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de drains and mitre drains

EARNING ELEMENT OBJECTIVES

hen you have learned LE-3, you should:

know the function of side drains and mitre drains;know the correct dimensions of the drains;know how to construct the side drain/mitre drain and how to control the depth and width.

DE DRAINS

he function of the side drains (or ditches) is to collect the water from the carriage way and surrounding areas. When they arecavated the outcoming material serves to form the very important road surface drain, the camber.

he size of the side ditches must therefore be sufficient to cope with the run-off water. The outcoming material should be sufficientprovide a compacted camber of 5 per cent. When a road with 4 1/2 m formation width is to be provided with a 5 per centmpacted camber (7 per cent loose) the size of the ditch should be as shown in figure 6 (area = 0.16 + 0.2 = 0.36 m 2).

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hen there are ditches on both sides of the road the depth of each side ditch can be reduced to 0.3 m. The soil from the first ditchn partly serve to raise the elevation of the carriage way.

n flat or slightly undulating terrain you should aim to achieve a longitudinal gradient between 2 and 5 per cent (1:50 and 1:20).ith gradients less than 2 per cent silting occurs easily while with gradients steeper than 5 per cent the ditches will erode easily.

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onstructing the side ditches

he side ditches are constructed in two steps: first a rectangular trench is excavated (ditching) and then a slope is made from thettom of the ditch up towards the carriage way (sloping). The soil coming out of the ditches is placed in the middle of the road.

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he reason for excavating the ditch in this manner and shape is that it is easier to measure and control than a traditional V-shapedtch made in one operation.

he correct width and depth is controlled with a stick which has been cut to correct length. Each worker who works on drainageould have his own stick.

itre drains

he mitre drains (or off-shoot drains) lead away the water from the side ditches to lower areas.

here possible they should be placed at the intervals shown below. In principle, the more mitre drains the better!

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emember to make the gradient of the side drains a minimum of 2 per cent (even in flat terrain, see figure 7).

oad gradient Mitre drain intervals

should not exceed metre when discharging the water on to a piece of farmland metre

40

80

120 if exceeded 20 to 50 wherever possible onto a

160 scouring will occur boundary

200

2 50 if exceeded silting will occur

emember that these are maximum distances; the quicker the water is led off the road, the less damage (either from scouring orting) will occur. Wherever possible, the discharged water from the mitre drains should be channelled to land boundaries in order

avoid damage to farm land. The soil coming out when the mitre drain is excavated should be deposited on the downhill side ofe drain.

so the mitre drains should have a minimum gradient of 2 per cent.

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