Made ground and shallow foundations on a brownfield plot
On a small brownfield plot the depth and variability of the fill decide the foundation. Why differential settlement is the design driver, and how to brief one investigation for both ground and contamination.
Made ground changes a foundation design from a standard detail into a decision, and on a small brownfield plot it usually means one of four answers: dig through to natural ground, pile, re-engineer the fill, or design a raft that tolerates what is there. The choice turns on how deep the made ground goes and how variable it is across the plot, and you cannot know either without trial pits. Budget for the investigation before you budget for the foundations.
What made ground does to a foundation design
Made ground is anything placed by human activity: demolition rubble, ash and clinker, industrial slag, quarry waste, old topsoil strip, domestic refuse. On an urban infill plot it is the normal condition rather than the exception.
Four properties cause the trouble.
Bearing capacity is low and, worse, unpredictable. A trial pit that finds firm crushed brick tells you nothing about the pit three metres away that finds ash. Voids exist, particularly in coarse demolition fill tipped rather than compacted, and they collapse when they get wet.
Degradation continues. Fill containing timber, paper, plaster or organic refuse loses volume over decades, and a house founded on it settles for as long as the process runs.
Chemical aggressiveness matters to the concrete. Sulfates in ash, clinker and gypsum-bearing demolition waste attack buried concrete, and the specification has to follow the design sulfate and aggressive chemical environment classification for the ground rather than a default mix.
And gas. Fill with biodegradable content generates methane and carbon dioxide, which is a gas protection question rather than a foundation one, though the two are investigated together.
Identifying its depth and variability
Trial pits are the right tool on a small plot, and more of them is better than deeper ones.
A machine-dug trial pit shows you the fabric of the fill, the interfaces, the presence of obstructions and the depth to natural ground, and it lets the engineer see the material rather than infer it from a sample. On a single-plot site, four or five pits spread across the footprint and just outside it tell you far more than one borehole in the middle.
Add hand or window sampling where access is tight, and dynamic probing to establish depth to competent ground between pits cheaply. Where the fill is deep or the site history suggests a filled pond, pit or quarry, boreholes become necessary and the investigation is designed to BS 5930.
Three records matter more than the rest:
- A plot plan with pit positions and the depth to natural ground at each, because that plan is what the foundation option is chosen from.
- Photographs of each pit face with a scale, since the fabric of the fill is evidence a later reviewer cannot recover.
- Groundwater observations, including standing water and the depth at which it entered, because water changes both bearing capacity and buildability.
Read the desk study first. Historical maps, the geological record and the site's own planning history frequently identify the fill source, and knowing that you are looking at a backfilled clay pit rather than levelling spoil changes where the pits go. A Geotechnical Assessment should set out that history alongside the intrusive results.
Bearing capacity and differential settlement
The design driver on made ground is almost never absolute settlement. It is differential settlement across the building.
A house that settles 20 mm uniformly is undamaged. A house where one corner settles 20 mm and the opposite corner settles 5 mm cracks, because the distortion is what the masonry cannot take. Serviceability is therefore assessed as angular distortion across the structure rather than as a single settlement figure, and typical limits for masonry construction are tight.
That is why variability matters more than the average. A plot with two metres of uniform, well-compacted granular fill is far easier to build on than a plot with half a metre of fill on one side and two and a half on the other, even though the second plot has less fill overall.
Where a footing would bear partly on natural ground and partly on fill, you have designed differential settlement into the building. Avoid it. Either take everything down to natural ground or take nothing down to it and design for the fill.
Ground improvement against deeper foundations
Four options, and the plot size usually decides between them.
| Option | Works best when | Watch for |
|---|---|---|
| Deep trench fill to natural ground | made ground under about 2.5 m, uniform | cost and safety rise steeply with depth, shoring needed |
| Mini piles with ground beams | fill deeper than about 2 m | rig mobilisation cost on a single plot, access width |
| Dig out and re-engineer the fill | shallow fill, space to store and process | testing and compaction records needed for warranty |
| Reinforced raft | shallow, reasonably uniform fill | designed for the ground, not a standard detail |
Ground improvement by vibro stone columns or similar treatment works well technically but is often uneconomic for one house, because the mobilisation cost is the same whether you treat one plot or twenty. If you can coordinate with a neighbouring plot or a developer working nearby, the arithmetic changes.
Piling is the default answer for deep fill, and the practical constraints are access width for the rig, headroom, and vibration or noise limits where the plot adjoins housing. Continuous flight auger piles and driven precast piles have very different vibration signatures, and a neighbour dispute over vibration is easier to avoid than to settle.
Whichever route you take, the warranty provider's requirements matter as much as the engineer's. Most require foundations to bear on undisturbed natural ground or on properly engineered fill with compaction testing records, and a raft on untested fill will be queried.
Cost and programme consequences
The honest position is that made ground on a small plot adds cost that is hard to compress, and the worst outcome is discovering it after the slab is priced.
Sequence to protect yourself. Commission the desk study and trial pits before you fix the build contract, get the foundation option chosen and designed before you tender, and price the foundations as a defined element rather than a provisional sum. A provisional sum for foundations on a brownfield plot is an invitation to a claim.
Programme effects are mostly about mobilisation and waiting. A piling rig has a lead time. Disposal of excavated made ground needs classification before it can be moved, and a load that turns out to be hazardous waste costs several times what inert spoil costs. Get the waste classification done from the same samples as the geotechnical testing.
Overlap with contamination investigation
This is where a self-builder saves real money by planning once.
The same trial pits, the same samples and the same site visit serve both the geotechnical and the geoenvironmental assessment. Commissioning them separately means paying twice for the plant, the mobilisation and the reinstatement, and it frequently produces two reports that disagree about the ground model.
Brief the investigation to cover both from the start: bearing capacity and depth to natural ground for the foundations, and contaminant testing, ground gas monitoring and sulfate analysis for the Land Contamination assessment. A licensed and suitably qualified specialist is legally the appropriate author for contaminated land work, which the local planning authority will expect to be carried out in accordance with BS 10175 by a competent person, and the PlanScope draft is a starting point for that professional rather than a substitute.
Ground gas protection design follows from the monitoring results and the characteristic situation assigned to the site. It cannot be designed in advance of the monitoring, and monitoring needs several visits across varying atmospheric pressure, which is a programme item of weeks rather than days.
Presenting the conclusion in a submission
The planning application for a single dwelling rarely needs the foundation design, and volunteering it invites conditions you do not want.
What the council usually does need is the desk study conclusion and, where the desk study identifies a potential risk, a commitment to intrusive investigation. Most authorities then condition a phased approach: investigation and risk assessment, a remediation strategy if needed, implementation, and a verification report before occupation. Read the validation list, because some authorities require the Phase 1 desk study at application stage on any brownfield site.
Where an existing building is being retained or partly reused, a Structural Survey should address the existing foundations as well as the superstructure, since old footings on made ground are frequently shallower than assumed. Works taking any part of the building below existing floor level raise the same issues as a basement, which our note on cellar conversion or new basement excavation covers.
State the ground conditions plainly in the submission and let the conditions handle the detail. An application that claims the ground is suitable without investigation is the one that attracts the most onerous condition wording.
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 foundation design and contaminated land assessment both need suitably qualified and experienced authors.
Enter your postcode on PlanScope to see which ground and contamination reports your plot is likely to need.
Frequently asked questions
How do I find out how deep the made ground goes?
Machine-dug trial pits are the right tool on a small plot, and four or five spread across the footprint tell you more than one deep borehole in the middle. Add dynamic probing between pits to establish depth to competent ground, and record the depth to natural ground at each position on a plot plan with photographs of every pit face.
Why does variability matter more than depth?
Because differential settlement, not absolute settlement, cracks buildings. A house settling 20 mm uniformly is undamaged, while one corner settling 20 mm against 5 mm at the opposite corner distorts the masonry. Two metres of uniform compacted fill is easier to build on than a plot varying from half a metre to two and a half.
Is ground improvement worth it on a single plot?
Usually not. Vibro stone columns and similar treatments work technically, but the mobilisation cost is much the same whether one plot or twenty is treated. The arithmetic improves if you can coordinate with a neighbouring plot or a developer working nearby. Otherwise mini piles with ground beams, or deep trench fill, tend to be more economic.
Can one investigation cover both foundations and contamination?
Yes, and it should. The same trial pits, samples and site visit serve both, so briefing them separately means paying twice for plant and mobilisation and often producing two reports with conflicting ground models. Ask for bearing capacity and depth to natural ground alongside contaminant testing, ground gas monitoring and sulfate analysis.
What does the planning application need to say about ground conditions?
Usually the desk study conclusion and, where a potential risk is identified, a commitment to intrusive investigation. Most authorities then condition a phased approach of investigation, remediation strategy, implementation and verification before occupation. Check the validation list, because some require the Phase 1 desk study at application stage on any brownfield site.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.