Foundations and embodied carbon on a difficult site
On poor ground the substructure can carry as much upfront carbon as the frame. How to compare piling, rafts, stone columns and foundation reuse, and what to submit.
On soft alluvium, made ground or a former industrial site, the substructure can carry as much upfront carbon as the frame above it, and the decision is usually taken by a structural engineer months before anyone writes the sustainability chapter. Substructure embodied carbon is decided at options appraisal, not at planning submission. By the time a whole life carbon assessment is being prepared, the piling solution has normally been priced and the alternatives have gone.
Why substructure dominates early carbon on poor ground
Foundations are concrete and steel in bulk, placed below ground where nothing can be reused or demounted later. On a good site with shallow strip or trench fill footings, the substructure is a modest share of the building's upfront carbon. On a site needing 20 metre bored piles, a piled raft and a gas resistant slab, it can rival the superstructure.
Three things drive that. Concrete volume, because cement clinker is the carbon. Reinforcement, because steel content rises steeply with pile length and raft thickness. And spoil, because arisings from bored piling and basement excavation leave site by lorry and, if contaminated, go to landfill with a disposal and haulage burden attached.
The RICS whole life carbon methodology, now in its second edition, covers this under the product and construction modules A1 to A5. Those modules are where substructure choices land, and they are the modules a local planning authority looks at first because they are the ones the applicant still controls.
Comparing piling, rafts and ground improvement
There is no ranking that holds across sites. What holds is a way of comparing.
| Option | Carbon driver | Where it tends to win |
|---|---|---|
| Trench fill or strip footings | Concrete volume | Competent ground within about 2 metres |
| Raft or piled raft | Slab thickness and reinforcement | Variable ground, light structures, shallow services |
| Driven precast piles | Factory concrete, no arisings | Granular ground with no obstructions |
| Continuous flight auger piles | Concrete volume and spoil | Cohesive ground, urban sites, vibration limits |
| Rotary bored piles | Length, reinforcement, spoil | Deep founding stratum, heavy loads |
| Vibro stone columns | Aggregate haulage | Granular fill, low rise, moderate settlement tolerance |
| Cement or lime stabilisation | Binder content | Large areas of soft ground, platform construction |
Driven piles avoid arisings altogether, which matters most where the spoil would be contaminated. Stone columns move the carbon from cement to aggregate transport, so the haulage distance becomes the variable that decides them. Stabilisation looks attractive until you count the binder, because lime and cement added across a site by percentage of dry mass adds up faster than engineers expect.
The honest position is that the comparison depends on the founding depth, the loads and the haulage distances for your site, so it has to be run rather than assumed.
Concrete mix design and cement replacement
The lowest cost carbon saving in the substructure is usually the mix, not the geometry.
Replacing a proportion of Portland cement with ground granulated blast furnace slag or fly ash cuts the embodied carbon of the concrete substantially, and BS 8500 and BS EN 206 accommodate those combinations in designated and designed concretes. Below ground, the trade off is slower early strength gain, which affects the construction programme rather than the finished structure. In piles and rafts, where formwork is not being struck and reused on a cycle, that penalty is often acceptable.
Two cautions. Availability of slag is constrained and varies by region, so a specification written around a high replacement level needs confirmation from the local supplier before it goes into a planning document. And durability requirements for aggressive ground conditions, set by the design chemical class from the ground investigation, limit which mixes are permitted.
Specify a carbon figure rather than a mix. Setting a maximum embodied carbon per cubic metre for each concrete grade, verified by the supplier's environmental product declarations, leaves the contractor free to meet it with whatever is available.
Reusing what is already in the ground
On a redevelopment site with existing foundations, reuse is the largest single saving available and the one most often dismissed in a paragraph.
It needs evidence: record drawings or intrusive investigation to establish geometry and depth, integrity testing, and an assessment of capacity against the new loads with a view on residual life. Where existing piles can carry part of the new structure, or where an existing basement box can be retained and extended, the avoided concrete is measured in hundreds of cubic metres rather than percentages.
The obstacles are practical. Records are missing, the new grid does not match the old one, and the risk sits with a structural engineer whose professional indemnity position makes assumption the easier path. Where reuse is possible, it usually needs to be identified at the feasibility stage so the new grid can be set out to suit, which again is before the planning documents are drafted. Our note on carbon in the infrastructure a scheme has to build covers the same point for roads and drainage.
Ground investigation as a carbon decision
A thin site investigation produces conservative foundations. That is the mechanism by which saving money on boreholes costs carbon.
Where the founding stratum depth is uncertain, piles get designed long. Where contamination extent is unclear, the remediation strategy defaults to excavate and remove. Where groundwater is poorly characterised, the basement gets a thicker slab. Each of those is a rational engineering response to uncertainty and each adds concrete.
Additional boreholes, in situ testing and a proper chemical suite cost little against the concrete they can remove from the design. Say that in the options appraisal, because it is the argument that persuades a client to fund the second phase of investigation.
Presenting the comparison to the authority
Authorities with whole life carbon policies, including the Greater London Authority under London Plan policy SI 2, expect an assessment at application stage and an updated one after construction. What they want from the substructure section is short and specific:
- The options considered, with the founding depth and load assumptions behind each.
- The carbon figure for each option, with the boundary stated, so A1 to A5 rather than a bare number.
- The reason the chosen option was selected, including where cost or programme decided it.
- The mix specification, expressed as a carbon limit per grade.
- The reuse position, with the evidence relied on or the reason reuse was ruled out.
Write it as an appraisal rather than a justification. An officer reading a section that considers only the chosen solution learns that no comparison was made. The figures themselves belong in a Whole Life Carbon Assessment, with the narrative in the supporting sustainability documents, and off site manufacture changes the arithmetic in ways covered in our piece on modern methods of construction and the carbon case.
Where cost and carbon pull apart
Most substructure carbon savings also save money, because concrete costs money. Three do not.
Cement replacement can cost more where the supply chain is tight, and the programme cost of slower strength gain is real on a tight site. Reuse investigation costs money upfront to avoid concrete later, and the money is spent before the saving is certain. And a longer ground investigation delays the start of design.
Being open about that in the appraisal is better than pretending every choice was free. An officer who sees a scheme adopt two of the three, and explain why the third was not viable on this site, has a defensible record for the committee report. A scheme claiming it optimised everything invites a harder question.
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 substructure carbon figures need a structural engineer and a carbon assessor. This is general guidance and not legal advice, so check the position with your local planning authority or a qualified professional before relying on it.
Browse the 66 PlanScope report types to see which carbon and sustainability documents a scheme of this scale usually needs.
Frequently asked questions
Which modules does substructure carbon fall into?
Modules A1 to A5 of the RICS whole life carbon methodology: product stage, transport to site and the construction process, including spoil removal. Those are the modules an applicant still controls at design stage, which is why authorities look at them first. State the boundary whenever you quote a figure so the comparison is legible.
Is driven piling lower carbon than bored piling?
Often, because driven precast piles produce no arisings and use factory cast concrete with controlled mixes. That advantage grows where the spoil from bored piling would be contaminated and need landfill disposal. Driven piling needs granular ground free of obstructions and generates vibration, so it is not available on every urban site.
How much cement can be replaced in foundation concrete?
BS 8500 and BS EN 206 accommodate ground granulated blast furnace slag and fly ash combinations, and below ground work tolerates slower early strength gain better than a formwork cycle above ground. The practical limit is set by the design chemical class from the ground investigation and by regional supply, so confirm with the supplier.
Can existing foundations be reused on a redevelopment?
Yes, where geometry and capacity can be established by record drawings or intrusive investigation, integrity testing and a structural assessment against the new loads. Reuse has to be identified early enough for the new column grid to suit the old one, which usually means at feasibility rather than at planning stage.
Does a fuller ground investigation reduce carbon?
Usually. Uncertainty about founding depth, contamination extent or groundwater pushes engineers towards conservative designs, which means longer piles, thicker slabs and excavate and remove remediation. Additional boreholes and testing cost little against the concrete they allow to be taken out of the design.
What does a council want in the substructure section?
The options considered with their load and depth assumptions, a carbon figure for each with the module boundary stated, the reason the chosen option won including cost or programme grounds, the concrete specification as a carbon limit per grade, and the reuse position with evidence.
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.