Typical geology: London Clay, Lambeth Group, Gault, Weald Clay. The most shrinkable ground in the UK. Assume high volume change until the ground investigation says otherwise, and expect tree influence to drive depth.
Take this from the modified plasticity index in the ground investigation where you have one.
Indicative minimum founding depth
2250mm
Plan and price for up to 2550mm
Why this depth
- Shrinkable clay with high volume change potential sets a starting depth of 1000mm before any tree is considered.
- A high water demand tree of 24m mature height influences ground moisture out to roughly 30.0m in high volume change clay. At 8m the foundation sits inside that zone.
Before you break ground
- Deep trenches need a safe system of work: no unsupported entry, spoil set back, edge protection, and concrete placed the same day where possible.
- Book the excavation hold point with building control and the warranty inspector before you concrete — both want to see open trenches.
- Confirm drainage runs, service ducts and step positions before pouring, so nothing is chased through green concrete a week later.
- Check the sulfate and design chemical class from the ground investigation and specify the concrete accordingly.
Guidance only. This is not a foundation design and it does not replace a ground investigation, a structural engineer, or your warranty provider's own depth tables — where those tables give a deeper figure, the deeper figure governs.
Why clay moves, and why trees make it worse
Shrinkable clay changes volume as its moisture content changes. Through a dry summer the clay near the surface gives up water and shrinks; through a wet winter it takes water back and swells. A footing founded inside that zone rides up and down with it, and the cracking shows up in the brickwork long after the trades have left site. Founding below the zone of seasonal movement is what keeps the building still.
A tree changes the depth of that zone. Roots draw moisture out of the clay well beyond the canopy, and the drier, deeper soil profile persists year round. How far the effect reaches depends on how thirsty the species is, how tall it will grow, whether it stands alone or in a row, and how plastic the clay is. That is why a hazel at six metres and an oak at six metres are not the same question.
Shrinkage and heave are opposite problems
If the tree stays, you are designing against shrinkage — go deep enough that seasonal drying does not reach the bearing surface. If the tree has just come out, or is coming out as part of the enabling works, you are designing against heave. The clay that the tree kept dry for decades will slowly take moisture back and swell, lifting anything sitting on it and pushing sideways against anything in its way.
Heave is not solved by digging deeper. It is solved by giving the ground somewhere to go: compressible material against trench sides, void formers under ground beams and slabs, a suspended floor rather than ground bearing, and slip membranes to pile shafts so the swelling clay cannot grip and drag the pile upwards.
Worked scenarios
Mature oak, 8m away, London Clay
High water demand, high volume change potential, well inside the zone of influence. Expect a deep trench fill in the region of two metres or a short-bored pile and beam solution, plus heave precautions if the oak is not staying. Price the piling option before you commit to digging.
Silver birch hedgerow, 12m away, Midlands till
Moderate demand and medium volume change, but a row counts for more than a single stem. It may sit just inside the influence zone, so check the mature heights rather than what is there today before settling on a depth.
No trees, glacial till in Scotland
Shrinkage is not the controlling factor. Depth is driven by frost, soft deposits and getting onto competent bearing — and the Scottish technical handbooks apply, not the Approved Documents.
Questions site teams ask
- How deep do foundations need to be in clay?
- In shrinkable clay a low-rise strip foundation is normally taken well below the zone of seasonal moisture movement — typically at least 750mm in low volume change clay and deeper in medium and high volume change ground. Where a water-demanding tree sits inside its zone of influence the depth can increase to two metres or more, at which point a piled solution is often cheaper and safer than a very deep trench.
- How close can you build to a tree?
- There is no single distance. What matters is the species water demand, the mature height of the tree, whether it stands alone or in a group, and how shrinkable the clay is. A high water demand tree such as oak, willow or poplar can influence founding depth out to more than its own mature height, while a low demand species in non-shrinkable ground may have almost no effect on depth at all.
- What happens if the tree is removed before we build?
- Removing a mature tree from shrinkable clay lets the ground recover moisture and swell back over several years. That is heave, and it is a different problem to shrinkage: deeper footings on their own do not solve it. It needs designing for with compressible fill or void formers to the trench sides, a heave-resistant or suspended ground floor, and slip membranes to any pile shafts.
- Does the region of the country change the answer?
- Yes, because the underlying geology does. London Clay, Lambeth Group and Gault across London and the South East are among the most shrinkable ground in the country. Mercia Mudstone and Lias through the Midlands and South West are typically medium. Much of the North, Wales, Scotland and Northern Ireland is glacial till where shrinkage rarely controls the depth and soft deposits, peat or rockhead do instead.
- Does this replace a ground investigation?
- No. It gives you an indicative depth and the reasoning so you can plan, price and challenge a drawing that looks wrong. The definitive depth comes from the site-specific ground investigation, the structural engineer and your warranty provider technical standards — and where they specify deeper, deeper governs.