Dry stone causeways
Practical Action
with other building operations. The stone that fills the causeway should go up to, but not
above, the boulders that have been laid; in other words once this part is finished, there
should be an even, level layer.
Now the strings are repositioned. They are raised by about 30 centimetres, and they must
also be moved in towards the centre by 30 centimetres. This means that the next layer is
narrower than the one below. Now repeat the procedure as for the first level. Boulders are laid
in one by one, touching each other and pinned securely into place. On this level, there is no
need to leave a gap - simply continue across the drain section. There is one new point that
must be followed for this layer, however, and that is that as far as possible, try and do what is
known as 'breaking the joints'. This means that each boulder should sit on two below, just as
when laying bricks or concrete blocks (see Figure 5).
In order to keep the boulders the same height as the string, some will need to be a little
larger, others a little smaller. If the boulder is not tall enough to reach the string, try turning it
on its side and pinning it that way. With the two lines of boulders set in, fill the centre again.
Now you will have two level layers, and it will be time to raise the string again for the third
and final time. Follow exactly the same procedure as for the second layer. Raise the strings by
30 centimetres and move them in by the same amount. If anything, it is even more important
that they are level. It is very important that this final layer uses boulders that are just as big
as the first two layers, because this is the layer that has to resist the floods most.
If the approach to the causeway is gentle, it may be a good idea to cut away the banking at
the ends so that larger boulders can be used all the way along, as in Figure 6 below. Place
the line of boulders along each string, and fill up the centre until the causeway is flat. Do not
let the surface 'dip'
between the boulders; if
anything, the centre
should be a few
centimetres higher than
the edges. The final 15
centimetres or so should
be small stones and
chippings; they fill the
spaces, bind the stones
together better, and make
for a smoother crossing
either on foot or by vehicle
(see Figure 7).
Figure 6: Tying in the ends of the bank
Finally, the surface of the
causeway is 'blinded' with
a layer of earth and subsoil. This should be about 15 centimetres thick. The approach to the
causeway is often quite steep, and this provides an excellent opportunity for doing two jobs at
once. By removing soil from the banks of the watercourse, the approach is made much
shallower, and hence easier for traffic to use, especially in the rainy season. This soil then
makes a good smooth surface for crossing the causeway. The earth will bind with the top layer
of small stones to give a surprisingly strong finish.
If the causeway has to be built on an earth base, then measures will have to be taken to pre-
vent erosion on the downstream side. A mixture of large and small stones should be built at
an angle against the side of the causeway, to further break up the force of the water.
There is one more job to do before the finish, and that is to check every boulder on both
sides, to see that they are touching. If a gap is found, then a thin slice of stone must be
driven down hard between them with a hammer to lock them together. Once this is done, the
causeway is finished, and it is immediately ready for use.
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