Geotechnical Assessment: the ground investigation structure

A geotechnical assessment for a planning application is a ground investigation report followed by an interpretative report, and the geotechnical assessment structure that councils and building control both recognise runs from desk study to design recommendations in a fixed order. The order matters. Each section is the input to the next, and a report that jumps from borehole logs to foundation advice without a stated ground model has skipped the step where the engineering judgement lives.

For a single self-build plot the whole exercise is modest. It is still worth understanding what you are buying.

Desk study and the preliminary ground model

The desk study assembles what is already known before anyone puts a hole in the ground: published geological mapping, historical Ordnance Survey editions, records of previous uses, mining and coal authority information where relevant, borehole records held by the British Geological Survey, groundwater and flood data, and a walkover of the site.

From that the engineer writes a preliminary ground model, a short statement of what is expected to be under the site and in what sequence, with the uncertainties named. On a typical plot that might read as topsoil over made ground over firm clay, with the depth of the made ground unknown and a possible perched water table above the clay.

The model is the reason the investigation costs what it does. An investigation designed against a clear model targets the questions that matter. One designed without a model drills a fixed grid and answers whatever it happens to hit.

This is also where the ground investigation and the contamination work diverge. The chemical side follows its own route through a preliminary risk assessment and, where needed, a Contaminated Land Assessment, and it is prepared by suitably qualified and experienced persons in accordance with the relevant British Standard. Keep the two reports distinct even where the same fieldwork serves both.

Investigation methods and exploratory hole layout

The methods section says what was done, where and why. Trial pits give a direct view of shallow soils and are the workhorse for housing foundations. Window sampling and dynamic probing reach deeper with small plant. Cable percussive boreholes are the traditional method for deeper work with in situ testing. Rotary coring is for rock.

Look for a layout plan showing every exploratory hole with a reference and a coordinate, and a rationale for the positions. Holes placed under the proposed building lines, at the proposed soakaway location and at any suspected made ground hot spot are doing work. Holes placed in a tidy grid across the car park are not.

The section should also record what could not be done. Refusal at shallow depth, an abandoned hole, restricted access under a tree canopy: all of these are legitimate and should be stated rather than quietly omitted. And where infiltration testing has been carried out for drainage purposes, say which method was used and at what depth, because the drainage engineer will need it.

Laboratory testing schedule

The schedule lists the samples, the tests and the standards. For a low-rise residential scheme on clay the usual set includes moisture content, Atterberg limits to give plasticity and therefore shrinkage potential, particle size distribution, and shear strength. Chemical testing for sulphates and pH informs concrete specification.

Two checks are worth making as a client. That the number of tests is proportionate to the number of strata rather than concentrated in one hole. And that the plasticity results are reported in a form the foundation designer can use directly, because plasticity drives the foundation depth required near trees.

Ground and groundwater interpretation

This is the section that turns data into a model. It describes each stratum, its depth range across the site, its variability, and the groundwater regime including any standing water recorded and any response monitored over time.

Groundwater deserves more than a single reading. A water strike recorded during drilling on a dry August day is not the winter level, and a report that gives one figure without a date and without comment on seasonal variation is understating what it knows. Where a basement or a soakaway is proposed, ask for monitoring over a period rather than a spot reading.

A good interpretative section also says where the model is uncertain, and what would resolve it. That sentence is often the most valuable one in the report.

Geotechnical parameters and design values

Parameters are the numbers the structural engineer will design with: bulk density, undrained shear strength, effective stress parameters, stiffness, and the allowable bearing pressure or the ultimate capacity depending on the design approach.

Design to Eurocode 7, published in the United Kingdom as BS EN 1997, works with characteristic values selected by the geotechnical engineer, to which partial factors are applied. A report that gives a single "safe bearing capacity" figure with no stated basis is working in an older idiom, and the structural engineer will have to interpret it anyway.

Ask for the characteristic values to be tabulated by stratum with the basis for each, whether from in situ testing, laboratory results or published correlation. That table is what the foundation design is built on and it is what a checking engineer will look at first.

Foundation, slope and excavation recommendations

The recommendations section applies the parameters to the proposal. For a house it covers the foundation type and depth, any influence from existing or removed trees on shrinkable clay, floor slab type and whether a suspended slab is needed, ground gas protection where the contamination assessment requires it, and the specification of concrete for the ground conditions found.

Where the scheme involves a cut, a retaining structure or a basement, the section extends to temporary works advice, excavation stability, and the effect on neighbouring foundations. That is where the geotechnical report and the Structural Survey of adjoining property meet, and where a party wall process usually begins.

Tree influence is the commonest single issue on a self-build plot. Where trees have been removed, heave rather than shrinkage may govern, and the answer is often a suspended slab with a void former rather than a deeper strip footing. The report should name the trees, their species and their distance, and connect that to the depth recommended.

How the report feeds drainage and contamination work

The ground investigation is not a standalone document. Its infiltration testing feeds the Drainage Strategy, and where infiltration is unviable the report should say so in terms the drainage engineer can cite. Its chemical results feed the contamination assessment and any remediation strategy. Its groundwater data feeds any basement or dewatering design.

Commission it once, early, and brief the engineer on all three uses. A report scoped only for foundations that has to be revisited for soakaway testing costs a second mobilisation, which on a single plot can be a large share of the original fee.

Councils usually expect a suitably qualified author for a Geotechnical Assessment, and professional review is strongly advised. An AI-assisted draft can structure the reporting and flag missing inputs; the ground model, the parameters and the recommendations have to come from the engineer who signs them.

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. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.