Using geological mapping before a ground investigation

A geological map site check costs nothing and answers three questions that shape a small development budget: what the ground is likely to be, whether soakaway drainage has any chance of working, and which hazards deserve attention before anyone hires a rig. The British Geological Survey publishes mapping and borehole records that cover most of England, and a careful hour with them tells you more than a page of assumptions. It will not tell you what is under your particular corner of the plot. That is the difference between a desk study and an investigation, and the gap matters.

What published mapping actually shows

BGS mapping is normally viewed at a scale of 1:50,000, and it presents the ground in two layers.

Bedrock is the solid geology beneath everything: a chalk, a sandstone, a mudstone formation, named and mapped as a unit. Superficial deposits sit above it where they exist, and these are the recent materials that usually matter most to a shallow foundation and a drainage design. Alluvium along a watercourse, river terrace sand and gravel, glacial till, head deposits on a slope, peat in a hollow.

Two limitations follow from the scale. A boundary drawn on a 1:50,000 map has a real world uncertainty measured in tens of metres, so a site close to a mapped boundary could be on either unit. And thin or patchy superficial deposits are frequently unmapped, which is why a site shown as bedrock at surface can turn out to have a metre of clayey head over it.

Read the map alongside the descriptions BGS publishes for each unit. The lexicon entry for a formation tells you its typical composition, thickness and engineering behaviour, which is the part people skip.

Finding borehole records near the site

The more useful half of the exercise is the borehole record search. BGS holds scanned logs from investigations carried out by others, many of them decades old, and the locations are searchable on its onshore index viewer.

What you are looking for is a log within a few hundred metres that gives a stratigraphy: the depth and description of each layer, any groundwater strike and its date, and sometimes test results. Three or four logs around a site build a picture that mapping alone cannot.

Treat the records with appropriate suspicion. Older logs use superseded terminology, descriptions vary in quality, the recorded position may be approximate, and a borehole on the other side of a fault or a buried channel tells you about a different sequence. Note the date of any groundwater level, because a reading taken in August is not a winter level.

A quick check of the historic mapping alongside the logs is worth the time. If the site was a brickworks, a gravel pit or a pond, the borehole log may be the only record of what went back into the hole. The walkthrough on checking whether a site is on a brownfield land register covers the related question of formal designation.

Made ground, and why it is usually invisible on the map

Made ground is any material placed by people: fill in a former pit, levelling material across a slope, demolition rubble, ash, industrial waste. It rarely appears on the 1:50,000 mapping in any useful detail.

You infer it instead. A site shown on an older edition map as a quarry, a clay pit, a pond, a railway cutting or an embankment is a candidate. So is any level platform in otherwise sloping ground, and any site where the ground surface sits noticeably above the neighbouring land.

The consequences run in two directions. Structurally, made ground is variable and often compressible, so foundations either go through it or are designed for it. Environmentally, it can contain contaminants and can generate ground gas, which is where a contaminated land assessment begins. Phase 1 of that work is a desk study and a walkover producing a conceptual site model, and it must be prepared by a suitably qualified and experienced person working to the relevant British Standard, because the local planning authority will judge it on that basis.

What the geology suggests about infiltration

This is the question a self-builder usually asks first, because a soakaway is the least expensive surface water solution and the drainage hierarchy asks you to consider infiltration before a watercourse or a sewer.

Three constraints apply whatever the map says. An infiltration rate has to come from an in situ test rather than from a published table. There has to be sufficient unsaturated ground beneath the base of the device, which means a groundwater level measured at the wettest time of year rather than the day of the trial pit. And infiltration through contaminated ground, or within a groundwater source protection zone, may be unacceptable to the Environment Agency regardless of how well it drains.

So the map narrows the options and the test decides. A site on gravel with a high water table fails for a reason the geology alone would not have told you.

Hazards a desk study should flag

The datasets and mapping raise several hazards that are worth checking before design.

Shrink and swell potential on clay soils, which sets foundation depth, and interacts with trees on or near the site. Soluble rocks, including gypsum in some mudstone sequences and dissolution features in chalk. Compressible ground where peat or soft alluvium is present. Slope instability on steeper sites and where a landslide deposit is mapped. Running sand, which affects excavation method rather than design. And radon, where the site falls in an area requiring basic or full protective measures under the building regulations guidance.

Coal mining and brine extraction are outside BGS mapping and need their own searches, from the Coal Authority in coalfield referral areas and from the relevant local arrangements in the salt field areas of Cheshire.

Where any of these is significant, a geotechnical assessment is the document that turns the hazard into a design parameter, and it is work for a geotechnical engineer rather than a desk exercise.

Where the desk study stops

The desk study produces a conceptual model: what the ground probably is, where the water probably sits, what might be in it, and what the risks are to the proposed development. Its output is a set of questions, not a set of values.

An intrusive investigation answers them. On a single plot that usually means a small number of trial pits or window samples, in situ infiltration tests at the proposed soakaway locations and depths, groundwater monitoring where levels matter, and soil sampling where the history justifies it. Specifying it well is where the desk study pays for itself, because a well targeted investigation on a small site is a modest cost and a badly targeted one has to be repeated.

An illustrative example: a plot on the edge of a village, mapped as glacial till over mudstone, with two BGS borehole logs 150 metres away showing 1.2 metres of clay over weathered rock and no groundwater strike. Historic mapping showed a pond in the corner of the field. The investigation put one trial pit at the pond location, found 1.8 metres of soft fill, and moved the house footprint eight metres. Infiltration testing confirmed the clay would not take water, and the drainage design went to attenuated discharge instead.

Feeding the findings into the drainage strategy

The drainage strategy should carry the geological evidence through explicitly: the mapped units, the borehole evidence, the infiltration test results with locations and depths, the groundwater level and its date, and the reason the chosen discharge option sits where it does in the hierarchy.

Where the site is in a flood zone, the same ground information supports the flood risk assessment, particularly on groundwater flooding and on the feasibility of any proposed storage. Climate change allowances are revised periodically, so check the current published allowance rather than copying a figure from an earlier report.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which reports the constraint check points to, then commission the ground investigation with a clear brief.

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