Assessing Soil Types and Their Relative Properties
Soil types vary widely across the United Kingdom, and most will not have any adverse effects of paving projects, as they are not prone to changes in physical property in any way that could cause damage or degradation to paving materials including concrete and mortar.
However, certain soil types can pose problems by their properties and ability or propensity to contain destructive chemical elements or retention of water, causing freeze/thaw activity, or mechanical actions such as ground heave and contraction during temperature and moisture changes.
There are six main types of topsoil in the UK, each with their own make-up.
Clay – which is heavy, and may be considered to be ‘plastic’ in that it is the most likely soil to expand and contract. (This will be discussed in greater depth later on)
Sand – described as comprising particles of 0.05 – 2mm, being light, free draining and (potentially) acidic. This may become problematic if using reinforcing or metal reinforcing bars in damp conditions, as they may become weakened by rust.
Silt – with particles between 0.002 – 0.05mm, fertile, moisture retentive and easily compacted.
Loam – usually containing clay, sand and silt, 0.002 – 0.05mm particles. Fertile.
Chalk – Alkaline, water retentive, comprising particles of calcium carbonate or lime. Particles between 0.002 – 0.05mm and prone to retain heavy quantities of water.
Peat – High in organic matter and moisture, particles between 0.05 – 2mm.
None of the above soils should present any problems on landscaping projects involving foundations and construction issues. When excavating trenches, sub-bases and bases, always ensure to heavily compact the bottom of the site with a vibrating mechanical plate, applying a capping layer of crushed stone to the bottom of the excavation to enable the plate to move freely across the ground. This is essential to ensure a solid base for pouring concrete when building footings and foundations.
Chalk
In general, chalk should present few problems, as it is easily compacted (unlike sand, silt, loam and peat). However, an awareness that some soils may be described as ‘deep chalk’, meaning that there is very little vegetative matter in the material, usually showing as ‘white’ indicating a high level of limestone.
This chalk soil is highly water retentive, and can expand to several times its dry volume, and create great pressures and lift or disrupt shallow concrete foundations and cause heave through freeze/thaw. Chalk will lift tarmac, concrete and any unbound material, gently returning the ground to original levels once thawed out, leaving a fractured surface behind.
Drainage systems rarely work in chalk ground, as the material can easily retain moisture once exposed to its effects, especially and primarily on the top few centimetres of the surface where frost action is likely to occur during the winter
Clay
Clay is a member of the feldspar (Mother of Clay) group, comprising particles of hydrous aluminium silicates and other minerals. Grain sizes are 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 and expanding in volume. Most clays in the ‘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 if wet. It tends to be alkaline, dependent on the region, between 7.5 -pH 10. It is more stable than sandy soil, yet vulnerable to subside and shrink when dry.
As clay tends to shift around as it dries and moistens, causing heaving, cracking or subsidence, it has the greatest potential to cause problems on landscaping 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 ground during compaction.
Installing steel reinforcing mesh into the centre of the 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, 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 under the surface 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.
Always make a careful note of the type of soil you are to be working on, as lack of due diligence when planning can be very expensive to rectify.
