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  A
dam is built on a soil or rock foundation, depending on the
type of dam. The foundation
must be strong enough to carry the weight of the dam, and
the water pressures acting on the dam. The dam foundation
is below the original ground level. It is covered by weaker
soils or rock that have to be removed either by excavation
or by blasting with explosives.
Sometimes, areas of weaker soils or rocks
are found below the planned foundation level and these have
to be removed and replaced with stronger materials
Some rock foundations
have cracks and fissures in them. These have to be filled
with grout
to stop water leaking out from the reservoir
through the cracks once the dam is completed. This is done
by drilling holes down into the rock, and by pumping grout
into them, which spreads outwards to fill the cracks.
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