Trial trench evaluation: sampling levels and layout

Trial trenching answers one question: is there archaeology here, and if so, what is it, how well preserved is it, and how far does it extend. The county archaeologist sets the sampling level, commonly expressed as a percentage of the developable area, and negotiating that figure down without evidence is the fastest way to end up doing the work twice.

Two per cent, five per cent, ten per cent. Those numbers get quoted as if they were national standards. They are not, and the justification behind the figure matters more than the figure.

What an evaluation is meant to establish

Evaluation sits between the desk-based work and any mitigation. The Archaeological Desk-Based Assessment tells you what is recorded: previous finds, cropmarks, historic maps, the Historic Environment Record, and the archaeological character of the area. Evaluation tests that on the ground.

The output is a statement about presence, absence, date, character, extent, depth, preservation and significance. Those eight things drive every decision that follows, from whether the scheme needs redesigning to what the mitigation condition will say and what it will cost.

An evaluation that reports only presence or absence has not done the job. Depth and preservation matter as much, because features surviving 300 millimetres below a ploughsoil behave very differently from features sealed under a metre of alluvium.

Choosing between geophysics and trenching

Geophysical survey, usually magnetometry on open land, covers large areas quickly and at low cost relative to excavation. It works well on ditched enclosures, kilns and burnt features, and it is close to useless for timber structures, shallow features, burials without grave goods, and anything under made ground or on certain geologies.

Trenching is the definitive test. It is slower, it needs machine time and a supervising archaeologist, and it produces evidence that a curator will accept without further argument.

The usual sequence on a greenfield site is geophysics first, then targeted trenching over the anomalies plus a blank-area sample to test for features geophysics cannot see. Skipping the blank-area sample is a standard shortcut and a standard reason for a curator to ask for more work. On brownfield sites geophysics is often abandoned altogether, because services, made ground and rubble render it unreadable.

Ask the curator which they expect before commissioning either. The written scheme of investigation has to be approved by them in any event, and their view on methodology is the one that counts.

Sampling levels and how they are justified

The sampling level is the proportion of the developable area opened as trenches. It is agreed with the archaeological curator, and there is no national figure.

What moves it up or down:

• the archaeological potential shown in the desk-based assessment, which is the single largest factor
• whether geophysics has already targeted the trenching, which can allow a lower overall figure
• the geology and depth of overburden, since deeper deposits need larger or stepped trenches
• the scale and nature of the impact, with piled foundations treated differently from full excavation
• previous investigation on or adjacent to the site
• whether the site lies within an Archaeological Priority Area or similar local designation

Sampling levels rise sharply once something is found. A first phase at a modest percentage that turns up real features is often followed by a second phase targeted on them, and the budget should carry that contingency from the start.

Trench sizes are conventionally long and narrow, commonly 30 metres by 1.8 metres or 20 metres by 2 metres, because length intercepts linear features and width is governed by the bucket. State the dimensions in the scheme of investigation, because a percentage figure alone allows very different layouts.

Trench layout, access and constraints

Layout should be designed to test, not to avoid. A grid or a staggered array across the developable area is the default, adjusted to target geophysical anomalies and to sample areas the survey could not read.

Real sites impose constraints, and each one needs recording in the report:

1. Live services, which need a utility search and scanning before machines move.
2. Trees, with root protection areas to BS 5837 keeping trenches out of large parts of some sites.
3. Standing buildings and hardstanding, where breaking out adds cost and may need separate permission.
4. Protected species and habitat, since trenching in a bat foraging corridor or a reptile area needs its own method statement.
5. Ground conditions, with waterlogging closing sites in winter and shrinkable clay needing shoring.
6. Access for a tracked excavator and a dumper, which on a landlocked field is not a given.
7. Ownership and tenancy, because a farm tenant has rights and crops have value.

Where trenches have to be moved, say so in the report and explain what that means for confidence. A curator reading a report that quietly omits a third of the site will assume the worst.

Reporting and the significance judgement

The evaluation report sets out the methodology, the trench records, the finds and environmental assessment, and a statement of the significance of what was found. That significance judgement is what feeds into the planning balance under national heritage policy, and it should be expressed in those terms rather than in purely archaeological ones.

Standards matter here. Councils expect the work to follow the Chartered Institute for Archaeologists standards and guidance for field evaluation, and to be carried out by an organisation or individual on the appropriate register. In a World Heritage Site or its buffer zone the bar rises again, as the note on World Heritage Sites and their buffer zones explains.

The report should be deposited with the Historic Environment Record and an appropriate digital archive. That deposition is often a condition, and it is often forgotten until someone tries to discharge it three years later.

Outcomes and what each means for the scheme

Four broad outcomes, with very different consequences.

Nothing found, across an adequate sample. The curator will normally accept that no further work is needed, sometimes with a watching brief on the groundworks as a precaution.

Low significance remains. Mitigation by record, usually a strip, map and sample exercise or a watching brief, secured by condition. Manageable cost, manageable programme.

Significant remains that can be avoided. Preservation in situ becomes the preferred approach, which usually means redesigning layout, foundations or drainage around the area, and securing it through a condition and sometimes a management plan.

Nationally important remains. This is the outcome that changes schemes. Where remains are of schedulable quality, national policy treats the loss as substantial harm to a designated asset in practical terms, and the scheme may have to be reworked from the layout up, or not proceed.

That last possibility is why the timing question matters so much, and the argument on each side is set out in the note on archaeological evaluation before or after determination.

Programming evaluation before determination

Curators increasingly ask for evaluation before determination on sites of any potential, and there is a principled reason for it. A decision maker cannot weigh harm to a heritage asset without knowing whether the asset exists.

Build the programme backwards from the target submission date:

• utility searches and landowner access agreement, two to four weeks
• written scheme of investigation drafted and submitted, one to two weeks
• curator approval of the scheme, two to four weeks and longer in busy authorities
• fieldwork, depending on sample size and machine availability
• finds and environmental assessment, which is where reports usually slip
• reporting and curator review, two to four weeks

Six to twelve weeks is a realistic total for a modest site with no complications, and complications are the norm. Winter fieldwork on clay is a particular risk, and crop cycles restrict access on arable land.

Where a heritage asset is also affected above ground, the evaluation sits alongside the Heritage Statement and the two should describe the same site, with the same significance language.

Enter your postcode on PlanScope to see which heritage and archaeology documents a site is likely to need before you commission fieldwork.

This is general guidance, not legal advice. Archaeological evaluation should be carried out by a suitably qualified organisation working to an approved written scheme of investigation, and councils usually expect a suitably qualified author. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.