slab Culverts:
The slab culverts are constructed for a span of maximum 2.5 m in width.
To construct the slab culvert, a stone or R.C.C slab is directly placed over the simply supported beam on piers or
abutments.
These slabs form a super structure over the culvert. The parapet and wing walls am also provided in this culvert, similar
to the permanent bridge.

6. 5. Beam and Slab Culverts:
It’s layout is similar to the big girder bridge.
The difference is only in respect of span length, number of spans and size of different structural components of the
culvert, which are normally smaller than the girder bridge.

7. DESIGN PRINCIPLES OF CULVERTS
Design of culverts should be based on the following considerations:
1. Site Selection:
It is an important consideration in the view of economy and safety of the culvert/bridge.For this purpose the following points
should be considered for ideal location of culvert:
i. It should be in a straight reach of the stream.
ii. There should be a good soil strata for the construction of abutment and piers’ foundation.
iii. There should be firmed bank on either side of the stream for culvert’s foundation.
iv. The crossing width should be minimum at the site. This reduces the length of culvert and its cost of construction, also
v. At the construction site, the stream flow should not be in turbulent nature, because it causes soil scouring near the
foundation, which can cause damage the culvert.
vi. As far as possible the straight crossing of stream section should be considered.
vii. Approaches on both sides of the culvert should be in straight form, at feast for the distance of 15 m.

8. 2. Alignment:
After selecting the construction site of culvert, the next step is to decide the alignment of culvert.The exact alignment
should be done by considering the following points:
i. The alignment should be in square, i.e. the axis of culvert should cross the stream exactly at 90°.
ii. As far as possible the skew alignment should be avoided.
iii. Alignment should be such that a smooth entry and exit of the stream flow can be obtained.
iv. It should not be curved, because it requires a great care and maintenance. It addition, at the curved alignment the
culvert is resisted by an additional force due to centrifugal action.
3. Discharge:
The discharge rate to be passed through the culvert should be carefully estimated, because safety and stability of the
culvert depend on the discharge rate passing through it.
There are several empirical relationships have been developed to predict the discharge rate.

9. 4. Number of Spans:
The number of spans should be such that the cost of culvert cannot be high. Generally, 3 to 4 spans are preferred to use in
the culvert.
5. Foundation:
A foundation should satisfy the following points:
i. It should be sufficient deep, so that the soil below it can bear the load without settlement.
ii. It should be kept beyond the maximum scouring depth of the stream. In case of pile and well foundation, the depth
should be such that, there could be developed more frictional resistance so that the culvert can be safe against sliding.
6. Clearance:
The main purpose of clearance in the culvert is to eliminate the possibilities of striking the normal traffic with the culvert.
The superstructure over it should be such that, no part of its should fall beyond the culvert’s width.