Unexploded ordnance risk and when a survey is expected

Nothing on a constraints map tells you that a site was bombed in 1941. Unexploded ordnance risk is not a designation, it does not appear on a local plan proposals map, and no statutory consultee raises it. It arrives instead through the ground investigation, or worse, through a piling rig, which is why an unexploded ordnance risk assessment is best commissioned at the same moment as the Phase 1 desk study rather than after a contractor asks about it.

The legal framework is health and safety rather than planning. Under the Construction (Design and Management) Regulations 2015 the client has to provide pre-construction information about hazards on the site, and the principal designer and principal contractor have to plan the work around them. An unknown risk is not a defence.

Where wartime bombing and military use create risk

Two histories matter. The first is aerial bombardment. Docks, railway marshalling yards, gasworks, aircraft factories and city centres took concentrated bombing during the Second World War, and a proportion of dropped munitions failed to detonate. Some were recovered at the time. Some were recorded as recovered and were not.

The second is military use of the land itself. Former airfields, firing and bombing ranges, anti-aircraft and searchlight battery sites, ordnance factories, ammunition stores, practice trenches and coastal defence works all leave ordnance behind, and much of it is small arms ammunition, mortar rounds and grenades rather than air dropped bombs. Practice ranges are the awkward case, because the risk is shallow, widespread and not concentrated in a single strike position.

Rural sites are not exempt. A quiet field in Norfolk or Suffolk may have been a bomber base dispersal, and a stretch of coast in Kent may have carried anti-invasion defences.

Desk study first, intrusive survey second

The first stage is a preliminary written risk assessment, prepared by a specialist using records rather than fieldwork. The sources are wartime bomb census and air raid damage records, Ordnance Survey mapping from the 1930s to the 1950s, contemporary aerial photography, air ministry and war office records, and any local authority or fire brigade incident registers that survive. The National Archives holds much of the primary material.

A competent assessment does three things. It describes the ordnance types plausibly present, it states the likely depth range for each, and it relates that depth to the proposed works. A risk of shallow small arms ammunition on a site with no excavation below 600mm is a different proposition from a possible 500kg bomb at eight metres on a site with 15 metre bored piles.

Most sites end there. The assessment concludes low risk, recommends a toolbox briefing and an emergency procedure, and that is the whole output. Where the risk is assessed as medium or high, the next step is survey: non-intrusive magnetometer survey across the site to locate near surface ferrous anomalies, or intrusive magnetometer probing at each pile or borehole position to clear the ground to depth in advance.

Work to a recognised methodology. CIRIA C681 remains the reference guide for unexploded ordnance in construction, and a report that cites no methodology at all is hard to rely on when a contractor prices the risk.

The operations that trigger concern

Depth and energy are what change the answer. Ordnance sitting undisturbed for eighty years becomes hazardous when something hits it, vibrates it or heats it.

• Piling of any kind, driven or bored, and especially continuous flight auger piling that penetrates deep made ground.
• Deep excavation for basements, attenuation tanks, lift pits and pumping stations.
• Dynamic compaction, vibro stone columns and other ground improvement.
• Borehole drilling and cone penetration testing, including the investigation work itself.
• Grubbing out of old foundations and obstructions where nobody knows what is below them.

That last one is the practical trap on redevelopment sites. A demolition contractor removing a slab and the fill beneath it is doing exactly the operation the risk assessment would have flagged, and often earlier in the programme than anyone expected.

Who prepares it and to what standard

Specialist consultancies staffed by former military explosive ordnance disposal personnel do this work, and the desk study should be signed by a named assessor with relevant experience. It is not a task for a general geotechnical consultant to add as an appendix, although the two reports should be read together.

There is no statutory competency scheme comparable to the licensing regime for protected species work, which means due diligence sits with you. Ask what records the assessor used, whether they visited the site, how they derived the depth ranges and what the residual risk is after the recommended mitigation.

The ordnance assessment and the ground investigation belong in the same conversation as the Geotechnical Assessment, because the piling design that follows determines how much clearance work is needed.

Cost, programme and the watching brief

A watching brief is the usual mitigation for excavation on a medium risk site. An explosive ordnance disposal engineer attends while the machine works, monitors the excavation, and inspects arisings. The engineer can stop the work and has the authority to clear the area.

That attendance is priced by the day and it lengthens the excavation phase, because the machine works at the pace the supervision allows. Plan for it in the programme rather than discovering it as a variation. Where piling is involved, the probing of pile positions has to be sequenced with the piling rig, and a rig standing idle while probes are driven is the expensive version of this.

Illustrative example. A brownfield site in a bombed industrial district, cleared in the 1960s and used as a car park since. The desk study identifies recorded strikes within 200 metres, deep made ground, and a proposed basement at five metres with bored piles beyond that. The recommendation is intrusive magnetometer probing at each pile position and a watching brief during basement excavation. The programme effect is real but manageable when it is known at tender stage, and disruptive when it is discovered after the piling subcontractor is on site.

How it sits alongside contamination and archaeology

Three desk based exercises look at the same historic mapping, and it is wasteful to commission them separately without coordination. The Phase 1 desk study for land contamination reads the same industrial history. An archaeological desk based assessment reads the same maps for a different purpose, and where archaeological evaluation involves trial trenching, the trenching is itself an intrusive operation that the ordnance assessment has to cover.

Sequence them so the ordnance position is known before anyone breaks ground. The ordnance clearance of a trial trench line is a small job when planned and an awkward conversation when the archaeologist is already mobilised.

Recording residual risk for the contractor

The output that matters most is the least glamorous. After mitigation there is always residual risk, and it has to be written down in the pre-construction information so that the principal contractor can plan for it, brief operatives and put an emergency procedure in place.

That procedure should be one page. Stop work. Clear and cordon the area, with a stated exclusion distance. Call 999. Do not move or handle the item. Nominate who takes charge on site and who notifies the client. Include it in the Construction Management Plan where the authority has conditioned one, because that is the document the council will actually read.

PlanScope drafts the Geotechnical Assessment and the Phase 1 and Phase 2 land contamination narratives from your site information, and inserts a clear flag where a site-specific figure has not been supplied. An unexploded ordnance risk assessment is not one of PlanScope's report types, so commission that separately from a specialist. 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 land contamination reporting expects a suitably qualified and experienced author working to BS 10175.

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

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