A capping layer is laid over
a poor quality subgrade, ensuring that it will provide stiffer resistance to
the loads from the structure above. It also reduces the chance of differential
settlement in the slab by supporting it more homogeneously than an unimproved
subgrade, which may contain ‘soft spots’.
Another way in which a
capping layer benefits external slabs is that it forms a permeable layer directly
beneath, which helps reduce the chances of pumping. It is also much easier to
compact a sub-base on a capping layer than it is to compact it on saturated
clay, meaning that by installing a capping layer, delays in the construction of
the sub-base due to wet weather can be reduced.
The materials most commonly
used in a capping layer are crushed rock, hardcore or a hydraulically bound mixture.
Because the grading of the material in a capping layer doesn’t need to be
particularly tightly controlled, the material can usually be sourced locally.
This means that capping layers are usually relatively cheap to install.
They are constructed by:
- Laying a relatively thin layer of material (max. 250mm).
- Thoroughly compacting the layer.
- Repeating until the required thickness has been reached.