Ground investigation spend on a small site

Most ground investigations on sites of fewer than ten homes cost more than they need to, and the reason is almost always the same: the work was split into three visits instead of one. A geotechnical rig comes out for foundation information, an environmental team comes back for contamination samples, and a third party arrives to dig soakaway pits. Each visit carries its own mobilisation, its own traffic management and its own minimum charge.

Combine them and you pay for one mobilisation. That single decision usually saves more than any negotiation on rates.

What drives the scope

Scope is set by what the council will ask for and by what your designer needs, and those are not the same list.

The council's list comes from the validation checklist and from consultee advice. On a small residential site it typically covers past uses of the land, the risk of contamination reaching future residents, surface water disposal, and, in coalfield and other referral areas, ground stability. Your designer's list covers bearing capacity, foundation depth, shrink-swell potential in clay soils, groundwater level and the effect of nearby trees.

The overlap between the two is large, which is what makes combining the work sensible. The differences matter though. A contamination sample and a geotechnical sample are taken differently, stored differently and tested differently, and a driller who was not told to expect both will not have the right containers on the van.

Desk study first, and what it rules out

The Phase 1 desk study is the least costly part of the job and the part that determines everything after it. It assembles historical mapping, geological information, landfill and permit records, groundwater and source protection zone data, coal mining and other mineral records, radon data, and any previous investigation on or near the site, then builds a preliminary conceptual site model of sources, pathways and receptors.

A good desk study does two useful things. It can conclude that no intrusive contamination investigation is needed at all, which on a greenfield garden plot with no industrial history is a realistic outcome. And it targets the intrusive work where the model says the risk is, rather than spreading trial pits evenly across the site.

Land contamination assessment is one of the areas where the local planning authority expects reports prepared in accordance with BS 10175 by suitably qualified and experienced persons, and the responsibility for that sits with the developer. A Contaminated Land Assessment drafted by PlanScope is a starting point for that qualified professional rather than a substitute for their judgement or their signature.

Trial pits, boreholes and window samples

Three techniques cover almost all small-site work, and each answers a different question.

On a plot for two or three houses, a handful of trial pits and two or three window samples often answer everything. On a site with made ground of unknown depth, or where a soakaway is proposed, boreholes earn their cost because they tell you where the water is.

Access governs the choice more than geology does. If the only route onto the site is a garden gate, the rig has to fit through it, and that constraint should be checked before the investigation is specified rather than on the morning of the works.

Infiltration testing and its timing

If surface water is going to a soakaway, the infiltration rate has to be measured, and the method councils and drainage engineers expect is the soakage test described in the Building Research Establishment's digest on soakaway design. The essential features are that the test pit is dug to the depth and dimensions of the proposed soakaway, that it is filled and allowed to drain three times in succession, and that the design rate is taken from the slowest of the three.

Two things go wrong repeatedly. The test is carried out at the wrong depth, usually shallower than the proposed soakaway, which gives a rate for a soil horizon the soakaway will never sit in. And it is carried out in a dry spell, when the ground is unrepresentative and the result flatters the design.

Groundwater level matters as much as infiltration rate. A soakaway needs clearance between its base and the highest expected water table, so a test that ignores winter groundwater produces a design that floods. Where the result is poor, the answer is usually to redesign the drainage rather than to retest, and the Drainage Strategy should show the fallback option so the council is not left guessing.

Laboratory testing decisions

Laboratory scheduling is where costs escalate quietly, because the samples are already in the lab and adding another suite feels marginal.

Keep it disciplined. Schedule contamination testing against the contaminants identified in the conceptual site model, not against a standard suite chosen for convenience. Where the desk study identified a former filling station, test for hydrocarbons properly. Where it identified nothing but farmland, a broad metals and hydrocarbons screen on a small number of samples is usually proportionate.

Geotechnical scheduling follows the foundation question. Classification testing tells you what the soil is. Shrink-swell testing matters only where the soil is a shrinkable clay and trees are present or were recently removed. Sulphate and pH testing on made ground informs the concrete specification and costs very little against the risk of a redesign.

Ask the consultant for the schedule before it goes to the lab, and ask what decision each test will inform. Any test that cannot be tied to a decision can wait.

Combining investigations to save mobilisation

The practical version of this is a single specification issued to a single contractor, covering geotechnical logging, environmental sampling, in-situ testing and infiltration testing, carried out in one visit with one traffic management arrangement.

Three details make it work. Tell the contractor at tender stage that both contamination and geotechnical samples are required, so the right containers and preservation arrangements are on site. Specify the infiltration test positions and depths from the drainage layout rather than leaving them to the driller. And have someone competent on site during the works who can move a pit position when the ground turns out to be different from the model.

That last point is the one that pays. An investigation supervised by the engineer who will design the foundations produces usable information. One left to an unsupervised crew produces logs.

A Geotechnical Assessment written from a well-targeted single visit is generally more useful than one assembled from three partial datasets taken months apart in different weather.

Sequencing against the application

The desk study should be done before the application is submitted, because it may be a validation requirement and because it may change the layout. The intrusive work can often wait for a condition, and on many small sites that is the sensible cash-flow decision.

The exception is where the intrusive result could change the scheme. If the soakaway is the only viable surface water route, or if a contamination hotspot would sterilise part of the site, find out before you fix the layout. Discovering it at condition discharge means either a redesign or a non-material amendment, and neither is quick.

Set the investigation cost against the wider budget early, in the same exercise that covers the application fee, the supporting documents and the discharge stage, rather than treating it as a construction cost that appears later.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

See PlanScope pricing if you are costing the supporting documents alongside the site investigation.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.