Geotechnical advice and the Eurocode 7 categories

Eurocode 7 sorts geotechnical work into three categories, and the category you are in decides how much ground investigation the project needs and who has to be involved. A simple house on known ground sits in the first category and can be designed from experience and a qualitative look at the ground. A basement under an existing building on made ground next to a railway sits in the third, and everything about the investigation, the design and the supervision changes.

Architects usually meet this at the point where a planning condition requires further ground investigation and nobody has budgeted for it. Working out the category early tells you what the budget should have been.

What a geotechnical assessment covers

At planning stage, a geotechnical assessment answers whether the ground can support what is proposed, whether the proposal will destabilise anything around it, and what further work is needed before construction.

The content follows the questions the council is likely to ask. A desk study of published geology, historical maps, mining and quarrying records and groundwater information. A walkover. A ground model describing the expected sequence of strata and the groundwater regime. An assessment of the hazards that model implies, which might include slope instability, shrinkable clay, dissolution features, mining legacy, made ground or compressible deposits. And a conclusion on what investigation and design work follows.

It is not a foundation design. That comes later and belongs to the structural engineer working from the geotechnical parameters.

Where the ground is also potentially contaminated, the contamination assessment runs to its own standards and is a separate discipline, even though the same drilling rig usually takes both sets of samples on the same day.

How the design categories drive the level of input

Eurocode 7 sets out three geotechnical categories, and the classification should be stated explicitly in any report.

The first category covers small and relatively simple structures where the risk of ground-related failure is negligible and where design can rest on comparable experience and a qualitative investigation. Simple low-rise buildings on known competent ground, and low retaining walls, typically sit here.

The second covers conventional structures and foundations with no exceptional risk and no unusual ground or loading conditions. This is where most buildings sit, and it requires quantitative geotechnical data and calculation rather than experience alone.

The third covers everything outside the first two: very large or unusual structures, abnormal risk, unusually difficult ground, sites with a history of instability, and works that could affect neighbouring structures in an unusual way. Category three work needs alternative provisions and rules beyond the standard, and it requires specialist geotechnical involvement throughout.

The category is not fixed at the start. A category two project becomes a category three project the moment the investigation finds a solution feature or an unrecorded shaft, and the report should say what would cause a reclassification.

Who specifies and who supervises the investigation

Three roles are involved and they are frequently confused.

The geotechnical adviser specifies the investigation: the number, position and depth of exploratory holes, the sampling and testing schedule, and the in-situ tests required. That specification should follow from the ground model and the category, not from a standard package.

The ground investigation contractor carries out the work: drilling, sampling, logging, in-situ testing and laboratory testing. The contractor produces the factual record.

Supervision sits between the two. Someone with geotechnical competence should be on site while the holes are being put down, because the decision to extend a borehole, change a sampling interval or add a test has to be taken there and then. An unsupervised investigation delivers exactly what the specification asked for and nothing about what the ground actually turned out to be.

On a category one site, supervision can be light. On a category three site it is continuous, and the specification is expected to evolve during the works.

Reporting requirements at each stage

Eurocode 7 distinguishes between the record of what was found and the interpretation of what it means, and both have to exist.

The ground investigation report presents the factual data: borehole logs, test results, groundwater observations, and an evaluation of the information. It is the permanent record, and it should be legible on its own years later.

The geotechnical design report sets out the design assumptions, the characteristic values of the ground parameters, the design approach applied, the calculations, and the supervision, monitoring and maintenance required during and after construction. Eurocode 7 requires this report, and its absence on a category two or three project is a gap that will be found at building control if not before.

At planning stage you will usually have a desk study and perhaps a preliminary investigation. Say clearly in the submission what stage the work has reached, what remains to be done, and what could change as a result. Councils are far more comfortable with a staged approach that is described honestly than with a report implying a completeness it does not have.

Interaction with the structural engineer

The interface is the set of characteristic ground parameters and the design approach. The geotechnical adviser derives the parameters from the investigation and states the uncertainty in them. The structural engineer applies them to the foundation design under the design approach used in the United Kingdom national annex.

Problems appear when that handover is informal. A verbal bearing capacity passed between consultants, with nobody recording the assumptions behind it, is the origin of a great many foundation disputes.

Insist on a written parameter schedule with the assumptions, the limitations and the recommended further testing set out. It costs almost nothing at the time and settles arguments later. The same discipline applies to any commissioned technical work, and the points in writing a brief for a specialist planning report hold here.

Questions to ask before instructing

Ask these and you will avoid most of the common problems.

1. Which geotechnical category does this project fall into, and why.
2. Who will specify the investigation, and will that person be independent of the drilling contractor.
3. Will a geotechnically competent person supervise the fieldwork, and for how much of it.
4. Will you produce both a ground investigation report and a geotechnical design report.
5. Are contamination sampling and soakaway infiltration testing included, or are they separate instructions.
6. Has the site been checked against mining, quarrying and dissolution records.
7. What would cause you to recommend additional investigation, and what would that cost.

The infiltration testing question is worth asking twice, because a drainage strategy that assumes infiltration without testing is one of the most common reasons a drainage condition cannot be discharged.

Where the council conditions further work

Councils rarely refuse on ground conditions. They condition them, and the conditions have teeth.

Expect a condition requiring a scheme of investigation to be submitted and approved, a further condition requiring the results and any remediation or foundation strategy to be approved, and a verification condition after the works. On sloping sites a monitoring regime may be required. In several London boroughs a basement scheme triggers its own assessment regime with an independent audit paid for by the applicant.

Read the conditions in the order they have to be discharged, and note which of them prevent any work starting. A condition requiring approval of an investigation scheme before development starts means the investigation itself may need separate consent or may be permitted as an exception, and that question is worth asking at the time rather than on the day the rig is booked.

Reviewing the resulting reports takes the same approach as reviewing a specialist report you did not commission. You can find reviewers through the PlanScope professional directory, and see how a geotechnical assessment fits with the rest of an application pack in the full list of report types.

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, and this is general guidance rather than legal advice.