Geotechnical assessment at planning application stage

A geotechnical assessment planning stage report answers a narrower question than the ground investigation that follows it. The authority is not asking you to design foundations. It wants to know that the ground conditions are understood well enough for the proposed development to be feasible, that any ground related constraint has been identified, and that nothing about the ground will make the approved scheme impossible to build without changing what was permitted.

That distinction saves money. A full intrusive investigation to Eurocode 7 standards costs considerably more than a desk study with targeted sampling, and on most sites the planning application needs the second, with the first secured by condition.

What the planning stage assessment covers

The scope at application stage is usually a desk study, a site walkover, a preliminary ground model, and an opinion on feasibility and risk. Where the desk study flags a specific hazard, a limited intrusive phase may be needed before determination rather than after.

Four questions drive the content. Will the proposed foundations work in the expected strata. Is there a stability issue, from slopes, retaining structures, mine workings or soluble ground. Are there ground related constraints on the layout, such as areas that cannot be built on or basement depths that cannot be achieved. And does the ground interact with another discipline, usually contamination, drainage or trees.

Answer those four and the report has done its job. A Geotechnical Assessment prepared on that basis gives the officer enough to condition the detail.

Desk study sources before anyone digs

The desk study is where the value is, and the sources are largely public.

• Geological mapping and the national borehole record, which often holds logs from neighbouring sites.
• Historical mapping across several editions, which reveals quarries, ponds, infilled ground, railway cuttings and former industrial use.
• Coal mining records where the site falls in a referral area, and records of other mineral extraction.
• Groundwater and surface water data, including the position of source protection zones.
• Radon potential data for the locality.
• Previous site investigation reports for the site or its neighbours, often held by the council from earlier applications.
• Aerial photography, which shows made ground, tipping and drainage patterns that maps omit.

Ask the local authority what it holds. Contaminated land officers keep reports submitted with earlier applications, and a borehole log from the plot next door is worth more than any amount of inference.

The walkover is not optional. Slope angles, seepage, ground heave, cracked boundary walls, vegetation indicating wet ground and evidence of previous filling are all visible, and none of them appear in a data search.

Phasing the investigation sensibly

The sequence that works, and the sequence authorities expect, runs from the general to the specific.

1. Desk study and walkover, producing a preliminary ground model and a risk register.
2. Targeted intrusive work before determination only where a hazard could change the scheme outright.
3. Full ground investigation after permission, designed against the approved layout and secured by condition.
4. Foundation and retaining structure design, with validation and verification reporting.

Stage two is the judgement call. Where the desk study suggests shallow mine workings, a deep infilled feature, or slope instability that could sterilise part of the site, an authority will reasonably want evidence before granting permission, because the alternative is a permission that cannot be built. Where the risk is ordinary variable ground, a condition is the proportionate answer.

Combine investigations wherever you can. The same boreholes can serve geotechnical, contamination and infiltration testing if the scope is written that way, and three separate mobilisations cost far more than one.

Foundations and the planning consequences

Foundation choice is usually a design matter, but it becomes a planning matter when it changes what gets built or what gets harmed.

Piling is the response that most often attracts a condition, both because of construction effects and because piles can create a pathway through a low permeability layer into groundwater beneath. On a site with any contamination history, expect a requirement for a piling method statement and risk assessment to be approved before that work starts.

Trees and clay deserve a mention together. Where a scheme puts foundations near retained trees in shrinkable soil, the depth needed for the foundation and the root protection area needed for the tree can conflict directly, and the resolution belongs at layout stage. The note on temporary works and tree protection on tight sites covers how that plays out during construction.

Slope stability and retaining structures

Sites on slopes need a stability opinion at application stage, not a calculation. The officer wants to know whether the proposed cut and fill is achievable, whether the retaining structures are of a scale the design can absorb, and whether anything is being proposed that would destabilise land in other ownership.

Three points recur. Temporary works during construction are often more demanding than the permanent condition, which is why a method statement is conditioned. Surface water management affects stability, so the drainage and geotechnical work have to agree with each other. And land in third party ownership above or below the site constrains what can be done, since you cannot rely on a slope you do not control.

In London, basement schemes have their own regime, and many boroughs require a Basement Impact Assessment covering hydrogeology, ground movement and structural stability to a prescribed structure. That is a more demanding document than a general geotechnical opinion.

Where geotechnics meets contamination and drainage

The three disciplines share the same holes in the ground and often the same consultant, which is efficient as long as the reporting stays clear about which standard each part follows.

Contamination assessment follows its own framework, with a conceptual site model, a preliminary risk assessment and, where needed, intrusive investigation and remediation strategy. That is the territory of a contaminated land assessment, and the local planning authority expects it to be prepared by suitably qualified and experienced persons.

Drainage depends on infiltration testing, which is a geotechnical measurement. Soakaway feasibility, groundwater levels and the seasonal variation in those levels all come from the ground investigation, and a drainage strategy built on an assumed infiltration rate is the most common cause of a drainage condition that cannot be discharged. Where asbestos or other hazardous material is suspected in made ground, that also shapes the demolition sequence, as set out in the note on demolition asbestos surveys and the planning submission.

What the authority is likely to condition

Expect conditions requiring a scheme of ground investigation to be submitted and approved, remediation and verification where contamination is present, a piling method statement and risk assessment where piles are proposed, approval of foundation and retaining wall details where stability is in issue, and a watching brief for unexpected ground conditions.

Read the condition wording before you price the work. A condition requiring approval of an investigation scheme before any development begins puts the investigation on the critical path, and discharging it takes its own determination period.

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 ground investigation, stability assessment and foundation design are engineering work that a draft cannot replace.