Archaeology and Foundation Design in York

Why York is different

Two thousand years of continuous occupation on the same spot have left deposits several metres deep in places, and the city centre sits on the York Moraine - the ridge of glacial material that made a crossing point here and gave the Romans somewhere to put a fortress.

What makes York exceptional is not just the depth of the archaeology but its condition. Waterlogged, oxygen-poor ground beneath parts of the city has preserved organic material - timber, leather, textile - that would have rotted away almost anywhere else. That is why excavations here have produced what they have, and it is why the deposits are treated as a resource to be conserved rather than simply recorded.

It also creates a specific engineering sensitivity: anything that dries those deposits out damages them. Lowering groundwater on a site can therefore be an archaeological problem as much as a construction one.

Preservation in situ, and what it does to a foundation

Where deposits are to be preserved in situ, the design objective changes from 'find a bearing stratum' to 'reach a bearing stratum while disturbing as little as possible'. That pushes foundation design in specific directions:

• Minimising the excavation footprint rather than the excavation depth - a small number of small-footprint elements can disturb far less than a continuous strip
• Piling in preference to mass excavation in some situations, because a pile disturbs a defined and limited plan area
• Reducing pile numbers and diameters where the structural design allows, since the metric that matters archaeologically is plan area disturbed
• Rafts and ground-bearing solutions where deposits are shallow enough to be spanned over rather than dug through
• Avoiding permanent groundwater lowering, because drying waterlogged deposits destroys exactly what preservation was meant to protect
• Designing service runs and drainage with the same discipline - a drain trench is an excavation like any other, and it is the element most often forgotten

What this does to a programme

More than it does to the design, usually, and this is the part that costs money when it is discovered late.

Ground disturbance in and around the historic core can attract archaeological conditions: an evaluation before work, a watching brief during it, or full excavation where the deposits warrant it. Each has a duration, each needs a specialist contractor, and none of them can be compressed by wanting them to be.

The workable approach is to establish the archaeological position at the start of a project, alongside the flood zone and the conservation position, and to build the time into the programme. A scheme that has been designed with the constraint is rarely in trouble. A scheme that meets it after planning permission, with a builder booked, frequently is.

The other two constraints that arrive with it

Archaeology rarely turns up alone in York. Most sites in and around the core carry at least one of the others.

Flood risk: substantial parts of the city fall within Flood Zones 2 and 3 along the Ouse and the Foss, and an assessment is routinely required. That drives floor levels, which interacts with everything above.

Conservation and listing: York has one of the largest central conservation areas in the country and a very high concentration of listed buildings. That governs the acceptable solutions rather than the structural standard - the Building Regulations apply the same way, and Building Control wants the same calculations, but the answer has to be one a conservation officer will accept.

Line all three up at the start and a York project runs like any other. Meet them one at a time as each becomes unavoidable and the design gets revisited three times.

Underpinning, basements and the historic core

Basement work and underpinning in the centre engage the archaeological question immediately, because the excavation is the whole point of the job rather than a means to an end.

That does not make it impossible, and it is done. It makes it a project where the archaeological consent, the structural design and the groundwater strategy have to be developed together, and where the sequence of work matters as much as the design of it. It is also a situation where the honest advice is sometimes that the space is not worth what it will cost to get.

Where we fit

We are structural and civil engineers, not archaeologists, and we say so. What we do is design foundations that respond to the archaeological brief - and, just as importantly, tell you early that there will be one.

On an existing building the same city produces its own workload: shallow Victorian footings in the terraces of Holgate, South Bank and Bishophill, timber-framed buildings in the core that have leaned for centuries and stopped, and made ground that changes bearing capacity noticeably over a short distance. A structural survey and report is a fixed £580 + VAT with the report inside 10 working days of the inspection, guaranteed.