Reading a geology map before a foundation decision

A published geology map tells you what to expect under a site and nothing about what is actually there. That is the right way to hold it. A geology map site check costs an hour, narrows the plausible foundation solutions from six to two, and tells you whether the ground investigation you are about to pay for needs two trial pits or six boreholes. What it cannot do is give you a bearing capacity, a depth to competent strata, or any number you could put on a structural drawing.

For a self-builder or a small developer, the value is in the questions the map lets you ask before anyone quotes.

What the published mapping shows at site scale

The British Geological Survey publishes onshore mapping through its GeoIndex viewer, and the standard product is derived from surveying at 1:50,000. At that scale, one millimetre on the map is 50 metres on the ground. A boundary drawn between two deposits is an interpretation, not a surveyed line, and it can be tens of metres out.

So treat every boundary near your site as a zone of uncertainty. If your plot sits close to the edge of an alluvium deposit, the honest conclusion is that you might be on it or off it, and that is exactly the kind of question one trial pit answers for very little money.

The viewer also shows the locations of recorded boreholes, mapped artificial ground, mass movement deposits and, in some areas, mining information. Turn the layers on one at a time. A site that looks unremarkable on the bedrock layer sometimes carries a large made ground polygon that only appears when you switch that layer on.

Superficial deposits against bedrock

Two separate layers, two separate questions.

Bedrock is the solid geology: chalk, sandstone, mudstone, limestone. It tells you the broad engineering character of the ground at depth, and it drives the questions about solution features in chalk and limestone, and about sulphates in some mudstones.

Superficial deposits sit above the bedrock and are usually the layer that decides a shallow foundation. Alluvium, peat, river terrace gravels, glacial till, head deposits and blown sand all behave differently, and several of them are variable within a single plot. Peat and alluvium are the two that most often turn a strip footing scheme into a piled one.

Read them together. A site mapped as one metre of head over Mercia Mudstone is a different proposition from the same head over alluvium, because the thickness and consistency of the superficial layer determines whether you can found on it, through it, or below it.

Where a site shows no superficial deposits at all, do not assume the bedrock is at the surface. Weathered rock, topsoil and previous cut and fill are all invisible on the map.

Borehole records near your site and their age

Recorded boreholes are the closest thing to real data you get for free. The viewer gives you their location and, for many of them, a scanned log you can read.

Three checks before you rely on one:

1. Distance. A log 40 metres away on the same deposit is useful. A log 400 metres away across a mapped boundary usually is not.
2. Date and purpose. A 1960s log drilled for a water supply may record only broad strata, while a 2015 log drilled for a building may record standard penetration test results and groundwater levels.
3. What it does not say. A log that stops at four metres tells you nothing about five.

Groundwater observations deserve particular attention, and particular scepticism. A water strike recorded on a single day in August is not the winter water table. If the log shows water within a metre or two of your proposed formation level, assume the design needs to cope with worse.

Read the logs alongside the historic mapping for the site. Our note on using historic maps as former land use evidence covers how to build that side of the picture, and the two together usually explain any made ground the geology layer shows.

Shrink swell potential and tree proximity

Clay soils that shrink in dry weather and swell when wet cause more foundation damage to small buildings in England than any other ground condition. The mapping will not give you a plasticity index, but it will tell you whether you are on a formation with a reputation: London Clay, the Lambeth Group, Gault, Oxford Clay, Kimmeridge Clay, Mercia Mudstone and several glacial tills among them.

The practical consequence for a self-build is foundation depth. On a shrinkable clay, foundation depths are driven by the desiccation zone, and the presence of trees, hedgerows or recently removed trees extends that zone considerably. The relationship depends on the species, its mature height and the distance from the foundation, and the industry reference point is the foundation depth guidance in the house building standards used by the main warranty providers, which your structural engineer will apply.

Two points people miss. Removed trees can cause heave as the clay rehydrates, so a cleared plot with stumps is a flag rather than a relief. And on a shrinkable clay site with trees, a deeper trench fill foundation often costs less than a shallow raft once the design is priced properly, which is an argument for getting the ground question settled before the structural scheme is fixed.

Tree constraints on the same site also feed the planning submission, because a Tree Survey (Arboricultural Impact Assessment) prepared to BS 5837:2012 will set root protection areas that interact with your foundation zones.

Made ground and its implications

Artificial ground, usually shown as made ground, worked ground or infilled ground, is the single most useful thing the map can tell you about a brownfield or urban plot. It means someone has moved material, and material of unknown origin and unknown compaction.

Three consequences follow. Made ground is rarely suitable to found on without treatment, so the foundation solution changes. It may contain contaminants, which brings the land contamination route into play and normally means a Phase 1 desk study before anything else. And it can generate ground gas where the fill included anything organic, which is assessed under its own methodology.

None of that is a reason to walk away. It is a reason to sequence the investigation so that the contamination and geotechnical work happen together, using the same trial pits, rather than paying twice for the same holes. The Contaminated Land Assessment and the Geotechnical Assessment draw on much of the same fieldwork, and briefing them as one exercise saves money.

When a ground investigation replaces the map

At the point where a number has to go on a drawing. A published map supports a site appraisal, a risk assessment and a scope of works. It does not support a foundation design, and no building control body or warranty provider will accept it as if it did.

The trigger points are straightforward:

• Any foundation design, because allowable bearing pressure comes from testing.
• Any basement or significant excavation, where groundwater and wall pressures govern.
• Any site with mapped made ground, mass movement deposits, peat or alluvium.
• Any site where contamination is suspected from previous use.
• Any site in a coalfield or mining referral area, where a mining report is a separate check.

Scope the investigation from what the map left uncertain. Naming the specific uncertainties in the brief, rather than asking for a standard investigation, produces a better proposal and usually fewer holes.

Feeding the answer into your design

Write the map reading up as a page, not as a folder of screenshots. Record the mapped bedrock and superficial deposits with the formation names, the boreholes you looked at with their reference numbers and dates, the layers you checked and found nothing on, and the three or four uncertainties you want the investigation to resolve.

That page does three jobs. It becomes the desk study section of the ground investigation brief. It gives your structural engineer a starting position for the foundation options. And it goes into the planning submission where the authority asks about ground conditions, stability or contamination, where a basement scheme needs a Basement Impact Assessment, or where a drainage strategy depends on whether infiltration is feasible at all.

On that last point, be careful. Infiltration drainage is often designed on an assumption drawn from the geology map and then fails a percolation test on site. If your surface water strategy depends on soakaways, test early, because the fallback position changes the whole layout.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

PlanScope can draft the supporting statements around the ground evidence once your investigation is done, flagging with a clear marker every figure that still needs confirming from the site data. For specialist technical reports, qualified professional input may be required.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.