Finding borehole and geology records for a site

Before anyone books a rig, an hour with the published geological mapping and the national borehole archive will tell you most of what the ground is likely to be under a site. That is not a substitute for investigation. It is what lets you write an investigation brief that asks the right questions, and it stops you paying for six boreholes where four and two trial pits would have answered more.

The records are free to search, they cover England thoroughly, and the main skill is knowing what each layer of information can and cannot support.

Searching by location

Start with the national geological map viewer, which works from a postcode, a place name or a grid reference. Zoom to the site boundary and turn on bedrock and superficial deposits as separate layers, because the two are mapped independently and reading only one is the commonest mistake.

The viewer also carries the index of archived boreholes, shown as points. Clicking a point gives the record identifier, the depth, the date and, in many cases, a scanned copy of the original driller's or engineer's log. Records from site investigations, water wells, mineral prospecting and old shafts all sit in the same archive.

Set a search radius rather than looking only within the red line. Three logs within 300 metres are usually more informative than one on the site itself, because they show how the sequence varies.

Note the drilling dates as you go. A cluster of logs from the same year almost always belongs to one investigation for one scheme, and reading them together tells you what that scheme was worried about.

Reading bedrock against superficial deposits

Bedrock gives you the long view: strength, weathering profile, likely groundwater behaviour, and hazards such as soluble rocks or steeply dipping bedding.

Superficial deposits give you the short view, and on most sites they are the ones that decide the foundation solution. Alluvium in a valley floor means soft compressible ground and a shallow water table. Glacial till means variable strength and cobbles that slow excavation. River terrace gravel means good bearing and fast infiltration. Peat means settlement, and it can be thin enough to be missed by a coarse sampling grid.

Where the map shows a boundary running through the site, treat that line as approximate. Mapped geological boundaries were drawn from exposures and augering at a survey scale that is coarser than a site plan, and the real contact may be tens of metres from where the line falls.

Using archived logs properly

Read the log as a document with an author and a purpose. A water well log from 1954 records what the driller thought mattered for the well, which is rarely what you need for a raft foundation. A site investigation log from a nearby scheme in 2008 is far more useful, and it will often include in situ test results.

Three things to extract from each log. The sequence, layer by layer, with thicknesses. Any groundwater strikes and the dates, since a strike in August means something different from one in February. And any laboratory or in situ results reproduced on the log.

Then build a simple cross-section, even a hand-drawn one, linking the logs across the site. The point of the section is not accuracy. It is to expose where you are interpolating across a gap, because that gap is where your exploratory holes should go.

What is not published

Confidentiality is the main gap. Records deposited with the archive can be held confidential for a period, so a recent investigation on the adjoining plot may exist and not be visible. It is worth asking the neighbouring landowner or developer directly, and worth asking the council whether a report was submitted to discharge a condition, because reports supporting applications are usually on the public register.

The second gap is made ground. Worked, made and infilled ground is mapped where it was recognised, but shallow fill and backfilled pits often are not. Cross-checking against successive historic Ordnance Survey editions usually finds more of it than the geological map does, which is why the constraints plan is built before the design starts rather than after it.

The third is anything recent. Ground changes. A site levelled and surcharged five years ago will not look like the mapped sequence.

There is also a scale gap that nobody flags on the viewer. National mapping was surveyed for regional interpretation, and a site of half a hectare can sit entirely inside the width of a mapped boundary line. Treat the map as a statement about the area, and the borehole logs as statements about points, and be honest in the report about which of the two any given conclusion rests on.

Hazard datasets that sit alongside

Several derived datasets are published from the same underlying geology and are worth checking in the same session.

Radon potential, which determines whether protective measures are required in new buildings under the building regulations, is derived from geology and measured indoor concentrations. Shrink-swell potential matters wherever there is a firm clay and a tree, because it drives foundation depth and it interacts with the tree survey and root protection areas. Published data on landslides, dissolution features, compressible ground and running sand each flag a specific mechanism to test.

None of these is a constraint in the planning sense. They are design inputs, and a planning officer will rarely ask about them. Your structural engineer will.

Turning the desk study into a brief

The output you want is a ground model: a written description of the expected sequence, the design issues it implies, and a numbered list of what you do not know. Something like this, for an illustrative example on a former mill site:

1. Made ground of unknown depth across the eastern half, from demolition of the mill.
2. Alluvium of variable thickness over river terrace gravel.
3. Mudstone bedrock at an uncertain depth, inferred from two logs 200 metres away.
4. Groundwater expected within 2 metres of the surface, seasonally variable.
5. Possible contamination associated with the former use, to be scoped separately.

Each of those five lines converts into an instruction: prove the depth of made ground on a grid across the eastern half, prove the alluvium thickness at the building footprints, confirm rockhead by two deeper holes, install standpipes for seasonal monitoring, and schedule a contamination suite. That is a brief a contractor can price, and it is a brief a reviewer can check against the recognised standards for ground investigation and for investigating potentially contaminated sites.

One honest caveat: desk study interpretation is where experienced engineers add most value and where the records mislead most easily. A sequence that looks straightforward from three logs can turn out to be a buried channel. Build contingency into the investigation programme rather than into the foundation design.

You can assemble the rest of the evidence base with PlanScope and enter a postcode on the home page to see which reports a site is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and ground investigation reporting is one of them. This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it.