Eurocode 7 and geotechnical categories in a submission

The Eurocode 7 geotechnical category is a risk classification, not a design method, and it decides how much ground investigation a project needs before anyone can design a foundation. Most housing and commercial development sits in Category 2. Category 1 covers small, simple structures on ground you already understand. Category 3 covers everything unusual, and a site that lands there will need a separate investigation regardless of how straightforward the building above looks.

At planning stage, the category matters less for the design than for the programme. Category 3 work takes months, and discovering it after permission is granted is how basements and mining-area schemes lose a construction season.

What the categories mean

BS EN 1997-1 sets out three categories, assigned to the whole structure or to parts of it.

Category 2 is where most spread foundations, piles, retaining walls, embankments and ordinary excavations sit. It requires quantitative data, which means an intrusive investigation with laboratory testing rather than a walkover and a borehole log from a neighbouring site.

The categorisation is provisional at the outset and gets confirmed or revised as information arrives. A site assumed to be Category 2 on the desk study can become Category 3 the moment a trial pit finds made ground of unknown provenance to five metres.

Eurocode standards are periodically revised, and the UK National Annex governs how they are applied here, so confirm the current version before writing a specification against a clause number.

Matching investigation scope to the category

BS 5930 is the code of practice for ground investigations and it sets the framework: desk study and preliminary sources, walkover, then intrusive work designed to answer identified questions.

For Category 2, the scope usually covers boreholes or trial pits to a depth related to the foundation influence zone, in situ testing, groundwater monitoring over a period rather than a single dip, and laboratory classification and strength testing. For Category 3, add whatever the specific hazard demands: rotary cores through rock head, inclinometers, instrumented monitoring, additional depth to prove voids, or pumping tests.

Two errors recur. Investigations sized by cost rather than by the questions they need to answer, so the boreholes stop above the layer that matters. And single-visit groundwater readings presented as a design water level, when the seasonal range is what governs.

Infiltration testing deserves its own mention, because the drainage design depends on it and the test method matters. Soakaway testing to the standard method, in the right season and to the right depth, is what the lead local flood authority will look for.

The ground investigation report and the design report

Eurocode 7 expects two distinct documents and they do different jobs.

The ground investigation report presents the factual record and the interpretation of ground conditions: what was done, what was found, the logs, the test results, the groundwater regime and the derived parameters. The geotechnical design report takes those parameters and sets out the design assumptions, the calculations, the supervision and monitoring required, and the checks needed during construction.

Keep them separate. A combined document that mixes factual logs with design assumptions makes it impossible for a reviewer to see which parameter came from a test and which came from judgement, and it is the commonest reason a report gets sent back for clarification.

Both should be signed, dated, and attributed to a named author with relevant experience. A Geotechnical Assessment drafted as a starting point gives you the structure and the policy framing, and councils generally expect a suitably qualified author for the interpretation, with professional review strongly advised before submission.

What belongs in a planning submission

Planning is not building control, and the temptation to submit everything is worth resisting. The case officer needs to know whether the ground presents a constraint that affects whether the development should be permitted, not how the piles are designed.

Five things earn their place:

1. A summary of ground conditions and the hazards identified, with the geotechnical category assigned and why.
2. The stability position, particularly on sloping sites, made ground, mining areas or land with a history of instability.
3. The groundwater regime, because it drives infiltration drainage, basement design and contamination pathways.
4. Any effect on neighbouring structures, highways or trees, with the mitigation principle stated.
5. What further investigation is proposed and how it will be secured, usually by condition.

Full factual logs go in an appendix. Design calculations generally do not go in at all.

Sites where category 3 is unavoidable

Six situations take a scheme into Category 3 more or less automatically, and all of them are worth spotting at acquisition.

Deep basements in urban areas with adjoining structures. Sites over shallow abandoned mine workings. Land with recorded instability, landslip or solution features. Deep made ground or landfill of unknown composition. Sites adjacent to tunnels, railways or major utilities with their own asset protection requirements. And tall buildings where settlement tolerances are tight.

Basements in particular attract a separate planning document in many authorities, and a Basement Impact Assessment covering hydrogeology, ground movement and structural stability is a validation requirement across a number of London boroughs. Where it applies, the geotechnical work has to be finished before the application rather than conditioned afterwards.

Budget realistically. A Category 3 investigation on a constrained site involves access, permits, monitoring periods and specialist testing, and the monitoring period alone can run to several months.

Coordinating with contamination and coal risk

Geotechnical and geoenvironmental work should be commissioned together and delivered as one programme, because they use the same boreholes.

Contamination follows its own route: a preliminary risk assessment, then intrusive investigation and risk assessment where the desk study identifies plausible pollutant linkages, reported through a Contaminated Land Assessment. That work has its own competency rules, and the local planning authority will expect reports prepared by suitably qualified and experienced persons.

Coal is separate again. Where a site sits in a development high risk area, a coal mining risk assessment is normally a validation requirement and the Coal Authority is consulted. Check the position on the published mining reports before designing anything, because shallow workings can rule out infiltration drainage and change the foundation solution entirely.

Where existing buildings are being retained or converted, the ground work has to be read alongside the Structural Survey, since the existing foundations are part of the problem rather than a given.

What councils actually read

Case officers read the summary, the constraints and the conclusion. Consultees read the part that concerns them: the lead local flood authority reads the infiltration testing, environmental health reads the contamination risk assessment, the highway authority reads anything affecting the highway, and the Coal Authority reads the mining risk assessment.

Write for that pattern. A one page summary at the front, stating the category, the hazards, the conclusion and the further work proposed, will be read by everyone. A ninety page factual appendix will be read by nobody unless the summary makes them curious.

One honest caveat. Ground investigation at planning stage is almost always partial, because access, demolition and funding all get in the way. Say what has not been investigated and why, and propose the condition that covers it, rather than implying a completeness the work does not have.

To see which ground, drainage and structural documents a site is likely to need, enter your postcode on PlanScope and work through the guided questions.

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