What is ground heave, and how do you prevent it?

Ground heave is a subtle yet powerful force that can challenge the integrity of foundations, slabs, and beams.

At Dufaylite, we believe in not just defining the problem, but equipping builders, engineers, and specifiers with clear, reliable, and effective ground heave solutions. Here, we’re going to unveil what ground heave is, why it occurs, how to prevent it, and why our Clayboard void former stands out in the market as one of the highest-performing and most eco-friendly options available for professional construction firms and their clients.

Written by Ashley Moscrop, Managing Director, Dufaylite Developments Ltd. Reviewed by Nigel West, Technical Manager, Clayboard. Last updated: 15 August 2026.

Key takeaways

• Ground heave is the upward movement of expansive clay soil as it absorbs water, which can crack slabs and foundations.
• It is the opposite of subsidence: instead of sinking, the ground swells and pushes up against the structure.
• Common triggers include removing large trees, seasonal moisture changes, and building on shrinkable clay.
• The most reliable prevention on clay is a compressible void former beneath the slab, giving the swelling ground somewhere to go.
• Clayboard is a BBA-certified paper honeycomb void former that weakens on activation, so the slab needs less reinforcement to resist heave.

What is ground heave, and why does it matter?

Ground heave refers to the upward expansion of soil. It is most associated with clay-rich materials – clays that swell when they absorb moisture and contract when they dry.

Although this movement may be modest (often tens of millimetres at a time), it exerts significant pressure on concrete slabs and foundations. Even a few millimetres of uplift can cause cracking, misaligned frames, lifted patios, warped doors, and long-term structural issues over time.

Common causes include:

• Moisture fluctuations in expansive clay soils, either from increased rainfall, broken drainage, elevated groundwater, or a lack of evaporation control.
• Tree removal or root decay occurs when previously dehydrated clay reabsorbs water and swells.
• Construction activities such as excavation, remodelling of soil, or trenchless processes disturb the natural equilibrium.
• External factors, such as broken drains or frost (frost heave), are especially problematic in cold climates, where freezing water pushes soil upward.

In plain terms, the meaning of ground heave is simple: the ground beneath a building lifts. It is the opposite of subsidence, where the ground sinks. Both are forms of ground movement, but they have different causes and different solutions — heave is driven by clay swelling as it takes on moisture, while subsidence is usually clay shrinking as it dries out. Getting that distinction right matters, because a foundation designed for one is not necessarily protected against the other.

Ground heave tends to become a live risk when something changes the moisture balance of the soil: a mature tree is removed and stops drawing water out of the clay, a leaking drain saturates the ground, or a long dry spell is followed by heavy, sustained rain. On the UK clay belt these triggers are common, which is why ground heave is a design consideration on so many sites across London, the South East, the East Midlands and East Anglia. The British Geological Survey maps large parts of southern and eastern England as having a significant shrink–swell hazard, so on those sites the question is not whether to plan for ground heave but how.

What happens when ground heave occurs?

Some of the obvious signs that ground heave is taking place are:

• Vertical or stepping cracks are appearing in building materials. The upward pressure causes foundations to shift unevenly, leading to damage to brickwork, blockwork, and concrete slabs.
• Load-bearing elements like columns are shifting.
• Doors and windows are sticking, jamming, or failing to close properly because the frames are not aligned properly.
• Internal finishes such as plaster, tiles, and joinery are warping or detaching, creating both cosmetic and functional issues.
• Patios are becoming raised, and pathways are becoming uneven.
• The pipes and drains running through or beneath the affected foundations are cracking or shearing apart under movement, causing leaks that further exacerbate the issue. Gas lines, electrical conduits and water services are also being affected.

It goes without saying that anything that has a detrimental effect on the structural integrity of the building will have serious consequences in terms of the value of the property, the reputation of the teams involved in the project, and the longer-term cost of repairs and maintenance. Damage caused by ground heave can also place some insurance claims in jeopardy, especially if adequate preventative measures were not put in place during construction.

Additionally, from an environmental perspective, clay heave can also leave soils permanently unstable if vegetation cannot regrow and restore some balance to the area.

The reason ground heave is taken so seriously is that the damage is progressive and expensive to put right. A few millimetres of uplift that cracks a slab today can become a structural repair costing tens of thousands of pounds once it has worked its way up through the building. Because the movement is seasonal — swelling in the wet, easing in the dry — cracks can open and close over the year, which both worsens the damage and makes it harder to diagnose. For developers and their insurers, an unaddressed ground heave risk is a liability that can surface years after practical completion. In short, ground heave is a problem that is cheap to prevent at foundation stage and expensive to ignore.

Can you avoid ground heave naturally?

In an ideal world, we’d all be building on cohesive soils with excellent stability and drainage properties, such as sand and gravel. But in the UK at least, this is certainly not always the norm.

In fact, according to Greenhouse Stores, approximately 25% of the country’s soil composition is clay, and 15% is silt. London and the South East are particularly known for their clay-heavy ground, which poses challenges for developers in the region and places ground heave mitigation firmly into focus.

If you’re working with suboptimal soil, taking steps to control moisture levels will help keep ground heave to a minimum. Additionally, ensuring there is enough uncompacted soil volume around the development will allow tree roots to grow without causing pressure.

Pruning trees regularly will help to reduce the amount of water they draw from the soil, which can sometimes mitigate heave, and root barriers can also be used to redirect roots away from foundations. But all things considered, if you’re serious about ensuring your build is not affected by ground movement, you need to take action to combat ground heave from phase one.

Natural mitigation only goes so far. Careful management of trees and drainage around a plot can reduce the moisture swings that drive clay heave, and building on well-draining granular soils avoids the problem entirely — but neither is an option on an established clay site with a fixed plot. In practice, most UK sites on expansive clay cannot be made safe by drainage and planting alone, because you cannot control the weather or fully predict how the clay will behave over the life of the building. For any structure that will sit on high-plasticity clay, the reliable answer is an engineered ground heave solution built into the foundation — a compressible layer that gives the swelling clay somewhere to go, so the movement is absorbed rather than transferred into the slab. This is the point at which passive precautions give way to active, engineered ground heave protection.

What strategies prevent ground heave?

Mitigating ground heave begins with understanding the soil, followed by selecting the most suitable method to stabilise the area.

Early geotechnical assessments

Before foundations are designed, it’s essential to carry out a soil survey to determine the plasticity index of the clay. High plasticity indicates a greater swelling potential, which in turn influences the foundation depth and the heave-control measures that need to be implemented.

Soil replacement or stabilisation

Excavating problematic clay and replacing it with non-expansive materials is effective – but unfortunately, it’s also costly and time-consuming. Stabilising the soil by mixing clay with lime or cement can help to reduce shrink-swell behaviour, although this takes time and demands rigorous quality control.

Prewetting

Allowing clay to swell pre-emptively via prewetting can reduce heave during later stages. However, this approach is slow (sometimes it takes years!), can weaken soils, and doesn’t guard against future moisture changes.

Drainage and moisture control

Controlling moisture is construction’s frontline defence against the destructive after-effects of ground heave. Depending on the project, steps include:

• Repairing leaks and broken pipes
• Improving surface grading
• Installing gutters, French drains, and foundation perimeter drains to divert water away from foundations
• Landscaping thoughtfully – for example, planting moisture-absorbing shrubs (not large trees close to foundations) to regulate soil moisture naturally.

Underpinning or deep foundation methods

In some cases, underpinning (either mass concrete underpinning, beam-and-base, or mini-piling) provides stability by transferring load to deeper, unmovable soil strata. But though these methods seem straightforward, they can be expensive and disruptive.

Compressible void formers

Perhaps the most efficient way to combat ground heave is to use compressible void formers. These are boards placed beneath slabs or beams that compress in response to ground heave, preventing any upward force from affecting the structure.

The most widely used engineered strategy, and the one most relevant on suspended-slab and ground-beam foundations, is to install a compressible void former beneath the concrete. This creates a controlled void under the slab, so that when the clay swells it expands into the void instead of pushing up on the structure. Paper honeycomb void formers such as Clayboard are engineered for exactly this: they support the concrete pour while dry, then, once water is introduced after the slab has set, the paper core weakens and switches off its load-bearing strength, opening the void that absorbs the heave. Compared with rigid polystyrene alternatives, this means the slab does not have to be designed to resist the full uplift of the clay, which can reduce the reinforcement steel a foundation needs. For most clay-belt projects a BBA-certified void former is the ground heave solution specifiers reach for first, with the right grade and void depth following from the soil’s plasticity and the loads the slab will carry.

Comparing ground heave solutions according to project requirements

Your needs will vary from build to build – but here’s a quick overview of when certain ground heave products and solutions are the most appropriate, according to what you want to achieve and/or the challenges you’re facing at your site.

When weighing up ground heave solutions, it is worth looking beyond the headline price. A void former’s thickness affects how much you have to dig; its certification affects whether it can be specified at all; and its behaviour under load affects how much reinforcement the slab needs. A slightly dearer void former that is thinner, certified and cuts the steel can be the cheaper option once the whole foundation is costed — which is why the comparison below weighs more than cost alone.

Introducing the most reliable and eco-friendly ground heave product on the market: Clayboard

Until now, in cases where they are the most suitable products for the job, EPS void formers have been the go-to for construction professionals.

They offer impressive protective qualities, but because they are manufactured from expanded polystyrene, they are both bulky and non-biodegradable – traits that make them expensive to transport and install, and a significant environmental issue.

What makes Clayboard distinct?

• Uniquely lightweight and thin, Clayboard is 30% slimmer than traditional alternatives, which eases transport, handling, and excavation constraints.
• It boasts an eco-conscious design. It’s made from a paper honeycomb core using 100% recycled materials, and, refreshingly, is fully recyclable post-use.
• Clayboard’s controlled structural performance supports thousands of kg per square metre when dry, then weakens selectively under minimal pressure once moisture is introduced, allowing intentional void creation.
• Clayboard has been awarded trusted BBA certification and is backed by decades of engineering honeycomb expertise.

How Clayboard works on site

1. Prep the subgrade (sand blinding or pea gravel)
2. Lay Clayboard on a 500g polythene membrane (Visqueen) with taped joints and a polythene overlay
3. Fix Voidpack pipes, usually at the time of steel fixing, before pouring the concrete
4. After the concrete is self-supporting, introduce water via Voidpak or hose to activate the weakening
5. After curing (~24–28 hrs), strike through the lower facing to allow drain-off

This process creates the void exactly where it’s needed without manual removal, safeguarding the structural integrity of the build while maintaining efficient and clean installation.

Clayboard excels as a viable alternative to EPS void formers because:

• Its slim profile means lower excavation costs and smaller overall sections.
• Its environmental integrity is unmatched, thanks to the fact that it is made from fully biodegradable materials.
• Its moisture-induced deformation ensures predictable protection.
• Its lightweight nature means it’s easier to transport, simple to install on site, and it produces much less wastage overall.

Why we’ll champion Clayboard – every time

At Dufaylite, our aim is to engineer solutions that combine durability, efficiency, and sustainability. Clayboard embodies our values completely – and it’s been uniquely designed to meet modern-day construction demands.

• Smart design: Thin, compressible, and durable
• Eco-aligned: Made from recycled paper honeycomb and fully recyclable
• Performance-focused: Controlled weakening, BBA-certified reliability
• Installation-friendly: Weighs less, handles easily, fewer logistics constraints

More builders and engineers are choosing Clayboard not simply as a ground heave prevention product, but as an intelligent investment in building longevity and site efficiency.

Ground heave FAQs

What is ground heave?

Ground heave is the upward movement of the ground caused by expansive clay soil swelling as it absorbs moisture. The pressure can crack concrete slabs and foundations if it isn’t accommodated.

What causes ground heave?

Common causes include removing mature trees (which stops drawing moisture from the soil), seasonal wet and dry cycles in shrinkable clay, and leaking drains. It mainly affects clay-rich soils.

How do you prevent ground heave?

On clay sites, the most reliable method is installing a compressible void former beneath the concrete slab, so the swelling ground heaves into the void instead of loading the structure.

Is ground heave worse than subsidence?

They are opposite problems: subsidence is the ground sinking, heave is the ground rising. Both damage foundations, but heave on clay is best managed with a compressible void former such as Clayboard.

The final word

Ground heave might be virtually invisible, but its consequences are anything but. Through advanced engineering, careful planning, and the incorporation of high-quality products like Clayboard into your ground heave protection strategy, you can mitigate ground movement, reduce risk, keep your project on budget, and minimise the environmental impact of your work.

Whether you’re tackling site-specific soil challenges, logistics constraints, or sustainability goals, Clayboard stands ready as the answer to all your heave-related problems.

Contact the team here at Dufaylite for more information on how Clayboard can be used to avoid ground heave (and, in fact, for a variety of other less conventional applications). You can also learn more about how our sustainable void former is helping trades build smarter here.