Site investigation scope for a constrained urban site

Scoping ground investigation on a tight urban site is an exercise in getting the most information from the fewest holes, in the smallest plant that will reach the depth you need. The governing document is BS 5930, the code of practice for ground investigations, and the discipline it imposes is that the scope follows the desk study and the conceptual site model rather than a standard package quoted by the metre. On a constrained site that discipline is what keeps the cost sensible.

Two decisions drive everything else. Whether geotechnical and contamination sampling can be combined in one visit, which usually they can and should be. And whether gas or groundwater monitoring is needed, because monitoring runs over months and becomes the long-lead item on the planning programme.

Why the desk study comes first

Without a desk study you cannot locate a hole intelligently. The desk study tells you the geological sequence, the historical land uses that might have contaminated the ground, the depth to which made ground might extend and where the previous buildings stood.

It also tells you what you are testing for. A former coachworks, a filling station, a dry cleaner and a printworks leave different contaminants, and a laboratory suite chosen without that knowledge either misses the relevant substances or tests for everything at several times the price.

The output that matters is the conceptual site model: named sources, plausible pathways, identified receptors. Each exploratory hole should exist to test a specific part of that model, and the scoping note should say which.

Scoping the intrusive works

Work from the model to a scope, and write the justification down. On a typical 0.2 hectare urban redevelopment plot the scope might comprise a small number of deeper holes to establish the sequence and bearing stratum, a spread of shallower holes to characterise made ground across the footprint, and inspection pits at the existing foundations.

Spread the locations rather than clustering them where access is easiest. An investigation with every hole in the car park because the rig could reach it tells you nothing about the ground under the proposed building.

And agree depths against the structural concept, not against a round number. If the engineer is considering piles, the holes have to reach below the anticipated toe level with adequate margin.

Access constraints and investigation methods

This is where urban sites bite. Run through the practical limits before the scope is written.

• Gateway and alley widths, which determine whether a tracked rig can enter at all
• Overhead constraints, including building overhangs, scaffolding and cables, which may force a low headroom rig
• Headroom inside retained buildings, often under 3 metres, which restricts casing lengths
• Proximity to party walls and the need for notices under the Party Wall etc. Act 1996
• Underground services, with detection to a recognised specification before any mechanical excavation
• Working hours and noise limits, especially where residential neighbours surround the site
• Highway occupation, which needs a licence from the authority where plant or welfare stands in the street

A low headroom rig costs more per metre and achieves less depth, so where a building is to be demolished anyway the sensible answer is sometimes to investigate after demolition and accept a condition in the meantime. That is a programme decision rather than a technical one, and it should be taken deliberately. The licence requirements for occupying the street are set out in scaffold licences and highway occupation.

Asbestos in made ground deserves its own mention. Where the history suggests demolition rubble, treat the ground as potentially asbestos-containing from the outset, with the controls that implies under the Control of Asbestos Regulations 2012 and a laboratory suite that includes it.

Combining geotechnical and contamination sampling

One mobilisation, two purposes. The economics are straightforward: the cost of bringing a rig and crew to a city centre site dwarfs the marginal cost of taking additional samples from the same hole.

Doing it well takes planning. The geotechnical and environmental scopes need agreeing together, with sample types, containers, preservation and chain of custody set out in advance. Geotechnical samples for classification and strength testing come from defined intervals; chemical samples need appropriate containers and prompt despatch to the laboratory. A hole logged for one purpose and sampled as an afterthought produces data that satisfies neither discipline.

Install monitoring wells in the same visit. Retrofitting them means a second mobilisation, which on a constrained site means a second licence, a second traffic management plan and a second round of neighbour notification.

A Geotechnical Assessment and a Land Contamination assessment are separate documents drawing on the same fieldwork. Contamination reports are expected to be prepared in accordance with BS 10175 by suitably qualified and experienced persons, and the responsibility for land affected by contamination rests with the developer under national policy. PlanScope's draft is a starting point for that professional, not a substitute.

Timing against the planning programme

Gas and groundwater monitoring is the item that catches developers. A meaningful monitoring record needs several visits spread over a period, including readings taken when atmospheric pressure is falling, and that is months rather than weeks.

So there are two strategies, and the choice should be conscious.

1. Investigate and monitor before submission, submit a full phase 2 report with the application, and ask for a lighter condition
2. Submit a phase 1 desk study with the application, accept the standard contaminated land condition, and investigate during the determination period or after consent

Route one gives certainty and a cleaner planning position, and suits a site where contamination is likely to be the controlling issue. Route two protects the programme on a site where the ground is probably straightforward. Where a basement is proposed, route one is effectively compulsory, because a Basement Impact Assessment needs ground water information that only monitoring provides.

Where existing foundations might be reused, inspection pits have to happen early enough to influence the structural scheme, and the practicalities are set out in reusing existing foundations on a redevelopment site.

Reporting for a planning audience

Planning officers and environmental health officers are not geotechnical engineers, and a factual report with 200 pages of logs and no interpretation will sit unread while the application stalls.

Write the interpretative report with a short summary that answers four questions directly. What is the ground made of. What risks does it present to people, buildings, water and vegetation. What has to be done about them. And is there any residual risk that needs managing in occupation.

Keep the factual data as appendices. Include a conceptual site model diagram, a plan of exploratory hole locations, a summary table of exceedances against the assessment criteria used, and a clear statement of the remediation strategy where one is needed.

Where further investigation is conditioned

The standard contaminated land condition comes in four parts: a site investigation and risk assessment to be submitted and approved, a remediation scheme based on it, a verification report confirming the remediation was carried out, and a provision dealing with contamination found unexpectedly during the works.

Treat each part as a separate submission with its own lead time. The verification report is the one that holds up occupation, because it cannot be written until the remediation is complete and the validation sampling has come back from the laboratory.

An illustrative example. A 0.15 hectare former workshop site in a terraced street has an access 2.4 metres wide and party walls on three sides. The investigation used window samples and hand-dug pits inside the footprint, one low headroom cable percussion hole for depth, two gas monitoring wells installed on day one, and a utility detection survey beforehand. Monitoring ran across the determination period, and the verification condition was discharged after demolition.

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