Using the BGS GeoIndex for borehole and ground data

The British Geological Survey publishes a free map viewer called the GeoIndex that shows what is under your site and where somebody has already drilled nearby. For a self-builder, an hour in the BGS GeoIndex looking at borehole records answers the two questions that most often derail a budget: what the foundations are likely to cost, and whether a soakaway stands any chance. It does not answer either question definitively. It tells you which of them to worry about.

Start with the geology, then the boreholes, then the caveats. In that order, because a borehole log read without knowing the mapped geology around it is a number without a context.

Reading the geology before the boreholes

The viewer shows two geological layers that matter. Bedrock is the solid rock beneath, mapped as formations with names like London Clay, Mercia Mudstone or Chalk. Superficial deposits are the younger material sitting on top: river terrace gravels, alluvium, glacial till, made ground where it has been mapped.

Superficial deposits usually decide the engineering. A site on 4 metres of alluvium over gravel behaves nothing like the plot across the road on bedrock at 300mm, and the boundary between them can run through a garden. Alluvium in particular flags soft, compressible ground and a high water table, which in turn points at raft or piled foundations and at surface water that will not go anywhere.

Zoom in and note the mapped boundaries near your plot. Where your site sits close to one, assume the boundary is approximate. Geological mapping is drawn at a scale that was never meant to resolve individual house plots.

Finding and downloading the borehole records

Switch on the boreholes layer and the map fills with markers, dense in urban areas and sparse in open countryside. Click one and the record panel gives a reference, the depth, the date, sometimes the purpose, and a link to a scanned log where the record is open. Many logs are available as free downloads. Some are confidential at the depositor's request and show location only.

What a good log gives you: a stratigraphic description layer by layer, the depth to each change, standard penetration test results where the drilling was geotechnical, the depth at which water was struck, and the ground level at the top of the hole.

What it will not give you, and where people go wrong:

• The datum is often unclear on older logs, so the stated ground level may not tie to current levels on your site.
• Descriptions follow the conventions of the day. A 1960s driller's "clay" covers a range that a modern engineer would separate into several materials.
• Water strikes record conditions on one day in one season. A dry log in August proves nothing about February.
• The hole was drilled for someone else's purpose, often a road or a sewer, so its depth may stop well above the level your foundations need.

Take three or four records around the site rather than one. If they disagree, that disagreement is itself the finding, and it is the argument for an intrusive investigation early rather than late.

Turning the data into planning documents

Ground information feeds several parts of an application at once, which is why it repays an hour at the start.

A geotechnical assessment built on archive records alone is a scoping document. It sets out the conceptual ground model and recommends what the investigation should test. The same archive search is the backbone of the desk study stage of a land contamination assessment, where BGS records sit alongside historical mapping, landfill registers and the Environment Agency's public register. Contamination work carries a competence expectation, since the Framework places responsibility on the developer and authorities expect investigations following BS 10175 by suitably qualified and experienced people.

For surface water, the mapped superficial geology gives the first indication of whether infiltration is on the table. That answer flows straight into the drainage strategy, and, where the site sits in Flood Zone 2 or 3 or exceeds a hectare, into the flood risk assessment as well. Groundwater flooding is the mechanism most often missed on small sites, and a shallow water table in the archive logs is the earliest warning you will get.

What the viewer cannot see

Mine workings are the biggest gap. BGS holds shaft and plan information for some areas, though coal records sit with the statutory mining body and other minerals are inconsistently recorded. If your site is in a coalfield referral area, the archive search is a supplement to the formal mining report, never a replacement.

Made ground is under-mapped almost everywhere. A plot that was a Victorian brickworks or a wartime dump may show nothing on the superficial layer, because the mapping predates the activity or the deposit was too small to map. Historical Ordnance Survey editions catch what the geology map misses.

And the viewer says nothing about what a neighbour did last year. Recent site investigations are commercially held and only reach the BGS archive where a depositor sends them, so the newest and most relevant data for your street is often the data you cannot see.

An illustrative search

Illustrative example. A plot for a single house on the edge of a market town. The GeoIndex shows Mercia Mudstone bedrock with a thin band of river terrace gravel following the stream 80 metres to the east. Three archive boreholes lie within 400 metres, all drilled for a 1970s sewer scheme. Two sit on gravel and record water at 1.8 metres. One, on the same side of the contact as the plot, records firm clay from 0.6 metres with no water strike to 6 metres. The conclusion is not a foundation design. It is a scope: trial pits on the plot itself, infiltration testing treated as unlikely to pass, and a drainage strategy drafted around attenuation and a controlled discharge. The figures are illustrative.

Where to stop

The desk study earns its keep by narrowing the questions. Once you know the likely ground model, you can brief a ground investigation that costs a fraction of a speculative one, and you can tell your designer whether to draw a soakaway at all.

What it cannot do is satisfy a condition. Authorities that require ground investigation, remediation strategies or verification reports want site specific data from someone competent to interpret it. Archive records go in the appendix.

Enter your postcode on PlanScope to see which ground related reports a site is likely to need, and use the BGS search to fill in the site description before you commission anyone.

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