1_Basic of Culvert,Bridge, Drift, Causeway

8/11/2019 1_Basic of Culvert,Bridge, Drift, Causeway

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arning Element: LE-1 Nature, definition and types of structures

ARNING ELEMENT OBJECTIVES

fter you have learned this element, you should be able to:

explain why the location and design of water-crossing structures is important;

describe which factors should be considered before a particular type of structure is chosen;

describe four different types of structures and explain when, in principle, they are used.

ructures such as bridges, drifts and box culverts usually account for a high proportion of the total cost of a road. They are th

eak links in a road system, because the damaging effects of floods or high rainfall are concentrated at the points where the

osses the road. Their failure will not only lead to high replacement costs, but may also make the road useless for long perio

or these reasons extreme care should be taken that suitable structures are constructed in the best place possible and that th

ght type and design of the structures is chosen.

he location of the structures should be considered at the time of the first survey when the preliminary alignment is chosen.

he choice of the type and design of water crossings for rural roads should be governed by:

) The nature of the river or stream

s water flowing throughout the year or is the stream/river dry and flooded periodically?

What are the maximum and minimum quantities of water flowing through the cross-section at the point of crossing?

What is the profile of the cross-section; narrow with high banks or wide and flat?

) The cost of construction and maintenance of alternative structures

s local material available?

What are the transport costs for materials not locally available?

Which skills are necessary?

s any sophisticated equipment necessary?

) The expected traffic density

What will be the effects if the stream/river cannot be used for certain periods per year? What are the costs to the road user

ansport is delayed?

What is the quality and design of the rest of the road?

he following structures are widely used as rural road water crossings:

Culverts. These can be subdivided in log culverts, concrete pipes, box culverts and corrugated-steel culverts of various shape

zes.

Drifts or fords. (For dry rivers) These are structures which provide a firm place to cross a river or stream. When a river/strea

ontains water, it flows over the drift, so that the vehicle will have to pass through the water. This implies that usually for

ermanent streams other solutions are preferable. Because drifts are cheap and easy to construct, they are well suited to cro

de, normally dry rivers which are periodically flooded.


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Causeways, culvert drifts or submersible bridges. These are different names for structures which are designed and in such a

at the normal dry-weather flow of the river passes through culverts below the roadway. The occasional floods pass both thr

e culverts and over the road, which means that - similarly to the drifts described above - the road is not always passable.

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Bridges. These are structures which are constructed above the maximum flood level, so that the road is always passable.

any different types of bridges exist:

Single span girder bridges; wood, steel, reinforced concrete

Multi-span girder bridges; wood, steel, reinforced concrete

Masonry arch bridges; wood, steel, reinforced concrete

Bailey bridges;

Floating bridges;

Suspension bridges.

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idges are usually more expensive than other types of structures described above. However, when we have a permanent na

ream with steep rocky embankments the construction of a simple girder bridge may be an economical solution, because the

pproaches to the crossing do not have to be excavated as would be necessary in the case of a drift.

his exemplifies that it is very important:

) to locate the structure at the best available crossing place, and

) to examine which type of structure is technically and economically preferable.

TRUCTURES

ructures are costly and difficult to replace. Their failure will make a road useless for long periods. Therefore:

pend enough time in determining the best place for the crossing;

ensure that the right type of structure is chosen;


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ensure that the structure is designed to carry the expected traffic loads and to resist scour;

ensure that the construction is carried out according to the design.

efore you choose a particular type of structure, assemble the following data:

what are the maximum and minimum quantities of water flowing through the cross-section at the point of crossing?

how often and for how long a period is the stream flooded?

what is the situation at various points of crossing? Can natural supports (rocky embankments, stony sections of the river bed

ilised? What type of soil is found at the various alternative places of crossing?

which local materials are available (lumber, rocks)?

what will be the function and design of the road leading to the crossing?

which skills are required and available to construct a particular structure?

epending on the circumstances, you may decide to construct culverts (concrete pipes, box culverts, corrugated steel culvert

ift (non-surfaced, surfaced), a causeway or a bridge (timber, masonry, concrete, steel).

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1_Basic of Culvert,Bridge, Drift, Causeway