Geotechnical assessment and Eurocode 7 expectations
Ground investigation at planning stage answers whether the site suits the proposal. Eurocode 7 sets the categories, the design approach and the two reports councils confuse.
Ground investigation at planning stage answers one question: is the site suitable for what you are proposing to put on it. National policy puts that question squarely on the applicant, and an authority that has doubts about slope stability, mining legacy, made ground or shrinkable clay will either ask for evidence before determination or attach a condition that stops work until it has been supplied. The second route is more common and more expensive, because by then the layout is fixed.
Eurocode 7 governs how the investigation is designed and how the results are turned into a design. It is a structural code rather than a planning one, and the gap between what the code expects and what a planning officer asks for is where most schemes lose time.
When a council asks for ground evidence
Requests cluster around a handful of triggers. A site within a coalfield referral area attracts a request for a coal mining risk assessment. Sloping ground, a quarry edge, a landslip record or a made ground fill attracts a request for stability evidence. A basement brings a Basement Impact Assessment covering groundwater and the effect on neighbouring structures. Infiltration drainage on any site brings a request for permeability testing.
Beyond those, requests are usually driven by the authority's own knowledge of local ground rather than by a policy threshold. A district with a history of swallow holes or peat will ask on almost every application. That is worth establishing at pre-application stage, because it changes the programme by months.
The desk study that has to come first
Eurocode 7 part two treats the desk study as the foundation of the investigation, and doing intrusive work before it is finished is the most common way to waste money on boreholes in the wrong place.
The desk study should assemble published geological mapping and borehole records, historic Ordnance Survey editions back to the earliest available, mining and mineral records, groundwater and source protection zone data, landfill and waste site records, flood and surface water mapping, and any records from neighbouring development. A walkover follows, looking for cracking, seepage, settlement in adjoining structures and evidence of previous ground works.
The output is a preliminary ground model: a written description of what you expect to find at what depth, with the uncertainties named. The intrusive investigation is then designed to test that model rather than to produce a generic set of holes at 20 metre centres.
Geotechnical categories and proportionate investigation
Eurocode 7 sorts structures into three geotechnical categories, and the category sets the level of investigation and the qualifications of the people doing it.
| Category | Typical scheme | Investigation expectation |
|---|---|---|
| 1 | Small, simple structures on known ground with negligible risk | Routine methods, qualitative assessment, experience |
| 2 | Conventional buildings, retaining walls, shallow and piled foundations | Quantitative geotechnical data and calculation |
| 3 | Unusual structures, very difficult ground, abnormal risk, sensitive neighbours | Specialist investigation and design, often instrumented |
A five-storey block on glacial till is category 2. The same block adjoining a railway cutting or over shallow mine workings is category 3, and the investigation cost changes accordingly. Getting the category wrong at the start is what produces the second round of fieldwork after the design team sees the first factual report.
Design approaches and what the reports are called
The United Kingdom National Annex adopts Design Approach 1, which requires two combinations of partial factors to be checked: one applying factors principally to actions, the other applying them principally to ground material properties. The governing case differs by problem, so both have to be run rather than assumed.
Two deliverables matter. The ground investigation report presents the factual data and the interpretation of ground conditions. The geotechnical design report sets out the design values, the assumptions, the supervision and monitoring requirements, and the items to be checked during construction. Councils frequently ask for "a geotechnical report" without distinguishing the two, and submitting the factual report alone is a common reason for a condition not being discharged.
Where the structure above ground is also uncertain, for instance in a conversion, a Structural Survey runs alongside rather than inside the geotechnical work.
Foundations near trees and buried services
On shrinkable clay, tree proximity dictates foundation depth, and the relationship runs on soil plasticity index, the species water demand and the mature height of the tree. The depths involved are substantial, and a scheme designed on a standard 900 millimetre trench fill that then meets a mature oak at eight metres will need redesign.
This collides with tree protection on the planning side. Root protection areas calculated under the British Standard on trees in relation to design, demolition and construction restrict where you may excavate, while the foundation design pushes you deeper exactly where the roots are. Resolve both at layout stage, with the arboriculturist and the geotechnical engineer looking at the same drawing.
Buried services are the other late surprise. Statutory undertaker records are indicative, and a service trench through the footprint can require a redesign of the drainage strategy as well as the foundations.
Conditions, and running contamination work at the same time
A typical condition requires a site investigation and, where needed, a remediation strategy to be submitted and approved before development starts, with a verification report on completion. Two points make discharge easier.
First, run the geotechnical and contamination fieldwork as one exercise. The boreholes and trial pits are the same holes, and the difference is in the sampling, the testing schedule and the reporting standard. A Contaminated Land Assessment follows BS 10175 and the land contamination risk management framework, and it has to be prepared by suitably qualified and experienced persons. Splitting the two exercises doubles the mobilisation cost for no benefit.
Second, agree the scope with the authority's contaminated land officer before mobilising. A short scoping email that sets out the number and depth of exploratory holes, the testing suite and the reporting standards costs nothing and prevents a rejected report.
Where a basement is involved, the groundwater regime drives both the structural design and the flood case, and basement flood risk measures depend on the water levels the investigation records.
Who is competent to sign it
Eurocode 7 requires geotechnical design to be carried out by appropriately qualified and experienced personnel, and the higher the category the more specific that becomes. For category 2 and above, expect a chartered engineer or engineering geologist with geotechnical specialism, and on larger schemes a professional on the ground engineering register maintained by the civil engineering institution.
A Geotechnical Assessment drafted with AI assistance can carry the desk study, the site history, the regulatory framework and the structure of the report, and it will flag where site data is missing rather than inventing it. The ground model, the design values and the sign-off are the engineer's, and the authority will expect a named author with the relevant competence.
Book the desk study before the layout is fixed. That is the single decision that saves the most money on a difficult site.
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.
Frequently asked questions
When does a planning application need a geotechnical assessment?
Most often where the site lies in a coalfield referral area, on sloping or filled ground, over shallow workings or a former quarry, or where a basement is proposed. Infiltration drainage also brings permeability testing. Beyond those triggers, requests are driven by the authority's knowledge of local ground conditions rather than by a national threshold.
What is the difference between a ground investigation report and a geotechnical design report?
The ground investigation report presents the factual data and the interpretation of ground conditions encountered. The geotechnical design report sets out design values, assumptions, and the supervision, monitoring and construction checks required. Eurocode 7 expects both. Submitting only the factual report is a common reason a ground conditions condition is not discharged.
Which design approach applies in the UK under Eurocode 7?
The UK National Annex adopts Design Approach 1, which requires two combinations of partial factors to be checked. One applies factors principally to actions, the other principally to ground material properties. Which combination governs varies by problem, so both have to be run rather than assumed from a previous project.
Can geotechnical and contamination investigation be combined?
Yes, and they usually should be. The exploratory holes are the same holes, and what differs is the sampling regime, the testing schedule and the reporting standard. Contamination work follows BS 10175 and the land contamination risk management framework and must be prepared by suitably qualified and experienced persons. Splitting the two doubles mobilisation cost.
Who can prepare a geotechnical report for planning?
Eurocode 7 requires appropriately qualified and experienced personnel, and the expectation rises with geotechnical category. For category 2 and above, a chartered engineer or engineering geologist with geotechnical specialism is the norm, and on larger or riskier schemes a professional on the ground engineering register maintained by the civil engineering institution.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.