Preservation in situ and foundation design that works

Preservation in situ archaeology foundations work is an engineering problem dressed as a heritage one. The planning outcome you are aiming for is that the buried remains stay where they are, undamaged, under your building, and that the curatorial adviser accepts your design as achieving it. That is a different exercise from excavating the site and recording what is there before it goes. It usually costs less, it is almost always slower to design, and it fails most often because the drainage was drawn after the foundations.

What preservation in situ means in planning terms

National policy treats archaeological remains as a heritage asset whose significance can be harmed by development, and it distinguishes between remains of the highest importance and everything else. For remains of national importance, whether scheduled or demonstrably of equivalent significance, the expectation is preservation in situ. Works affecting a scheduled monument need separate scheduled monument consent under the Ancient Monuments and Archaeological Areas Act 1979, which is not part of the planning permission.

Below that threshold, the decision is a balance. Where harm is justified, national policy expects the loss of significance to be offset by recording and by making the evidence publicly accessible, which is the route through excavation, post-excavation assessment, archive and publication. Where the remains are important enough that harm is hard to justify, the argument shifts to whether harm can be avoided instead, and that is where foundation design earns its place in the planning file.

Be honest with the client about the framing. Preservation in situ is not a way of avoiding archaeological cost. It moves the cost from excavation into structural design, monitoring and programme certainty, and on a site with deep stratified deposits that trade is often worth making.

Deposit modelling before layout

You cannot design around remains whose height and extent you do not know. The deposit model is the document everything else depends on, and it should exist before the building is positioned, not afterwards.

A usable model draws on the Historic Environment Record, previous interventions on and adjacent to the site, borehole and trial pit logs from your own geotechnical work, British Geological Survey records, and where the deposits are alluvial or peat based, dedicated geoarchaeological boreholes with recorded sequences. What comes out of it is a surface: the height of the archaeological horizon across the site in metres above Ordnance Datum, with the thickness of made ground above it and a confidence statement for each area.

Two numbers govern the design. The depth from finished level to the top of archaeology tells you how much construction can happen without touching anything. The variation in that depth across the footprint tells you where the difficult corner is. On urban sites the horizon frequently rises towards one boundary because of historic terracing, and the whole scheme can hinge on a two-metre strip.

Commission the archaeological desk-based assessment early enough to inform the model. Councils expect a suitably qualified author working to the Chartered Institute for Archaeologists standards, and professional review of this work is strongly advised.

Piling layouts and quantified deposit loss

Piling is the standard answer, and it is not automatically a benign one. Every pile removes or displaces the deposits within its shaft, and the design question is how much loss the adviser will accept.

Advisers ask for volumetric deposit loss to be quantified as a proportion of the archaeological horizon within the footprint, and the acceptable figure is agreed case by case rather than set nationally. Expect to present a plan with the pile positions overlaid on the deposit model and a calculation of area and volume affected.

Design moves that reduce loss:

• Fewer, larger piles rather than many small ones, because the total disturbed plan area for a given capacity usually falls.
• Pile positions coordinated with the known extent of features, so that piles fall in areas already disturbed by modern intrusions such as former basements, wells or service trenches.
• Sleeved or cased piles in soft deposits, which reduce lateral displacement and soil mixing compared with unsupported augering.
• Avoiding driven piles where vibration and displacement would disturb a wider zone than the shaft itself.

The piling platform is the part that gets missed. A granular working platform placed and compacted over the site loads the deposits beneath it, and on compressible organic deposits that loading causes settlement and crushing. Design the platform as a structural element, with a geotextile separator, a stated thickness and a limit on plant loading, and include it in the mitigation you present.

Rafts, ground beams and made ground

A raft spreads load without deep excavation, which sounds ideal until you consider the formation level. Forming a level raft base on a sloping site means cutting, and cutting is exactly what you are trying to avoid.

Rafts work where there is enough made ground above the archaeological horizon to accommodate the slab, the build-up and the drainage falls without reaching it. On many urban sites there is: a metre or more of modern levelling material is common. Where the made ground is thin, a ground beam and pile solution spanning between limited bearing points usually causes less loss than a raft that requires a cut formation.

Two details decide whether a raft actually delivers preservation. The first is whether ground improvement is proposed beneath it, because vibro stone columns or dynamic compaction will destroy far more of the deposits than piles do. The second is the edge condition, since perimeter thickenings and toe beams are effectively strip foundations and they tend to sit exactly where wall lines of earlier buildings survive best.

Services and drainage, where the loss happens

On most schemes the foundation design is careful and the services are not. A 1.2 metre deep gravity drain run across a site cuts a continuous slot through every deposit it crosses, and a cluster of manholes removes more volume than a dozen piles.

Treat drainage and services as part of the archaeological mitigation from the first coordination meeting:

1. Route all services in a single shared corridor, positioned over a low-significance or already-disturbed area.
2. Keep gravity drainage within the made ground by pumping where necessary, accepting the maintenance consequence.
3. Avoid soakaways, infiltration trenches and deep attenuation tanks over the archaeological horizon entirely.
4. Raise finished floor level where the scheme can absorb it, since 300 mm of extra build-up can lift every service run clear.
5. Lay shallow services within the internal build-up rather than below slab.
6. Check the incoming utility connections, which are outside your control and frequently outside the mitigation strategy.

An illustrative example. A four-storey building over Roman and medieval deposits at 1.8 metres below existing ground level, with 1.5 metres of Victorian made ground above. Foundations were 600 mm diameter cased piles on a grid coordinated with a former basement, with a suspended slab and ground beams within the made ground. Foul drainage was pumped from a shallow collection chamber rather than run to a deep gravity connection, and the surface water attenuation was placed in a shallow geocellular tank inside the made ground. Deposit loss was calculated, tabulated and agreed before the application was submitted.

Groundwater, and the slower harm

Waterlogged deposits survive because they are waterlogged. Organic remains such as timber, leather and environmental samples degrade once oxygen reaches them, so anything that lowers the water table or introduces a preferential pathway causes harm that no site inspection will detect for years.

That brings three things into scope. Temporary dewatering during construction, which can draw down levels across a wider area than the excavation. Permanent cut-off walls and basements, which alter groundwater flow and can leave one side dry. And piles themselves, which can connect aquifers through an impermeable layer.

Where waterlogged deposits are present, expect the adviser to ask for a hydrogeological assessment, baseline groundwater monitoring before construction, and a monitoring regime continuing afterwards. Historic England has published research and advice on preserving archaeological remains that sets out what that monitoring should measure. This is the part of the strategy most likely to be resisted on cost and most likely to be the reason a scheme is later said to have failed.

Agreeing the approach

The curatorial adviser is usually the county or borough archaeologist, with the Historic England advisory service taking that role for parts of London. Engage before the layout is fixed, and bring the deposit model rather than the drawings.

What tends to secure agreement is a written scheme of investigation covering the evaluation, a foundation design statement with the deposit loss quantified, a services and drainage strategy, a monitoring proposal where hydrology matters, and a construction sequence with named hold points for archaeological attendance. Expect the permission to carry pre-commencement conditions requiring approval of those documents, and expect the foundation design to be one of them.

Where preservation in situ cannot be achieved, the argument moves to justification, and the harm tests set out in substantial harm and the justification required for it become the centre of the case. Either way, the heritage statement has to explain the design decisions rather than describe the site.

Enter your postcode on PlanScope to see which reports your site is likely to need. 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.