Archaeology and foundation design: preservation in situ

Where archaeological remains matter enough to justify keeping them, the mitigation is usually a foundation design rather than an excavation, and the decision is made by the piling layout and the slab build-up rather than by the heritage statement. Preservation in situ means leaving deposits undisturbed in the ground, in a burial environment that continues to preserve them. It is normally less expensive than full excavation, and it is what the local authority's archaeological adviser will ask for first on a site of real significance.

The catch is that preservation in situ is a construction commitment, not a planning position. It binds the substructure design, the ground investigation, the dewatering strategy and the monitoring regime, and none of that can be resolved after the foundations are priced.

Preservation in situ as a mitigation strategy

National policy expects significance to be understood before an application is determined, and it expects harm to designated and non-designated assets to be avoided or minimised. Where remains are of high significance, avoiding harm means not digging through them.

The alternative, preservation by record, means excavating and recording the remains before they are destroyed. It is a legitimate outcome, and on many sites it is the right one, but it is slower on site and the post-excavation analysis, publication and archive costs continue long after the building is finished.

In situ preservation also has an obligation attached that people miss: the deposits have to remain in a condition that preserves them. Leaving them physically undisturbed while drying them out through dewatering is not preservation, and for waterlogged organic remains it is close to destruction.

An Archaeological Desk-Based Assessment establishes what might be there and at what depth, and an Archaeological Field Evaluation establishes what actually is. You need both before the substructure design is fixed.

How foundation choice affects deposits

The engineering question is how much of the archaeological horizon is removed, disturbed or compressed, and over what plan area.

A traditional strip foundation for a low-rise building removes a continuous ribbon of ground at exactly the depth where remains commonly survive, which is why it is frequently the worst option on an archaeologically sensitive site. A raft spreads load across a wide area with a shallower excavation. Piles remove or displace a small percentage of the plan area but reach far deeper, and pile caps and ground beams then become the main disturbance.

Compression matters as well as removal. Loading a deposit, or driving a displacement pile beside it, compresses and deforms stratigraphy even where nothing is excavated, and that damage is not recoverable.

Set the design against the evaluation results. Where remains lie at 0.8 m and the proposal is a raft at 0.5 m over a granular capping layer, you may have a solution. Where the same remains sit under a basement, you do not, and the honest answer is a full excavation strategy with the cost and programme that go with it.

Piling layouts that reduce disturbance

Piling is the usual answer on constrained urban sites, and the archaeological objective is to minimise the proportion of the deposit disturbed while avoiding the drilling of drainage paths through a sealing layer.

The levers are:

• Fewer, larger piles rather than many small ones, which reduces the number of penetrations even if each is wider
• Pile positions coordinated with the evaluation results so they fall in areas already disturbed by modern services or earlier basements
• Bored and cased piling in preference to driven displacement piling where compression of surrounding deposits is the concern
• Avoiding pile caps and ground beams within the sensitive horizon by raising them into made ground above it
• Agreeing a maximum permitted percentage of the archaeological horizon that may be disturbed, agreed with the curator and reported against

Historic England has published guidance specifically on piling on archaeological sites, and it is the reference the curator will use. Read it before the structural engineer commits to a solution.

Rafts, ground beams and void formers

Above the piles, the slab build-up decides whether the sensitive layer is reached at all. A suspended slab with void formers over a granular blinding raises the construction above the horizon and leaves a monitored void. A raft on a capping layer spreads the load and can avoid excavation entirely where levels allow.

Levels are the constraint. Raising the structure to protect deposits raises the finished floor level, which affects access thresholds, street frontage relationships and sometimes the overall height. Resolve that at design stage with the architect, because a level change agreed with the curator in month eight is a redesign, not a detail.

Services are the quiet problem. Drainage runs, attenuation tanks, substation bases, lift pits and tree pits all dig deeper than the slab, and a scheme that protects deposits under the building while trenching through them in the landscape has not protected anything.

Agreeing the approach with the curator

The local authority's archaeological adviser, or the county archaeologist where the authority buys the service in, is the person whose advice the council will follow. Engage early, share the evaluation results, and present the substructure strategy as a drawing with the archaeological horizon shown on section.

What the curator needs to see is a level, not an intention. A section drawing showing the top of the archaeological horizon, the underside of the slab, the capping layer and the pile positions answers the question in one sheet. A written commitment to minimise disturbance does not.

Expect the approach to be secured by condition, usually requiring a written scheme of investigation covering the mitigation strategy, monitoring during construction and the reporting arrangements. Where the remains are of national importance and scheduled, a separate consent regime applies under the Ancient Monuments and Archaeological Areas Act 1979.

Monitoring during construction

Two kinds of monitoring apply. Archaeological monitoring, meaning a watching brief during ground works so that anything encountered is recorded properly. And deposit monitoring, meaning instrumentation to show the burial environment has not changed.

Deposit monitoring typically uses piezometers to track groundwater levels, sometimes with redox measurements, installed before construction to establish a baseline and read through the works and for an agreed period afterwards. Where dewatering is proposed, that baseline is what proves whether the surrounding deposits were affected.

Write the trigger levels and the response into the scheme. A monitoring regime that records a falling water table without requiring anything to happen is a record of harm, not a mitigation.

Cost comparison against excavation

Preservation in situ usually costs less overall than open-area excavation on a complex urban site, but the savings are unevenly distributed. You pay more for the substructure, the ground investigation and the monitoring. You avoid the excavation, the post-excavation analysis, the specialist reporting, the publication and the archive deposition.

The programme difference is often larger than the cost difference. An excavation sits on the critical path in a way that a piling layout does not.

Price both, with the curator's input on the likely scope of each, before you decide. And build the Geotechnical Assessment and the archaeological evaluation into the same site investigation where you can, since both want trial pits and borehole logs.

PlanScope drafts archaeological desk-based assessments and heritage statements structured to UK planning policy, and a qualified archaeologist should review before submission.

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.