Clay Ground
Clay is a member of feldspar (Mother of Clay) group, comprising particles of hydrous aluminium silicates and other minerals. Grain sizes are particles of less than 4 micrometres, and when exposed to increments of water, tend to soften, and even liquify. Some clays are more ‘expansive’ than others, absorbing water more easily and expanding in volume. Most clays in their ‘normal’ state, are 40% water, very soft when wet, and very solid when dry (think pottery, which is hard fired clay).
Clay will settle very quickly once spread or moved around, often within a year or two – sometimes after a few months if the ground is wet. It tends to be alkaline, dependent on the region, between 7.5 to Ph 10. It is more stable than sandy soil, yet vulnerable to subside and shrink when dry.
As clay soil tends to shift around as it dries and moistens, causing heaving, subsidence or cracking, it has the greatest potential to cause problems on landscaped sites with foundations. For this reason, houses on clay sites are built on substantial concrete ‘strip foundations’, at least 750mm and more often, 1000mm deep. Unless you are building retaining walls, (in which case you should be using specification drawn up by a qualified structural engineer, who will be looking at the effects of pressure causing pivoting), the majority of foundation work landscapers are likely to be constructing will involve raft foundations, i.e. under paving or other horizontal surfaces.
Clay ground can compress under the weight of a concrete slab, and leach moisture into the concrete resulting in uneven curing and brittleness. For this reason, it is essential to lay concrete over a bed of crushed concrete conforming to C4 and/or MOT Type 1, both well compacted. This will keep the wet concrete separate from the clay soil. We would further advise that a geotextile membrane is fitted under the crushed concrete to prevent it from being forced into the soft clay ground during compaction.
Installing steel reinforcing mesh into the centre of a concrete slab/raft (minimum 100mm deep C20 or C25 strength), or reinforcing bars within a strip foundation, is essential to provide as much strength as possible into the structure. On large areas, it is advisable to consider expansion joints (see Expansion Joints) separating each section using fibre board or similar, especially when working on driveways or lengthy ‘strip’ projects which may not be suited to a monolith, thereby allowing some movement within the foundation. Ensure the expansion joint is reflected in the surface pattern to avoid cracking or movement in the paving or blockwork away from the joint location.
If the area is substantial, consider installing flexible drainage pipes covered in shingle as land- drains running under the concrete to ensure that moisture can be returned to the areas capped off by concrete and reduce the likelihood of the ground from drying out too quickly causing shrinkage of the clay soil.
By designing in shrub beds or borders, or areas of free draining shingle, thus controlling the balance of moisture around and under the foundation (by means of soaker hoses during hot weather if necessary) in an attempt to replace the volume of water that was previously entering the ground before works started. Strategically placed, these pipes can prevent the site from undue and unwelcome ground movement.
