Geotechnical assessment: what Eurocode 7 reporting requires
Planning wants enough ground evidence to show stability and a workable drainage design. Here is what to investigate and how the reporting should read.
Planning rarely asks for a full geotechnical design. What it asks for is enough evidence that the ground can support what you propose, that stability and land contamination have been considered, and that the drainage strategy rests on real infiltration data rather than an assumption. A geotechnical assessment for a planning application therefore sits between the desk study and the engineer's design, and on a single plot it is often the piece of work that determines the foundation budget.
Eurocode 7 is the framework the reporting follows, and it sets expectations about what has to be recorded and how parameters are chosen.
When planning needs geotechnical evidence
Four situations bring it into scope on a small site.
Sites in a coalfield referral area, where the council will expect a coal mining risk assessment before determination. Sloping sites or land with a history of instability, where stability has to be addressed. Sites where a soakaway or other infiltration drainage is proposed, since the design cannot be justified without testing. And former industrial or agricultural land, where ground conditions and contamination overlap.
Elsewhere the council may leave it to building control and condition a site investigation instead. Ask at pre-application which route your authority takes, because commissioning a full investigation you did not need is an expensive way to be thorough.
Desk study sources worth using
Start with published records. They cost little and they shape the intrusive work that follows.
- British Geological Survey mapping and archived borehole records for the area.
- Historic Ordnance Survey maps at several dates, which reveal filled ponds, quarries, tanks and former buildings.
- Coal Authority mining reports where the site falls in a referral area.
- Environmental data on landfill, radon and made ground.
- Groundwater and surface water information, including any records from adjoining development.
- Records of trees, both present and removed, since a felled mature tree on a shrinkable clay site sets up heave rather than subsidence.
Write down what the desk study predicts before you dig. A ground model stated in advance, then tested, is far more convincing than one assembled after the results arrive.
Intrusive investigation and what Eurocode 7 expects
Eurocode 7 divides work into geotechnical categories according to risk, with a simple category for straightforward structures in known ground and higher categories where conditions or loads are unusual. Part 2 deals with ground investigation and testing.
The reporting expectation is a ground investigation report recording the factual data, and a geotechnical design report interpreting it. On a small residential scheme the two are often combined, and that is acceptable provided the factual and interpretative parts are distinguishable. A reader must be able to see what was measured and what was inferred.
Scope the investigation to the ground model and the structure. Trial pits are usually enough to establish foundation depth in shallow made ground. Boreholes or window samples are needed where the sequence is deeper or where groundwater has to be monitored over time. In situ testing and laboratory work should be chosen to produce the parameters the design actually needs, including sulfate and pH results where concrete specification depends on them.
Ground model and parameter selection
The characteristic value of a geotechnical parameter, in Eurocode 7 terms, is a cautious estimate of the value affecting the occurrence of the limit state. That word cautious is the whole point, and it is where reports diverge.
Show the data spread and say how you arrived at the value used. A single triaxial result quoted as though it characterises a stratum is not a cautious estimate of anything. Where the data are limited, say so and choose accordingly, then explain what further testing would refine.
Present a ground model that names each stratum, its depth range across the site, its expected variability and the groundwater regime. That model is what the structural engineer, the drainage engineer and the contractor will all read, and inconsistency between their assumptions is where cost overruns start.
Infiltration testing for drainage design
Where infiltration drainage is proposed, the council and the lead local flood authority will expect testing carried out to a recognised method, with results at the depth and location of the proposed system.
Three points recur.
Test at the right depth. An infiltration rate measured at 600 mm in permeable topsoil tells you nothing about a soakaway founded at 2 m in clay. Repeat the test as the method requires, since a single fill and empty cycle is not a result. And record the groundwater level, because a system founded within a metre of the water table will not work regardless of the measured rate.
Where infiltration fails, the strategy moves to attenuation and a controlled discharge, which changes the layout and sometimes the developable area. Establish that early. The division of work between the flood and drainage documents is set out in what goes in a flood risk assessment and what goes in a drainage strategy, and the Drainage Strategy has to be built on the geotechnical results rather than the other way round.
Coalfield and shrink-swell clay sites
In a coal referral area, the council will consult the Coal Authority and will normally want a coal mining risk assessment identifying shallow workings, mine entries and any need for treatment. Mine entries carry standoff distances that can sterilise part of a site, so establish their position before you fix a layout.
On shrinkable clays, the controlling factors are soil plasticity and the influence of trees. Foundation depths are set from the volume change potential and the proximity and species of vegetation, using established guidance, and the presence of a tree removed within the last few years matters as much as one still standing.
Neither issue is normally fatal. Both are expensive when discovered after the foundations are priced.
Where the report belongs in the condition sequence
On most small sites the sequence runs like this. A desk study and any assessment required at application stage. A condition requiring intrusive investigation and a remediation or foundation strategy to be approved before work starts. Then verification reporting before occupation.
Because contamination and geotechnical work share the same fieldwork, commission them together. Reports on land contamination must be prepared by suitably qualified and experienced persons, and a contaminated land assessment is one of the areas where that is a legal expectation rather than a preference.
An illustrative example. A single plot on a former paddock in an area of London Clay, with a mature oak 12 m from the proposed corner. Trial pits at four positions, plasticity testing, a soakaway test at the proposed invert depth and a foundation depth recommendation referenced to the tree influence would be a proportionate scope. Those figures are illustrative only.
A Geotechnical Assessment is work councils expect from a suitably qualified author, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval, and for specialist technical reports qualified professional input may be required.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified engineer before relying on it.
Frequently asked questions
Does a planning application always need ground investigation?
No. Councils commonly condition intrusive investigation rather than requiring it before determination. It is normally needed at application stage on coalfield referral sites, sloping or unstable ground, sites relying on infiltration drainage, and former industrial or agricultural land. Ask at pre-application which approach your authority takes.
What reports does Eurocode 7 expect?
A ground investigation report recording the factual data, and a geotechnical design report interpreting it. On small residential schemes the two are often combined, which is acceptable as long as a reader can tell what was measured from what was inferred. Part 2 of the standard covers ground investigation and testing.
What is a characteristic value?
It is a cautious estimate of the parameter value affecting the occurrence of the limit state being considered. The word cautious is what distinguishes it from an average. Show the spread of results and explain how the value used was selected, because a single test result quoted as representative of a stratum is not a cautious estimate.
How should infiltration testing be done?
At the depth and location of the proposed system, using a recognised method with the required number of fill and empty cycles, and with groundwater levels recorded. A rate measured in permeable topsoil says nothing about a soakaway founded in clay two metres down, and a system within a metre of the water table will not work.
What extra work does a coalfield site need?
Within a coal referral area the council consults the Coal Authority and normally expects a coal mining risk assessment covering shallow workings, mine entries and any treatment required. Mine entries carry standoff distances that can sterilise part of a site, so establish their position before fixing a layout.
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.