Archaeology on a brownfield site with made ground

Brownfield does not mean no archaeology. It means the archaeology is buried under made ground, partly destroyed by whatever stood on the site last, and harder to predict than on a greenfield site. On an urban site with 2 m of nineteenth century made ground over river alluvium, the deposits beneath the Victorian cellar floors can be better preserved than anything in a ploughed field.

The commercial question is therefore not whether archaeology is present. It is how much of it survives, at what depth, and whether your foundations will reach it.

Why brownfield does not mean no archaeology

Three mechanisms work in favour of survival on previously developed land.

Made ground raises the modern surface above the archaeological horizon, so later activity happens higher up. A yard surface laid in 1870 protects everything below it from twentieth century services and landscaping.

Buildings seal deposits rather than destroying them uniformly. A floor slab, a cellar floor or a cobbled yard caps the surface it was laid on, and the backfill of a demolished building often contains the material that the demolition removed from elsewhere.

Waterlogging preserves organic remains. Urban sites on floodplains and former watercourses hold waterlogged timber, leather and environmental evidence that dries out and disappears on free draining ground.

Against that, nineteenth and twentieth century basements, deep foundations, service trenches and wartime damage remove archaeology outright. The pattern is patchy rather than uniform, which is exactly why a desk study alone cannot answer the question.

Truncation, survival and deposit modelling

Deposit modelling is the technique that turns a vague risk into a mapped one. A geoarchaeologist takes the available subsurface records, logs them to a consistent scheme, and produces a model of the deposit sequence across the site: made ground thickness, the level of the first undisturbed horizon, the presence of alluvium or peat, and the depth to natural geology.

The inputs usually already exist.

• Geotechnical borehole and trial pit logs from any previous ground investigation on or near the site
• Records held by the national geological survey for boreholes in the vicinity
• Contamination investigation logs, which often record made ground depth in detail
• Historic mapping showing building footprints, basements, yards and watercourses by date
• Records of archaeological work on adjacent sites, held in the historic environment record
• Wartime bomb damage mapping in cities that have it

The output is a set of levels you can compare against your proposed foundation design. That comparison is the whole point. A scheme with a piled raft at 1.5 m below existing ground level, on a site where the first archaeological horizon sits at 2.8 m, has a very different mitigation problem from one with a two storey basement.

Made ground depth and what sits beneath

Made ground varies within a single site more than most developers expect. A 40 m frontage can have 0.8 m at one end and 3.5 m at the other, where a former dock, pond or quarry was infilled.

Three questions to put to the geoarchaeologist.

What is the depth and character of the made ground, and does it contain redeposited archaeological material? Redeposited finds are not in situ archaeology, and they are frequently misreported as though they were.

What is the first horizon of archaeological interest, and is it a surface, a cut feature level or a palaeoenvironmental deposit? These need different mitigation.

Where has truncation removed the sequence entirely? A mapped truncation footprint is the most valuable single output of the exercise, because it is where you can build without archaeological constraint.

The findings belong in an Archaeological Desk-Based Assessment with the deposit model as an appendix, and councils expect the work to follow the professional standards published by the Chartered Institute for Archaeologists.

Combining contamination and archaeology fieldwork

The efficiency that most schemes miss. A single ground investigation can serve both purposes if it is designed that way from the start.

How it works in practice:

1. Agree the investigation layout jointly between the geotechnical engineer, the contamination consultant and the archaeologist, so that hole positions answer all three questions.
2. Have a geoarchaeologist present to log the cores or window samples as they come up, or arrange for cores to be retained and logged later.
3. Record the made ground and underlying deposits to an archaeological as well as a geotechnical scheme, which costs almost nothing extra at the time and a great deal to repeat.
4. Use the results to target any trial trenching rather than spreading trenches evenly across the site.

Two cautions. Contamination restricts archaeological excavation, because working in made ground with hydrocarbons or asbestos requires controls that slow fieldwork and raise cost, so the contamination findings need to reach the archaeologist before the evaluation is priced. And a watching brief during contamination work is not an archaeological evaluation, a distinction we set out in an archaeological watching brief or an evaluation.

Basements and historic cellars

Cellars are the most misread feature on urban brownfield sites.

A Victorian cellar removed the archaeology within its footprint, down to its floor. Below that floor, however, deposits can survive intact, and the cellar floor itself seals them. A site that reads as fully truncated on a historic map may retain a sequence at 3 m that nobody has reached.

Cellar backfill is also informative. Demolition rubble tipped into a cellar in the 1960s contains the building that stood there, and recording it can satisfy part of the mitigation requirement for the lost structure.

Where your scheme includes a basement, the position reverses. A new basement across the site removes everything, which means the mitigation is preservation by record through excavation, with the cost and programme that implies. Design the basement footprint with the deposit model in front of you, because pulling a basement back from the most sensitive area occasionally removes the need for full excavation altogether.

Human remains add a further layer where a burial ground, a plague pit or a churchyard is indicated, and the licensing and procedural requirements are strict, as covered in human remains on site, licence and procedure.

Evidence that reduces risk

What actually shifts the position with a county archaeologist or a city archaeologist:

• A deposit model, with the borehole logs it was built from
• A foundation design drawing showing the depth of every intrusion, overlaid on the model
• A truncation footprint plan, mapped rather than described
• A record of previous archaeological work within a defined radius, with the results summarised
• A statement of what has been considered and excluded, with reasons

What does not shift it: an assertion that the site is brownfield, a general statement that archaeology is unlikely, and a desk study that lists the historic environment record entries without analysing them.

Where the scheme also involves demolition in a conservation area, the consent position for the demolition itself runs alongside, as set out in conservation area demolition and relevant demolition.

Sequencing investigation efficiently

The order that keeps the programme intact.

1. Desk-based assessment and deposit model, before the layout is fixed. Weeks rather than months, and it can be done at any time of year.
2. Pre-application discussion with the archaeological advisor to the authority, with the model in hand. Agree the evaluation scope in principle at this stage.
3. Targeted evaluation, usually trial trenching, designed against the model and combined with the ground investigation where possible.
4. Mitigation strategy, choosing between preservation in situ through foundation design and preservation by record through excavation.
5. Application, with a written scheme of investigation offered in draft so that the condition can be drafted around it.
6. Discharge of condition, fieldwork, and the post excavation programme, which is the stage most often underfunded.

Post excavation deserves a final word. Analysis, specialist reporting, archive deposition and publication can take longer than the fieldwork and are usually secured by the same condition. Budget for it at the outset, and agree the archive deposition arrangements early, because museums with no space to accept an archive have delayed more than one scheme at the final discharge.

Enter your postcode on PlanScope to see which heritage and archaeology documents your site is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Councils usually expect archaeological work to be carried out by a suitably qualified author, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.