Scoping a ground investigation for a planning application

Most small schemes need a desk study at application stage and the intrusive ground investigation secured by condition. Getting the scope right means deciding three things before anyone books a rig: what the desk study has told you to look for, which technique reaches the depth that matters, and whether the same holes can serve the contamination assessment as well as the foundation design. Spend a day on the desk study and you usually halve the fieldwork.

The expensive mistake runs the other way: booking a day of trial pits, finding made ground to four metres, and having to go back for cable percussion boreholes because the first visit could not reach the bearing stratum.

Desk study before anything is dug

The desk study establishes the conceptual site model, which is the thing every later decision rests on. For a small site it takes a few hours of searching and a few more of writing.

The sources worth working through:

• Geological mapping, including the superficial deposits and the bedrock, from the British Geological Survey
• Historical Ordnance Survey mapping across every available edition, looking for pits, ponds, infill, tanks, railway sidings, works and watercourses
• Coal mining information where the site sits in a coalfield referral area, including whether it is in a development high risk area
• Landfill and waste site records, and permitted or historic industrial uses nearby
• Radon potential for the area
• Groundwater vulnerability, source protection zones and surface water features
• Flood maps, and any record of previous ground investigation on or adjoining the site

The single most informative document on a brownfield plot is usually a historical map. A pond on an 1890s edition that has disappeared by 1950 is an infilled hollow, and infilled hollows are where the soft ground and the ground gas sit.

Write the output as a conceptual site model with sources, potential sources of contamination, pathways and receptors, and the geotechnical questions the investigation has to answer. Then scope the fieldwork against that model rather than against a standard template.

Choosing between trial pits, window samples and boreholes

Each technique has a reach and a purpose, and the common error is to choose on day rate.

For a single dwelling on a greenfield plot in firm clay, machine trial pits plus a couple of window sample holes answer most questions. For an infill plot with made ground, a small terrace or anything with a basement, expect boreholes.

Trial pits have one advantage nothing else matches: you can see and photograph the soil profile, and you can expose an existing foundation to measure it. Where the project involves building up to or under a neighbour, that matters, and the position on existing foundations and basement depth is settled with an inspection pit rather than an assumption.

Depth and spacing for a small scheme

Depth is driven by the loaded zone, not by convention. The investigation should reach far enough below the proposed foundation level that the stress increase from the building is no longer meaningful, which for strip footings on a house usually means a couple of metres below founding level, and for a raft or a piled solution means considerably more.

Three site-specific factors extend the depth. Shrinkable clay with trees on or adjacent to the site, where foundation depths have to account for soil desiccation and the influencing distance of each tree species. Made ground of unknown thickness, where you need to prove the base of it. And any basement, where the depth of interest extends below formation level by enough to characterise the founding stratum and the groundwater regime.

Spacing is a matter of judgement rather than standard. Common practice on a small scheme is a minimum of three exploratory holes, at least one within the footprint of each proposed building, plus holes positioned to test the features the desk study identified: the line of the former pond, the area of the old workshop, the position of a proposed soakaway.

Put one hole where you least want to find a problem. Investigations designed to confirm a comfortable assumption are the ones that produce surprises during groundworks.

Combining geotechnical and contamination sampling

One visit can serve both purposes, and it only works if the brief says so from the start.

Geotechnical sampling wants disturbed and undisturbed samples, in situ testing, and a logged profile. Contamination sampling wants representative samples taken with clean equipment, in the right containers, kept cool, with a chain of custody, and analysed by an accredited laboratory against the right suite. Lubricants and fluids used for drilling can compromise chemical results, so the method has to be agreed in advance.

Ground gas is the item most often forgotten, and it cannot be done in a single visit. Characterising ground gas requires standpipes installed during the investigation and monitored on several occasions across a period, including visits during falling atmospheric pressure. That means the monitoring programme has to start early enough to be complete before the information is needed, which on a site with made ground or a former landfill nearby can be the critical path item.

Infiltration testing for a soakaway follows its own method, with repeated fillings of a trial pit and a measured fall, and it needs to happen in the same visit as the trial pits. A Drainage Strategy that assumes infiltration without a test is an assumption the lead local flood authority will not accept.

Standards the work should follow

Four reference points cover a small investigation.

BS 5930 is the code of practice for ground investigations and sets out the planning, execution and reporting of the work. Eurocode 7, in the BS EN 1997 series, governs geotechnical design and the ground investigation and testing that supports it, and its geotechnical categories are a useful way of justifying the scope: a straightforward house in known ground sits in a lower category than a basement next to a party wall.

For contamination, BS 10175 covers the investigation of potentially contaminated sites, and the local planning authority will expect reports prepared in accordance with it by suitably qualified and experienced persons. Ground gas assessment follows the published industry guidance on the subject.

Specify the standards in the brief. A quotation that does not name BS 5930 and BS 10175 is a quotation for something less than an investigation.

What the planning submission needs

For most householder and small residential applications, the submission is a phase 1 desk study and preliminary risk assessment. It should conclude with one of three positions: no further work required, further intrusive investigation required, or further investigation required for a specific identified risk.

Where the site has a known constraint, more is needed up front:

1. A site in a coal mining development high risk area normally needs a coal mining risk assessment with the application.
2. A site on or adjoining a former landfill, infilled ground or a known industrial use usually needs at least a preliminary intrusive investigation to show the scheme is feasible.
3. A basement or a retaining structure affecting a neighbour needs the ground model established before the design can be assessed, which means intrusive work before determination.
4. A site where infiltration drainage is proposed needs the infiltration test results.

The authority's local validation list governs, so check it before you decide. A Geotechnical Assessment and a land contamination assessment are different documents with different authors, and both are types where the local planning authority expects a suitably qualified person. The contamination reports in particular are ones where a professional is required rather than merely advisable.

Where investigation can wait for a condition

The standard contaminated land condition sequence is well established, and it exists precisely so that intrusive work can follow permission: submission and approval of a site investigation scheme, then the investigation and risk assessment, then a remediation strategy, then a verification report, with a further clause dealing with unexpected contamination found during works.

Deferring is sensible where the desk study shows a low or moderate risk, where access for a rig is not available until the site is cleared, or where the building to be demolished sits over the area of interest.

Deferring is a mistake where the finding could change the scheme. If a contamination result might require a membrane, a cover system or a change in foundation type, or if ground conditions might rule out the basement, you are deferring a decision rather than a survey. The same applies to groundwater: whether a basement can be built as drawn depends on the water regime, and groundwater control for a basement below the water table is not a question to leave to a condition.

Agree the scope with the case officer and, where contamination is involved, with the authority's contaminated land officer. A scope agreed in writing before the fieldwork is the best value insurance on the project.

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