Eurocode 7 and geotechnical evidence for planning

A geotechnical assessment planning application does not need a full foundation design, and submitting one wastes money. What the authority needs to know is narrower: whether the ground conditions make the development achievable without unacceptable effects on neighbours, on stability, on drainage and on the wider environment, and whether the mitigation can be secured by condition. The detailed design follows later, under the Building Regulations and BS EN 1997, the Eurocode covering geotechnical design.

But the evidence you gather for planning sets the ground model that every later decision depends on, so it is worth doing properly the first time.

What a geotechnical assessment establishes at planning stage

Three questions, in this order. Is the site stable, and will it remain stable during and after construction. Can the proposed development be founded and drained in the way the scheme assumes. And will the works affect neighbouring land, structures, groundwater or surface water.

National policy expects sites to be suitable for their proposed use, taking account of ground conditions and any risks from land instability, and places the responsibility for securing safe development on the developer. Local plans add detail in coalfield areas, in districts with landslip histories and in boroughs with basement policies.

A geotechnical assessment at this stage is a risk document. It states what is known, what is assumed, what remains to be investigated, and what will be done about each.

The desk study and the ground model

The desk study is the part most often rushed and the part that decides everything else. It draws on published geological mapping, historical maps showing previous uses and made ground, borehole records from nearby sites, mining and quarrying records, groundwater and hydrogeological data, and a walkover.

From that material you build a conceptual ground model: a description of the strata expected, their likely depths and properties, the groundwater regime, and the uncertainties. Draw it as a section through the site with the proposed development superimposed. A ground model that exists only as text is one nobody will test against the scheme.

The model does two jobs at planning stage. It supports the statement that the development is feasible, and it justifies the scope of the investigation that will follow, which is what a condition will require.

Deciding the intrusive investigation scope

Eurocode 7 sorts structures into geotechnical categories, from small conventional structures in straightforward ground through to unusual or complex projects, structures in difficult ground and sites with abnormal risks. The category drives the investigation effort, and stating it in the assessment gives the officer a defensible basis for the scope.

Investigation techniques follow BS 5930 for ground investigation practice. The usual combination is window sampling or cable percussive boreholes for shallow work, rotary boring where rockhead or mining legacy matters, trial pits for foundations and services, in situ testing, and monitoring installations for groundwater and ground gas left in place for repeat visits.

Two scheduling points are worth planning around. Groundwater monitoring needs several visits over months to capture seasonal variation, so a single set of readings taken in August proves very little. And infiltration testing for drainage, carried out to the established soakaway test method, has to be done at the depth and location the drainage design assumes, which means the drainage strategy and the ground investigation should be commissioned together rather than in sequence.

Slope stability, made ground and mining legacy

Three ground conditions turn a routine site into a planning issue.

Slopes. Any development that loads the head of a slope, cuts into the toe, or changes drainage across it needs a stability assessment, and inherited landslip is more common in parts of England than the flat mapping suggests. Where a scheme relies on retaining structures, their height, position relative to boundaries and long term maintenance all become planning matters.

Made ground. Former industrial sites, infilled pits, railway embankments and reclaimed land carry variable, often compressible fill. The planning consequence is usually foundation depth, which then interacts with trees, service runs and neighbouring structures.

Mining legacy. In coalfield referral areas a coal mining risk assessment is normally a validation requirement, and shallow workings, shafts and adits can require treatment before development. Check the referral mapping early, because the treatment cost can change the viability of a small site.

Where contamination work overlaps

Geotechnical and contamination investigations use the same holes and different logic. The geotechnical scope is driven by the structure and the ground it bears on. The contamination scope is driven by the source, pathway and receptor model for the site's history.

Combining them is normal and sensible, provided the specification recognises both. Samples for chemical testing have different handling and preservation requirements from those for geotechnical testing, and ground gas monitoring installations need a screened response zone in the right stratum.

The contamination side follows BS 10175 for the investigation of potentially contaminated sites, with gas risk assessment to the recognised guidance where landfill, made ground or mining is present. A land contamination assessment is a report a suitably qualified and experienced person prepares, and national policy places responsibility for securing safe development on the developer, so treat an AI-assisted draft as a starting point for that professional rather than a substitute.

Where the scheme includes a basement, the two assessments feed a third, and the sequencing is set out in basement impact assessment: scope and stages.

What the planning application actually needs

For most schemes the submission is short: a desk study and ground model, a statement of the geotechnical category and the risks identified, the proposed investigation scope, and the mitigation principles.

Add more where the risk is visible to the officer. On a sloping site, a stability appraisal with sections. On a site next to existing buildings, a statement on the proposed foundation and excavation method and its effect on neighbours. On a coalfield site, the coal mining risk assessment. In a borough with a basement policy, the basement impact assessment the policy specifies.

Expect conditions requiring the full investigation, a remediation or mitigation strategy, and verification before occupation. Those conditions are normal and manageable. What is not manageable is a condition drafted on the assumption of a risk that does not exist, so respond to the consultee comments in writing before the decision rather than accepting a condition you will later have to vary.

Carrying the model into detailed design

The planning stage ground model becomes the ground investigation report and then the geotechnical design report as the project develops, which is the structure Eurocode 7 expects. Keep the documents as one lineage rather than commissioning a fresh investigation for the contractor, because inconsistent ground data between planning and construction is a familiar source of dispute.

Two handover points matter. The designer's duties under the Construction (Design and Management) Regulations 2015 include passing on the ground risks identified, so the residual risk register should name them. And the verification report that discharges the condition has to demonstrate that what was built matches what was approved, which is easier when the sampling locations were fixed against the approved drawings.

An illustrative example. A 25 unit scheme on a former brickworks has made ground of variable depth over firm clay. The planning submission carries a desk study, a section through the site, a category 2 classification and a proposed scope of twelve window samples with three monitoring installations. The condition requires the investigation and a remediation strategy. Total planning stage cost is a fraction of a full site investigation, and the developer keeps the flexibility to design foundations once the layout is fixed.

Enter your postcode on PlanScope to see which ground related reports your site is likely to need, then commission the desk study before the layout is finalised.

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.