Field Instructions

DRAINAGE

Remember that:

- drainage is one of the most important construction activities;

- drainage consists of road surface drainage (camber), side drains, mitre drains and cross-drains (culverts);

- drainage works should be carried out as soon as possible after the excavation. Catchwater drains should preferably be provided before the excavation is done;

- side drains have a double function:

(a) to collect and lead away the surface water; and

(b) to provide sufficient material for the formation of the camber of the road (road surface drainage);

- side drains are best done in two separate stages:

(a) trenching (digging of a rectangular or square ditch); and

(b) sloping (completing the ditch by providing the slope towards the centre-line of the road);

- scour checks will protect the drainage system and the embankments against erosion;

- observe closely how the drainage system performs during the first rains. Correct inadequacies immediately!

EXCAVATING THE TRENCH FOR THE SIDE DRAIN

Note: The soil must be thrown on to the centre-line.

Method

- Calculate the volume (v) of the excavation by multiplying the area (a) of the trench by the length (1).

- Calculate the number of men needed for the trenching by dividing the volume by the productivity rate for the type of soil concerned.

- Set out, together with the headman/gang leader, the width (w) of the trench with pegs and strings for the distance to be excavated during the day.

- Instruct the headman/gang leader on how many men are needed and which tools to use.

- Give the headman/gang leader measuring sticks to check the width and the depth of the trench.

Control

Before approving the work, ensure that the answer to the following questions is “Yes”:

- are the dimensions of the trench correct? - has the soil been placed on the centre-line?

Tools

For excavation: hoe, forked hoe, mattock, fork, spade, pickaxe and shovel.

For setting out and control: strings, pegs, measuring tape and measuring sticks.

SLOPING OF SIDE DRAINS

Note: As for trenching, the soil should be thrown on to the centre-line.

Method

- Calculate the volume to be excavated by multiplying the area of the triangular section by the length to be covered during the day.

- Calculate the number of men required by dividing the volume by the appropriate task rate.

- Set out the width of the slope with pegs and strings, i.e. a string parallel to the excavated trench.

- Instruct the headman/gang leader and workers; the workers should loosen the soil into the trench from where it should be thrown on to the centre-line.

Control

Before approving the work, ensure that the answer to the following questions is “Yes”:

- are the measurements correct? Check with a wooden mallet having the shape of the ditch cross-section;

- is the surface between the string and the bottom of the slope straight and without a “hump”?

- has the soil been placed on the centre-line?

Tools

- As for trenching plus a mallet showing the shape of the completed ditch.

CAMBER FORMATION

The formation of the camber follows immediately after the sloping and consists of spreading the soil heaped along the centre-line towards the sides of the road. The spreading should be done in such a way that a camber of 5-7 per cent is obtained.

Method

- Calculate the volume to be spread by adding the volumes for the ditching and sloping over the distance in question.

- Calculate the number of men needed by dividing the volume of work by the task rate for spreading.

- Instruct the gang leader/headman on the tasks, the number of workers and the tools to be used.

- Instruct the workers that the soil has to be spread (raked) from the heap along the centre-line towards the edges of the road.

- Tell the workers why the camber is necessary, explaining that rain-water must run off the road surface quickly because standing water weakens the road and very quickly makes it impassable to traffic.

- Inspect the work and measure the camber with a camberboard (a straight-edge board of a predetermined shape). The camberboard is used in combination with a spirit-level.

Control

Before the camber formation can start:

- is the soil from the trenching and sloping placed as a ridge along the centre-line?

Before approving the work:

- is the gradient of the camber correct?

- are there any depressions in the road surface?

- is the crown (the highest point) of the road on the centre-line in the middle of the road?

Tools

For spreading: spreader (heavy rake, hoe or shovel).

For control: camberboard (straight-edge board and spirit-level).

SCOUR CHECKS

Scour checks reduce the speed of water in the ditches and thus reduce the scouring. They should be provided whenever the gradient of the side drain is greater than 6 per cent. Some soils erode more easily than others and on such soils scour checks might be necessary even at lower gradients. (You can see this when inspecting the road after the first rains.)

Method

- When the side drain has been excavated, measure the gradient with a dumpy level, abney level or a measuring tape/spirit-level.

- Find the scour check interval from the learning text table; the steeper the gradient, the shorter the distance between the scour checks.

- Deepen and widen the ditch at the place of the scour check to be able to properly place and secure the boulders or stakes used to construct the check.

- When stakes are used, they should be driven deep into the ground and supported with stones on the lower side.

Control

- Ensure that the scour checks have been placed at the correct intervals and that they are properly constructed.

LAYING CULVERTS

Method

- Set out the position of the culvert lines on the lowest points in the alignment and on additional places as required.

- Determine how deep the culvert trench will have to be (the diameter of the ring + 15 cm gravel bed if necessary + overfill). Do not forget that the outlet should be deeper than the inlet (gradient multiplied by length of trench).

- Calculate the number of rings needed by dividing the width of the road (at the level where the culvert will be laid!) by the length of one ring.

- Calculate the volume of the trench (length × width × depth).

- Transport the rings to the site, keep one ring in reserve on the site in case a ring breaks during the laying.

- Set out the outlet ditch (mitre drain) and the culvert trench with pegs and strings.

- Excavate the outlet ditch before the culvert trench is excavated to avoid the ponding of water.

- Excavate the trench; remember that not more than 3 to 4 people can work effectively at the same time in the trench. With trained workers, the work can be done as task work.

- Prepare the culvert bed, using gravel if necessary. Ensure that the culvert bed is laid to the correct gradient; check the gradient with a string or straight-edge board, combined with a spirit-level and measuring tape.

- Lower the pipes with ropes and place them in the correct position.

- Cover the joints with tar paper, backfill and compact in thin layers; add water if necessary to obtain proper compaction; the layers of backfill should not be thicker than 15 cm.

- Construct aprons and headwalls if required.

Control

- Before starting the work on the trench:

- is the outlet or mitre drain ready? - are the rings on site and undamaged?

- Before laying the rings:

- is the gradient of the bed correct (3-5 per cent)? - is the bed properly shaped and compacted?

- Before approving the total work:

- is the backfill properly done? - is the compaction satisfactory? - are both inlets and outlets cleared?

Tools

- For excavation: hoes, forked hoes, mattocks, forks, pickaxes and shovels.

- For lowering rings: strong ropes, car tyres as “cushions”.

- For properly placing the rings: wooden stakes (not crowbars!).

- For compaction: hand-rammers.

- For setting out and control: strings, straight-edge board, spirit-level, tape measure