In this article we will discuss about:- 1. Meaning of Culvert 2. Types of Culvert 3. General Design Principle 4. Design.
Meaning of Culvert:
A small bridge may be of 3 to 4 spans with the length of span not more than 3 m is known as culvert. In case of highway where culverts are constructed to cross the small distributary etc., the span length may be about 4.5 m. Similarly, in case of railway track the maximum span length can be about 6 m and should not exceed this limit.
The culvert mainly consists of aboutment, wingwalls, arches or desk slab, parapet and foundation. Floor pitching from inside the culvert in the bed of drain may or may not be provided, it depends on the soil condition and flow velocity of the stream water.
Types of Culvert:
The culverts are generally of five types.
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These are given as under:
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.
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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.
3. Arch Culverts:
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These culverts involve the construction of super structure in the form of arches. The> are similar to the Masonry bridges. The arch culverts are not provided with the piers and batters to the sides of aboutment.
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 aboutments. These slabs form a super structure over the culvert. The parapet and wingwalls am also provided in this culvert, similar to the permanent bridge.
5. Beam and Slab Culverts:
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It’s layout is similar to the big grider 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 grider bridge.
General Design Principle of Culvert:
The design of culvert or bridge 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.
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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 aboutment 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.
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.
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.
Design of Culvert:
It consists of following parameters for determining:
1. Wingwall:
The function of wingwall in the culvert is to retain the earth materials behind it, and also to save the soil against flood water. There are various types of wingwalls such as straight, return, curved, splayed etc., are used in the culverts. The return type wingwalls are furnished at 90° with the aboutment, while splayed types are at 45°.
The wingwalls are constructed, using the brick or Masonry materials. The thickness of it varies from 30 to 40 cm at the top and 0.3 H to 0.4 H at the bottom (H is the height of the wingwall). The length ranges from 1.5 to 2 times the height of aboutment.
2. Piers:
All intermediate supports between the aboutments are known as piers. The top thickness of pier for different span lengths and culvert types, is given in Table 20.1.
3. Aboutment:
End supports of the culvert/bridge are known as aboutments.
Its main functions are given below:
i. It carries the loads of the culvert/bridge; and —
ii. It retains the earth fill behind it.
The design of aboutment is given as under:
Thickness of Aboutment:
The thickness of aboutment is different for different types of culvert and length of the span, used.
These are given below:
1. For Slab Culvert:
In case of slab culvert, the top thickness of aboutment varies according to the length of span, given as under:
The base thickness of aboutment is fixed as per its height. It should be kept in the range of 0.4 H to 0.45 H, in which H is the height of aboutment.
2. For Arch Culvert:
The top thickness of aboutment in case of arch culvert can be calculated by using the following formula –
Where,
t = top thickness of aboutment, cm
R = soffit radius, cm
r = rise of arch, cm
S = span of arch, cm
The top thickness of aboutment for different arch angles can also be calculated by using the following formulae:
In which, C is the constant, which depends on the length of the span. The values of C are given in Table 20.2.
For making the aboutment stable, it should be furnished at some slope from its back, depending on the arch angles.
The recommended slopes for different arch angles are given as under:
Using these slope values of the aboutment, its base thickness can also be calculated by using the following equations for different arch angles –
For 60°, 75° and 90° arches –
T = t + (H/2) … (20.2)
For 120° to 180° arches –
T = t + (H/3) … (20.3)
In which, T is the base width and H is the height of aboutment above the foundation.
The passage through which water flow takes place is known as water-way, and length of this passage is known as linear water-way of the bridge. In case of arch culvert, the segmental type arches are mainly used. The thickness of it can be computed by using the following formula –
Where,
R = radius of soffit
S = arch of span
N = constant, its values are given as,
= 0.4 for a culvert, which has single span
= 0.45 for a culvert, which has multi spans
Haurst formula gives a satisfactory result, and is commonly used for computing the thickness of aboutment (T). When culverts are covered under large embankment, then 50% additional thickness should be provided to the aboutment.
Depth of Foundation:
It is determined on the basis of maximum load acting on it and safe bearing capacity of the soil. For calculating the depth of foundation, the following formula can be used –
Minimum depth of foundation = 1.13 D
In which, D is the maximum depth of scour. For small culverts, constructed in non-erodible beds, 45 to 60 cm deep foundation from the bed level is used.