Trial trenching strategy in an archaeological evaluation

An archaeological field evaluation trenching strategy exists to answer one question: what is actually in the ground, and does it change what can be built here. Everything else in the design of the evaluation follows from that. The trench percentage, the geophysics, the timing and the reporting programme are all instruments for producing an answer the county archaeologist will accept, at a cost proportionate to the risk being retired.

Developers who treat the evaluation as a box to tick end up paying twice, because a poorly targeted evaluation produces an inconclusive result and an open-ended mitigation condition.

What the evaluation is meant to answer

National policy expects an applicant to describe the significance of any heritage assets affected, including any contribution made by their setting, and where a site has the potential to include assets with archaeological interest, to submit a desk-based assessment and, where necessary, a field evaluation. The August 2026 Framework carries that requirement forward under its policy-coded structure, so quote the current code from the published document rather than an older paragraph number.

An evaluation should establish presence or absence, and where remains are present, their date, character, extent, depth, state of preservation and significance. That last word is the one that matters commercially. Significance drives whether the response is preservation in situ, which can sterilise part of the site, or preservation by record, which is a cost and a programme item but not a design constraint.

Write the objectives into the scheme before anyone specifies a trench. An evaluation designed to test a specific model of the site produces evidence that supports a mitigation strategy. An evaluation designed to cover a percentage produces a percentage.

Geophysics first, trenches second

Geophysical survey is fast, non-intrusive and low cost by comparison, and on the right geology it maps ditches, pits and enclosures across a whole field in days. Magnetometry is the usual technique for open sites. Ground-penetrating radar and resistivity have narrower uses, mainly masonry and buried structures.

It is also blind on some sites. Heavy clay, made ground, extensive modern services, ferrous fencing and hard standing all degrade the results, and a survey over a former industrial yard tells you very little. Where the geology is unhelpful the honest answer is to skip the survey and put the money into trenches.

Where geophysics does work, it changes the trench design completely. Anomalies get targeted trenches, blank areas get a sample to test whether the survey missed anything, and the resulting layout is defensible in a way that a plain grid is not. A prior desk-based assessment baseline that identified the likely period and character of activity makes the interpretation of the geophysics far more reliable.

Sample percentages and how they get negotiated

Trial trench evaluations in England are usually specified as a percentage of the developable area. Common practice runs from around 2 per cent on lower-risk sites to 5 per cent or more where the desk-based work indicates high potential, and the curator sets the expectation rather than the applicant.

Three variables are negotiable in practice:

• The percentage itself, where the desk-based assessment shows truncation or low potential across part of the site.
• The area the percentage applies to, since access roads, retained open space and areas of no ground disturbance can sometimes be excluded.
• The phasing, so a first tranche of trenches informs whether the rest are needed.

Trench dimensions are conventional rather than fixed: long, narrow trenches of the order of 20 to 30 metres by 1.8 to 2 metres are standard, machine-excavated under archaeological supervision to the top of the first archaeological horizon or to natural, whichever comes first.

Put the negotiation in writing before mobilising. A written agreement from the curator on the percentage, the layout and the acceptance criteria is worth more than a phone call when the results are equivocal and someone wants a second phase.

Writing the written scheme of investigation

The written scheme of investigation is the controlling document. It is submitted to the local planning authority, approved on the curator's advice, and then binds the contractor. Write it to the Chartered Institute for Archaeologists standard for field evaluation and make it specific to the site.

A usable scheme covers the aims and objectives, the site background from the archaeological field evaluation brief and the desk-based work, the trench plan with grid references, the excavation methodology including machine type and supervision, recording systems, the sampling strategy for environmental and dating evidence, the treatment of human remains and the licence position, health and safety, the finds and archive policy including the receiving museum, the reporting timetable, and the arrangements for monitoring visits.

The archive clause is the one most often left vague, and it causes trouble years later when a museum will not accept a bulk assemblage without a deposition fee that nobody budgeted.

Programme risk

Evaluation programmes fail on three things. Ground conditions in winter, when trenches flood and the machine cannot work without damaging the site. Access and biosecurity on farmland, where a standing crop can delay work by months. And post-excavation, which is invisible on a programme drawn up by someone who thinks the job ends when the trenches are backfilled.

Allow real time for specialist assessment of finds and environmental samples, for radiocarbon dating if the site produces datable material, and for the curator to read and respond to the report. On a site with a moderate assemblage that sequence runs to weeks rather than days.

If a planning committee date is fixed, work backwards from it and add contingency. An evaluation report that arrives after the committee papers are published is a report the members will not have read.

Turning results into a mitigation strategy

The evaluation report should end with a clear recommendation, not a list of options. Where remains are absent or of low significance, say so and recommend no further work. Where they are present, propose the proportionate response: a watching brief during groundworks, targeted excavation of defined areas, a strip, map and sample exercise across the footprint, or design changes to preserve remains in situ under a protective layer.

Preservation in situ is the outcome that changes the scheme, and it is worth modelling before the layout is fixed. Raising levels, relocating attenuation basins and using piled rather than strip foundations are all recognised responses, though piling through archaeology is itself a matter the curator will want to assess.

When a condition beats pre-determination work

Not every site needs trenching before the decision. Where the desk-based assessment characterises the resource well enough for the authority to judge the application, a condition requiring a written scheme of investigation and subsequent mitigation is proportionate and keeps the programme moving.

Pre-determination evaluation is the right call when the potential significance is high enough that the answer could change the layout, the quantum or the acceptability of the scheme. Paying for trenches to inform a design is money spent once. Discovering a scheduled-quality site after permission, with a mitigation condition of unknown scope, is money spent repeatedly. Where heritage significance is the driver, the setting analysis in a heritage statement should be prepared alongside the archaeological work rather than after it.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Browse the report types on PlanScope to draft the archaeological documents your application needs, then have them reviewed by a qualified archaeologist. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.