Hard Landscaping on Clay Ground: Preventing Heave, Subsidence and Structural Failure

Hard Landscaping on Clay Ground: Preventing Heave, Subsidence and Structural Failure

Hard Landscaping on Clay Ground

Recent winters have produced greater volumes of rainwater, with the soil regularly deemed to be at field capacity, or waterlogged and unable to cope with any more precipitation. Recognising the potential problems such conditions can place on contractors undertaken landscaping projects cannot be underestimated, especially on frequently impervious clay ground.

Clay minerals derive from weathered feldspar (Mother of Clay) group, comprising particles of hydrous aluminium silicates and other minerals. Grain sizes are particles of less than 2 micrometres, and when exposed to increments of water, tend to soften and even liquefy. Some clays are more ‘expansive’ than others, absorbing water easily and expanding in volume. Most clays in their ‘normal’ state may be 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 mildly alkaline, dependent on the region, between pH6.5 to pH8. It is more stable than sandy soil, yet is vulnerable to subsidence and shrink when dry.

As clay soil tends to shift around as it dries or moistens, causing heaving, subsidence or cracking, it has the greatest potential to cause problems with foundations. For this reason, houses on clay sites are built on substantial concrete strips, at least 750mm and more usually, 100cm deep. Unless you are building retaining walls (in which case you should be using specification drawn up by a 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 and 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. Direct contact with wet clay can affect curing conditions and long-term durability if adequate separation and drainage are not provided. For this reason, it is essential to lay concrete over a bed of crushed concrete and/or MOT Type 1, both well compacted. This will keep the concrete separate from the clay soil. I 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 Reinforcement Measures

Installing steel reinforcing mesh into the centre of a concrete slab/raft, 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 that the expansion joint is reflected in the surface pattern to avoid cracking or movement in the paving or blockwork away from the joint location.

Similarly, ground mesh matting such as biaxial sheets, which help to spread the lateral loading of the sub-base and base materials by preventing sideways movement, are extremely useful in stabilising the ground under pavements.

I always strive to imagine what effect my work will have when capping off ground that was previously exposed to the weather, and attempt to mitigate any reaction from heave or shrinkage by ensuring that sufficient water is penetrating the ground – at least as much as before I started on the project.

Design Methods of Alleviation

By designing in shrub beds or areas of free draining shingle, and controlling the balance of moisture around and under the foundation by means of soaker hoses or flexible land-drains laid under the sub-base, I attempt to replace the volume of water entering the clay ground as though it had not been capped off. These non-solid areas can dissipate water into the ground and be channelled into the pipework laid under the MOT and feed directly into the clay ground.

These measures are particularly relevant when working on new build sites, as you will have little idea of the previous land formation. This is where some research will be required, and the best source is Google Earth. Try to obtain some aerial photographs of the site as it exists today, then compare with previous photographs (usually available with two-year gaps in between i.e. 2021, 2019, 2017 etc) and look in particular at any tree cover that may have existed. If aged images show dozens of oak trees near to the site you are working on, you may be sure that the amount of moisture they were taking up is likely to cause problems with ground heave due to the lack of tree roots.

Equally, if the land layout has been altered, and new levels and falls indicate that water has been drained away from the site (perhaps under the local SuDS plans) there may be cause to consider shrinkage as being a potential issue.

Driveways and Car Parks

If you are working on designing and installing a new – or replacement – car park, perhaps changing from a non-permeable surface to a permeable on to comply with the SUDS Regulations, always ensure that you consider the effect of rainwater percolating through the permeable jointing compound into the base and sub-base, and ensure that this water can ‘escape’ from under the new surface. All too often, new permeable surfaces become waterlogged or clay ground become overly ‘plastic’ with ground water rising from under the soil, and seeping downwards through the permeable joints causing problems with sinkage and undulations.

Consider thermal movement, not only freeze/thaw, not also expansion and contraction, especially on clay ground. After a wet winter, it is advisable to build in contraction joints within any concrete foundations, to cope with settlement and the effects of drying out during warm weather.

It is advisable to conduct percolation tests combined with a California Bearing Test (or ‘heel’ test) to examine the likely plasticity of the soil and ground conditions to ensure adequate drainage through percolation, otherwise a drainage system may have to be designed and installed to remove subterranean water from building up.

Edging restraints are very important when paving on clay ground, especially in respect of driveways and car parks, where constant vehicle movements, braking and power steering are likely to put pressure on certain areas of the scheme. I have including two detailed cross-sectional drawings in the Hints & Tips Section of The Library – Resin Bound Driveways and Block Paving Driveways on Clay Ground, highlighting the importance of strong edging.

Walling Projects

Turning now to walling projects, especially retaining walls which are backfilled with clay soil. As previously mentioned, all retaining walls – certainly those above 900mm high – should be designed and specified by a qualified Structural Engineer. The designer will usually only pay attention to the strength and integrity of the wall, with no regard to the soft landscaping elements.

Assuming that you have properly tanked the rear of the wall with bituminous paint, you should have no problems with the longevity of the project as far as clay is concerned.

However, it is important to be aware of the deleterious effects that clay can have on cement and concrete. Sodium, magnesium and calcium sulfates and/or sulfides are found in many sedimentary clay soils (including the famous ‘London’ clay). These sulfides can inhibit the normal hardening of cement, leading to loss of durability of the hardened material.

The presence of sulfate ions, when subjected to high moisture content in wet clay soil will have a deleterious effect on the hardening or ‘setting’ of mortar, and unless sealed and protected by a waterproof barrier (bitumen) can cause seepage and weeping of sulfides from between the joints of brickwork and blockwork. If necessary, I would suggest that you examine the potential problems on any given site, and check to see if your local Builders Merchant recommends using a sulphate resistant cement on at least certain areas of your project.

Summary

So many areas of Britain are affected by clay soil, and all have successful schemes completed every day, and there is no reason to single clay ground out for being any more problematic than other soils. Simply being aware, and taking careful precautions when specifying for a project, highlighting your research and qualifying your specification when presenting your Construction (Design Management) Regulations 2015 (CDM) plan to your customer, you will have acted in a professional manner.

There may well be additional costs included in your quotation that may have been omitted by a competitor, but by your due diligence in recognising the potential problems that are likely to be discovered during the works, you will have proven your professionalism and no doubt stand a better chance of being awarded a project because of that fact.