School buildings in developing countries
Practical Action
base, the ditch can be dug deeper then backfilled with brick and stone rubble before
levelling. Note, however, that in cold climates the top of the footing of the foundation needs
to lie below the frost line. If ground frost of below - 15°C might be expected the depth of the
ditch would need to be greater than 1.2 metres, rising to 1.6 metres for ground frost of -
30°C.
The footing of the foundation is usually made of a relatively rich concrete mix, for example of
one part cement to three of sand to six of river or lake bed gravel or crushed rock, which
needs to be well-compacted. The width of the footing would be determined by local ground
conditions, so the dimensions for other well-constructed buildings in the area would be a
useful guide. Conventionally 45 or 60 cm are used for relatively small buildings. The height
or thickness of the footing can be taken as the same as the width of the foundation wall
above the footing. The foundation wall needs to be as wide, or slightly wider, as the wall
above. It is unlikely that foundation wall widths below 20 cm could be considered completely
safe, and even for relatively simple buildings widths as high as 40 or even 45 cm have
sometimes been used. Again, experience from other buildings in the area would be a useful
guide.
The foundation wall needs to be built with a relatively strong, durable and water-resistant
material. These materials would include most hard ashlar or rubble stone (most soft stone as
well as some types of limestone might not be durable or strong enough), most stronger types
of fired clay bricks - generally not field fired bricks, dense concrete blocks or bricks, or cast
concrete. If stabilised soil blocks are used these need to be made with a low-shrinkage soil,
such as laterite, with extra stabiliser and waterproofer and preferably made in a machine
press that can apply higher pressures than manual presses. It is normal practice for the
foundation wall to be built to protrude above the ditch that was built for it to at least the level
of the internal floor. It is also advisable, unless in very dry climates where the water table is
always far below the footing, to cap the foundation wall with a damp proof course. This can
simply be a rich concrete mixed with a waterproofing compound.
The floor is made by partly filling the space between the foundation walls with pebbles,
broken brick, coarse sand, or even broken bottles provided that the glass does not protrude
from the floor. This then needs to be levelled. This is covered by a layer of concrete,
stabilised soil, or concrete or clay tiles. Timber floors are often used in Western countries,
but this is generally quite an expensive option and liable to be attacked by termites where
they occur.
External Wall
Damp proof course
Ground level
Floor Finish
Fill & Rubble
Foundation Wall
Foundation Footing
Figure 4: Construction of the floor
The above example of a foundation wall is the most common type of foundation, but other
types of foundation, e.g. arches, are also possible. Special care needs to be taken on sloping
ground to ensure that the trench for the foundation is level. If the slope is steep the trench
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