Special Public Works Programmes - SPWP - Anti-Erosion Ditches - Training Element and Technical Guide for SPWP Workers, Booklet No. 1 (ILO - UNDP, 84 p.) 5. Procedure 5.1 Site selection 5.2 Siting of the ditches 5.3 Plotting the dimensions of the ditch 5.4 Preparing the surface soil of the ditch site 5.5 Excavating the ditch 5.6 Checking the dimensions of the ditch 5.7 Constructing the retaining banks 5.8 Checking that the retaining banks are horizontal 5.9 Improving the waterways 5.10 Protecting the slopes

Special Public Works Programmes - SPWP - Anti-Erosion Ditches - Training Element and Technical Guide for SPWP Workers, Booklet No. 1 (ILO - UNDP, 84 p.)

5. Procedure

5.1 Site selection

The choice of site should be determined by the community of farmers, as a whole, with everyone working together to ensure the success of the project.

Work should begin with the protection of the highest-lying fields. Areas will be chosen which, although already suffering from the effects of erosion, can still be cultivated. It is pointless constructing ditches on land where all the topsoil has already been lost through erosion.

It is up to the engineer in charge of the works to advise the farmers on the selection of appropriate sites.

5.2 Siting of the ditches

MATERIALS AND MANPOWER REQUIREMENTS

This important task needs to be carried out by specialised personnel.

Generally speaking, a surveying team consists of:

- one gang foreman, with a knowledge of topography (this may, in fact, be the site foreman in many cases);

- two semi-skilled workers to transport the surveying equipment, It is a good idea to make the same two people responsible for this task throughout operations;

- a number of workers to carry the benchmark posts.

The equipment needed for this preparatory phase of operations has been described earlier.

SURVEYING THE LAND

Once it has been decided where the works will be sited, the first task of the siting team is to take note of the principal topographical features of the area.

- the high-lying areas, or crestlines;
- the flowpaths of run-off water, or thalwegs.

Figure

In this, the siting team will be greatly assisted by the farmers who, knowing well their own land, will be able to point out the various courses water takes (thalwegs) and each will be subsequently marked out using wooden posts. In the same way, the main crestlines will be plotted out. To avoid confusion, one could use yellow posts to mark the crestlines and red for the thalwegs.

SITING THE FIRST CONTOUR LINE

Siting starts from a crestline which has already been marked out.

Beginning from the first benchmark, the contour line is found by means of a spirit-levelling board or water level, plotting out a series of points which are on the same level - a procedure which has been explained previously.

Figure

These points are marked using wooden posts. Once the siting of the first contour line of posts has been completed, we go on to mark the second and so on, using different coloured posts for each line, to avoid confusion.

SITING THE SECOND CONTOUR LINE

Starting from the middle post, and using either a measuring chain or decametre, we measure the required distance down the steepest section of the slope. To achieve this, one of the workers holds the end of the measure against the middle post whilst a fellow worker unwinds the measure to find the desired distance. A wooden post is driven into the ground at the point which has thus been found.

Care must be taken to ensure that, while measuring, the decametre or chain is at a right angle to the first line of contour benchmarks.

WRONG

RIGHT

Using this post as a starting point, a new contour line is plotted out with the help of the spirit-levelling board or water level, in the same way as the first.

We continue plotting the retaining banks one after another, following this same procedure.

EVENING-UP THE CONTOUR LINES

Once all the retaining banks have been plotted, certain irregularities in their appearance may be noticed. Sometimes the posts are not in an even line. If this is the case, the contour line should be evened up by smoothing out any irregularities which occur along its length.

This evening-up is achieved by slightly modifying the position of those posts which are not in line with the form of the contour as a whole.

The post marking a point which seems to have been placed too low should be moved up slightly, and vice versa.

The difference between the real contour which was originally plotted out and the newly evened-up contour line should not, however, exceed 10 cm.

EVENING UP THE PLOTTED CONTOUR LINE

5.3 Plotting the dimensions of the ditch

This entails marking the position in which the ditch should be dug and the boundaries it should keep within.

Tools needed for this task are a tracing line and either a pickaxe or a plough.

The procedure to follow if a pickaxe is used is the following:

- The tracing line is stretched between the posts which should be approximately 30 cm apart.

- With the pointed end of the pick, trace out the furrow, using the tracing line as a guide.

- A second groove, parallel to the first, is now traced along the ground, again using the pickaxe.

- The two grooves correspond to the dimensions the ditch will have.

- Next, the posts are moved in such a way as to mark the total extent of the retaining ridge.

PLOTTING THE DIMENSIONS OF THE DITCH - 1

PLOTTING THE DIMENSIONS OF THE DITCH - 2

PLOTTING THE DIMENSIONS OF THE DITCH - 3

If an animal-drawn plough is used, the method is as follows:

- The plough traces out a furrow along the ground following the layout of the ditches. The operator must be sure to steer the plough exactly along the line of posts of the same colour.

- When the first furrow has been completed, the posts are moved across, keeping an equal distance between them, to mark the bounds of the ditch. A second furrow is made, using the posts as a guide. These two furrows mark the boundaries of the ditch. The ground between may then be ploughed to ease the task of digging.

If a tractor is used, the choice of implement depends on the nature of the soil:

- a disc plough is needed on soft ground;
- a subsoiler should be employed on hard ground.

The worker drives the tractor along the line of posts and then continues to plough back and forth within the bounds of the site so that the ground is prepared thoroughly for the next task which consists of excavating the ditch.

5.4 Preparing the surface soil of the ditch site

Once the site of the ditch has been traced out, the next task is to prepare the ground by scraping the surface, clearing it of vegetation and then breaking up the surface soil (unless a disc plough has been used, in which case this task will have already been done).

The tools required for this operation are the pickaxe and shovel.

The pointed end of the pickaxe is used to break up the surface layer of soil which is often hard and compact.

The cutting edge of the pick is useful for slicing through roots and for removing clods of earth.

Preparing the site to be backfilled

The site of the retaining bank is also worked over with the pickaxe in order to break up the soil. This is particularly useful if trees are to be planted on the bank as water is then more easily absorbed and it also encourages the roots to establish themselves.

5.5 Excavating the ditch

The tools used for this stage of the works are the shovel and pickaxe.

If the task is arranged as piecework, each worker has a set amount of ditch to dig. His section is marked out with posts. He should be equipped with both a shovel and a pickaxe.

If the digging is to be carried out by a line of workers, each worker is given either a shovel or pickaxe. The diggers form a line with the one using the pickaxe at the front, followed by the shoveller(s).

According to the type of soil, the following is a guide to the composition of the team:

- on hard ground:	1 pickaxe to one shovel;
- on normal ground:	1 pickaxe to every two shovels;
- on soft ground:	1 pickaxe to every three shovels.

The excavation of the trench is commenced at the middle of its plotted siting.

1. The soil is broken up, using the pickaxe, to a depth of about 20 cm and the earth removed by shovel. In a single movement, the earth is thrown by the shoveller to the place where the embankment is to be constructed. A worker is capable of shovelling earth a distance of 3 m in a single throw, on average.

Figure

Note: When constructing contour line ditches, the embankment should always be situated downhill from the ditch.

2. The middle of the ditch is dug out in the same way as before, in 20 cm sections until the required depth, as specified by the gang foreman, is reached.

Figure

EXCAVATION OF THE DITCH

CHECKING THE DEPTH OF THE DITCH

To make sure the ditch has been dug to the correct depth, the worker makes a mark on the handle of his shovel or pickaxe, representing the required depth and uses it to guide him during the course of the digging.

TOO SHALLOW

TOO DEEP

RIGHT DEPTH

With the middle of the ditch dug to the required depth, the sides will be more or less vertical so, in order to prevent them from caving in, they should be sloped off.

To do this, the bounds of the bank should be marked out. The gang foreman will work out what the distance between the vertical side of the trench and the edge of the bank should be.

Figure

The uphill bank (the side the water runs down) is levelled off first in the following way:

- The worker stands in the ditch, facing the bank which he then proceeds to level off using the cutting edge of the pickaxe. The earth is removed from the ditch by shovel.

- When this operation has been completed, the slope of the bank from the bottom of the ditch to the natural level of earth above should be level, with no hollows or bumps. Its level-ness can be checked with the handle of the shovel which should rest flat against the bank from top to bottom.

- The downhill bank is then levelled off in the same way.

CHECKING THE SLOPING OF THE BANK

5.6 Checking the dimensions of the ditch

Upon completion of the ditch-digging, the gang foreman should inspect the work and check that the actual dimensions of the ditch correspond to those which were foreseen.

In order to do this, a tracing line is stretched across the top of the ditch, with four coloured marks along it. The two outer marks indicate the overall width of the ditch and the two inner ones the width of the base of the ditch.

Using a graduated rule, or surveyor's rod, the gang foreman checks the depth of the ditch straight down from the two central marks.

If the ditch is too shallow, it must dug out to the required depth. Should it have been dug too deep, the ditch should be left as it is.

CHECKING THE DIMENSION

5.7 Constructing the retaining banks

The earth which has been excavated in the digging of the ditch should be backfilled downhill.

The purpose of a retaining bank, like that of a ditch, is to stop the flow of run-off water. It must, therefore, be strong enough to withstand the thrust of the run-off water. This is ensured by compacting the backfilling material.

Compaction consists of packing the earth down so as to give it more cohesion. This can be achieved using simple methods such as stamping by foot or working with a hand rammer.

Stamping the soil down involves men and animals passing on top of the embankment and to be most effective is best done when the soil is damp.

Compaction by hand rammer is achieved in the following way: a 10 cm layer of earth is placed on the bank and carefully packed down by pounding the area with the rammer. Another layer of earth is brought and packed down and so on, until the required height of the bank is reached.

Figure

To be most effective, compaction is best carried out on damp earth, although this is seldom possible since work is usually undertaken during the dry season.

Therefore, to make sure the retaining embankment is really solid, it should again be compacted after the first rains of the season.

5.8 Checking that the retaining banks are horizontal

The banks must either be horizontal (absorption network) or have a slight slope (diversion network). LEVELLING RODS are used to check this.

These rods are T-shaped, 150 cm high, with a 30 cm bar.

LEVELLING ROD

Three such rods are needed, two painted white (A and B) at the top and one black (C).

Rods A and B are positioned at two places where the height of the bank has already been checked.

Rod C is moved to various positions between the other two rods, the bank being horizontal when all three are on the same level. If any places along the bank are found to be too low, they should be levelled up with earth and compacted.

CHECKING THE HORIZONTALITY OF THE RETAINING BANKS

TOO LOW

TOO HIGH

LEVEL

5.9 Improving the waterways

At regular distances along the retaining banks (about 200 m), channels should be made to allow excess run-off water to flow into the waterways.

These waterways must be protected against erosion and this will be dealt with in another booklet.

Figure

5.10 Protecting the slopes

Vegetation will protect and consolidate the slopes between the ditches and retaining banks. Steps must be taken to encourage vegetation to establish itself where it is required. This can either be vegetation which seeds itself naturally, cultivated plantations or, alternatively, the area can be grass-seeded.

NATURAL VEGETATION

This consists of grasses and undergrowth which grow naturally on rain-moistened soil. Weeding of the soil should be avoided while the vegetation is becoming established.

Until the vegetation has become thick enough to protect the slope, it can be shielded from the drying effects of the sun by covering the surface of the soil with straw and dried grass - a technique known as mulching.

GRASS-SEEDING

Alternatively, the slopes can be sown with grass seeds. For grass-seeding to be effective, the soil needs to be moist enough to allow the seeds to germinate and the seedlings to take root. Therefore, the best time for grass-seeding to be undertaken is at the beginning of the rainy season.

The soil should be prepared by breaking up the topsoil to a depth of 5 cm and removing stones and the bigger clods of earth, using a RAKE.

RAKE

The grass-seeds should be BROADCAST over the slope. This entails taking a handful of seeds and releasing them in a single wide sweeping movement of the hand. In this way, the seeds are sown evenly over the surface of the ground. One handful of seeds (about 50 g) will cover an area of 3 m². One kilogram of grass-seeds should be allowed for every 20 m² of ground to be covered.

Here again, the slope needs to be protected by mulching while the grass is taking root.

PLANTING OUT

The planting out of plantlets and seedlings should be undertaken at the beginning of the rainy season once the soil of the slope has become moist to a depth of 20 cm, but only after the rains have become sufficiently well-established to avoid any risk of prolonged dry spells.

OBTAINING THE PLANTLETS

The plantlets are obtained from tree nurseries and are supplied either with bare roots or contained in plastic pots or bags.

YOUNG PLANTS

With bare roots

in containers

STORING THE PLANTLETS

If the plants are not to be used immediately, care should be taken to prevent them drying out.

To store the plantlets for three or four days, they should be kept out of the sun in a shed and covered with damp sacking.

If they have to be stored longer than a few days, the plants must be HEELED IN. This involves:

- digging a trench in the earth;

- placing the plants side by side in the trench, at an angle facing north, roots at the bottom;

- covering the roots with moist earth up to the first branches.

During very dry periods, the earth should be covered with dry leaves, branches or dampened sacking.

HEELING IN

PREPARING PLANTING HOLES

Planting holes are prepared on the sloping top-side of the retaining bank. The spacing of these holes will depend on the species of tree used. For shrubs, it will vary between 0.50 m and 2 m.

Figure

The hole is cube-shaped, at least 30 cm deep and dug out by means of a pickaxe and shovel. The topsoil (to a depth of 20 cm) is placed in a heap to one side of the hole and the remaining 10 cm of earth (unproductive soil) put in a separate heap on the other side.

Figure

TRANSPORTING THE PLANTS TO THE PLANTING AREA

The plants are brought from the sheds to the worksite only when the preparation of the planting area has been completed. They may be transported in wicker baskets or wheelbarrows. It is essential that the plants are not allowed to dry out so they should be covered with damp sacking during transport.

PLANTING OUT

Preparation

Bare-rooted plants:

- Check the condition of the roots (they should be supple).

- Cut off any roots which are too long and any which have dried out.

Plants in containers:

- Carefully cut away the plastic container.

- Handle the plant with care in order to avoid any soil being lost from around its roots.

- Take the plant in one hand, holding it by its collar (the part which separates the root from the stem).

- Place the plant in the hole which has been prepared, so that the collar is a few centimetres above ground level. A stick laid across the hole will help to position the plant correctly. Careful attention must be paid to check that the roots of the plant are not bent under.

- Start filling the hole with fertile soil from the first heap.

- Press the soil down lightly around the roots of the plant.

- Fill the hole up to the collar of the plant with the rest of the soil.

- Tread the earth down lightly.

PLANTING OUT - 1

PLANTING OUT - 2

PLANTING OUT - 3