2_Cause Way Design

8/11/2019 2_Cause Way Design

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Learning Element: LE-4 Causeways

CAUSEWAYS

Remember that:

- the culverts should be distributed evenly throughout the length of the structure;

- an apron of reinforced concrete or hand-packed stones is essential to break the energy of the over-flowing water;

- causeways should have a one-way camber of 3 per cent towards the down-stream edge and their up-stream edge should be rounded;

- road-side markers should be streamlined to limit their resistance to the over-flowing water;

- drifts and causeways should not be located near or in a river bend but in a stable portion of the river bed so that a good foundation can be constructed;

- the culvert bed should be a 15 cm gravel layer, preferably topped by a 7.5 cm concrete floor;

- the minimum width of the apron should be two times the height of the causeway from the river bed. This width should never be less than two metres;

- the depth of the foundation trench at the down-stream side of the causeway should be at least the height of the expected overflow of water over the structure.

Construction

- See FIELD INSTRUCTIONS - DRIFTS.

- Place reference pegs, survey pegs and multi-purpose pegs. Excavate the foundation trenches.

- Prepare the bed for the culverts. A layer of gravel (minimum 15 cm) preferably topped with a layer of 7.5 cm of concrete.

- Lay the culverts on the correct positions and at the correct gradient (3.5 per cent).

- Construct the headwalls up to road level.

- Backfill the area between the head walls with a suitable material and compact this material thoroughly.

- Pour a layer of concrete (10 cm) to provide a smooth surface for the traffic. The transitions between the horizontal stretch and the approaches should be gentle.

- Excavate the trench for the apron and place the stones (the largest at river bed level near the down-stream edge of the causeway).

After you have learned this element you should be able to:

- explain what a causeway is;

- describe which specific design points are particular to a causeway;


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- comment on how to locate a causeway;

- describe how a typical small causeway is constructed.

In engineering textbooks a water crossing having a number of culverts to receive regular waterflows and which is submersed during floods is described with a great number

of names; culverts drift, causeway, submersible bridge or vented drift. In this learning element the structure is described as a causeway.

A causeway can be defined as a structure which has a double function:

(a) it allows the normal dry weather flow of a river/stream to pass through the culverts below the roadway; and

(b) the occasional floods pass both through the culverts and over the roadway.

Fig 13A


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Fig 13B

Because they have this dual function causeways present hydraulic problems which are peculiar to this type of structure and great care should be taken with their

construction. Many causeways have failed because of a wrong location or a wrong design.

Design

If the culverts are concentrated in the centre of the causeway, the high speed water jets coming out of these culverts will cause heavy scour at the sides of the culverts

(figure 13A).

This implies that in designing causeways the culverts should be distributed evenly throughout the length of the structure (figure 13B).

When the water flows over the causeway it moves as shown in figure 14. The forward roller shown on the right moves the sand forward. The back roller makes the sand

surface on the left side steeper and steeper until it collapses. This effect causes the foundation to be exposed very quickly unless effective protective measures are taken.


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Fig 14

A suitable apron of concrete or well-graded stones is therefore absolutely necessary to break the energy of the falling water.

Fig 15

To improve the hydraulic properties of the causeway it is recommended to have a one-way camber towards the downstream end of the causeway. The rounding of the

upstream edge further contributes to a smooth discharge of the overflowing water.


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Fig 16

As regards the culvert capacity, when suitable divergences (flared end sections) are provided at the end of the pipes this can be increased by as much as 50 per cent. Such

sections are manufactured in steel or concrete and should be considered when more major causeways are constructed.

Fig 17

Plan

Elevation


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When roadside markers are provided to show the edges of the causeway and to indicate the stream level, care should be taken that these offer as little resistance as

possible to the flow. This means that such markers should be streamlined to the greatest possible extent.

Location

The location of a causeway is - as with all structures - extremely important. Efforts should be made to locate the most stable portion of the river bed, so that a better

foundation can be provided to the structure.

Also, avoid locating a drift, bridge or causeway near or in a river/stream bend. During floods the water will tend to cling to the concave bend of the river and causecontinuous erosion at that side. At the convex bank silting will take place.

When a lot of debris (tree trunks or branches, etc.) floats in the river there will be a danger that the culverts get blocked. If this happens, usually a transverse flow will occur

along the upstream side of the culverts creating a deep trench which can undermine the structure. To avoid this, debris diverting or collecting structures, such as debris

deflectors or racks, may be necessary.

Construction

Two trenches are dug at the up and downstream edges of the causeway. The trench at the downstream side should be at least 1.50 metres deep if an overflow of 50 cm

over the roadway is expected (see figure 16).

Between these trenches a suitable bed for the culverts is prepared of a well-graded gravel material. The provision of a 7.5 cm concrete floor is recommended. This floor

should be indented to accommodate the shape of the culvert, so that the culvert is evenly supported. After the culverts are laid the head walls are constructed up to the

road level and the area between the head walls is backfilled with a well-graded stony/gravelly material. Lean concrete may be used also as a backfilling material, although

the costs of the structure will be considerably higher in this case.


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Fig 18

A 1:3:6 concrete surface layer of 10 cm thickness is laid at a 3 per cent cross-fall, the highest point being at the upstream side. If possible, the upstream side edge is

rounded. The connections between the approaches and the horizontal stretch are rounded to provide a smooth riding surface. Finally, the aprons are constructed, as

shown in figure 18. For this type of small causeway the minimum width of the apron at the downstream side should be two times the height of the causeway from the river

bed.

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2_Cause Way Design