Trial trenching: sampling strategy and coverage
There is no statutory trenching percentage. What drives the figure in a curator's brief, why targeted trenching alone is rarely accepted, and how to programme it.
Archaeological trial trenching sampling is set by the local authority's archaeological adviser in a written brief, not by a national standard. There is no statutory percentage. What you get is a figure in a curator's brief, usually somewhere between 2 and 5 per cent of the area that will be disturbed, with more where geophysics or the desk study points to substantial remains, and a requirement that the layout tests the blank ground as well as the anomalies.
Your job is to meet that brief, or to argue a different one with evidence. Arguing without evidence is how a pre-determination evaluation turns into three months of correspondence.
What an evaluation is meant to establish
National policy expects a desk-based assessment where a site may contain heritage assets with archaeological interest, and field evaluation where that is necessary to understand the significance of what is there. Significance has to be understood before the decision is made, not after, which is why pre-determination evaluation is the norm on greenfield sites of any size.
The evaluation answers six questions: are there archaeological remains, what are they, how old are they, how extensive are they, how well preserved are they, and how significant are they. That last one drives everything, because significance determines whether the remains can be developed over with recording, must be preserved in situ, or make the scheme undeliverable as drawn.
Start from the Archaeological Desk-Based Assessment. A good assessment narrows the question enough that the evaluation can be targeted, and a thin one leaves the curator with no choice but to ask for blanket coverage.
Choosing a sampling percentage and justifying it
The percentage is calculated on the area to be disturbed rather than the red line, which matters on a scheme with large areas of retained open space.
What moves the figure up: a scheduled monument or find spot on or adjacent to the site, a cropmark complex visible on aerial imagery, a geophysical survey showing coherent anomalies, a location that archaeologists would predict for settlement, and ground conditions where remains survive well. What moves it down: deep made ground sealing or having destroyed the horizon, quarrying history, a geophysical survey with good coverage and nothing in it, and prior evaluation nearby that came back blank.
If you want a lower percentage, bring the evidence. A magnetometer survey over suitable geology with no anomalies is a genuine argument for reduced trenching. An assertion that the site has been ploughed for centuries is not, because ploughing truncates features without removing them.
Get the brief in writing before anybody prices the work. Three contractors pricing against three different assumptions produce three unusable quotes.
Targeted against systematic layouts
Three layouts and a hybrid.
A systematic array, laid out on a grid or in a staggered or herringbone pattern across the whole area, gives even coverage and tests ground where nothing is predicted. It is the default where geophysics is unavailable or unreliable.
Targeted trenching places trenches across identified anomalies to characterise and date them. It is efficient and it tells you nothing about the rest of the field.
The hybrid is what most curators require: trenches targeted on anomalies plus a systematic sample across the apparently blank areas, because geophysics misses whole classes of feature. Timber buildings with post holes, graves cut into chalk, and features on clay-with-flints or alluvium can all be invisible to a magnetometer.
Trenches are usually 20 to 50 metres long and around 1.8 to 2 metres wide, the width set by the excavator bucket. Longer trenches cross more boundaries and give better spatial information than the same area in short trenches, so if the brief states an area rather than a layout, favour length.
Constraints from services, trees and access
The layout that works archaeologically often cannot be dug.
- Live services. A full service search plus detection on the ground, with hand excavation over anything detected, and trenches moved rather than crossing a main.
- Trees. No machine excavation within a root protection area set out to the tree survey standard, which on a hedgerow boundary removes a band several metres wide from the plan.
- Access. A tracked excavator and a dumper need a route in, and a 360 machine working in a wet field in February creates damage that costs more to put right than the trenches.
- Overhead lines. Minimum clearance for an excavator boom, which usually means an exclusion corridor rather than a reduced depth.
- Crops and livestock. Standing crops mean compensation or a wait until after harvest, and stock has to be moved and fencing arranged.
- Ecology. Nesting birds in boundary vegetation, badger setts, and reptile habitat in rough grassland all constrain where and when machines can work.
- Contamination and made ground. Both change the method, the spoil handling and the health and safety case.
Topsoil and subsoil are stripped and stored separately and reinstated in order. Agree the reinstatement standard and any crop loss payment with the landowner before mobilising, in writing.
Interpreting negative results honestly
A 4 per cent sample examines four hundredths of the ground. That is a reasonable sample for detecting a settlement enclosure and a poor one for detecting a single inhumation or a small post-built structure.
So report the result for what it is. A negative evaluation substantially reduces risk and does not eliminate it. Say what the sample could reasonably have detected and what it could have missed, and state the residual risk plainly. A curator reading that will usually be content with a watching brief condition. A report claiming the site is archaeologically sterile on the strength of nine blank trenches invites a demand for more trenching.
Where remains are present but limited, the honest finding is often the most useful one: a dispersed scatter of plough-truncated features of low significance is a finding that supports a recording condition rather than a redesign.
When evaluation triggers excavation
Where remains of real importance are found and cannot be preserved in situ, the usual outcome is mitigation by excavation and recording, secured by condition and carried out before or during construction. That is preservation by record, and it is a cost and a programme item rather than a planning obstacle.
Budget for the whole sequence, not the digging. Excavation is followed by post-excavation assessment, then analysis, specialist reporting, archiving with a museum and publication, and those stages run for years after the machines leave. The obligations are set out in post-excavation assessment, archive and publication, and they are the part most often left out of a cost plan.
Where human remains are encountered the position changes immediately, with licensing and procedural requirements that stop work until they are dealt with, as covered in human remains on site: licence and procedure.
Nationally important remains, whether scheduled or not, are a different proposition. Scheduled monument consent is a separate consent and the policy presumption against harm is very strong, which usually means designing around rather than through.
Programming evaluation around seasons and weather
Trenching is a winter casualty. Waterlogged ground makes features invisible, trench sides unstable and machine movement destructive, and frozen ground cannot be cleaned by hand. A trench dug in the wrong conditions produces a report full of caveats and a curator who wants it done again.
Build the sequence and leave room in it:
- Desk-based assessment, which also identifies whether geophysics is worth doing on the local geology.
- Geophysical survey, outside the growing season on arable land where the crop prevents it.
- The curator's brief, then a written scheme of investigation approved in writing before anyone mobilises.
- Contractor procurement, with a registered organisation the curator will accept.
- Fieldwork in workable ground conditions, with time for an extension of trenching if something unexpected appears.
- Reporting, then the curator's response, then determination.
That is comfortably three to six months on a straightforward site and longer where geophysics has to wait for a harvest. Where the evaluation feeds a heritage case covering built assets as well, the findings belong in the Heritage Statement alongside the significance assessment rather than in a separate appendix nobody reads.
Enter your postcode on PlanScope to see which heritage and archaeology documents your site is likely to need.
This is general guidance, not legal advice; check the position with your local planning authority's archaeological adviser or a qualified archaeologist 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.
Frequently asked questions
What percentage of a site has to be trial trenched?
There is no national standard. The figure comes from the local authority archaeological adviser's written brief and commonly sits between 2 and 5 per cent of the area to be disturbed, with more where the desk study or geophysics indicates remains of real interest. Obtain the brief in writing before pricing the work.
Can I rely on geophysics instead of trenching?
Rarely on its own. Magnetometer survey performs well on some geologies and poorly on others, and it misses whole classes of feature including post-built timber structures and graves cut into chalk. Most curators require a hybrid layout: trenches targeted on anomalies plus a systematic sample testing the apparently blank ground.
Should evaluation happen before or after planning permission?
Before determination, in most cases. National policy expects the significance of archaeological remains to be understood when the decision is taken, so conditioning the evaluation defers the very information the decision maker needs. Post-permission conditions are appropriate for mitigation recording and watching briefs, not for establishing significance.
How should a negative evaluation be reported?
Honestly, stating what the sample could reasonably have detected and what it could have missed. A four per cent sample is reasonable for finding an enclosure and poor for finding a single burial. Describing the residual risk plainly usually leads to a watching brief condition, whereas claiming sterility invites a demand for more trenching.
What does excavation cost beyond the digging?
Post-excavation assessment, analysis, specialist reporting, archive deposition with a museum and publication, all of which run for years after fieldwork finishes. Those stages are the part most often omitted from a cost plan, and they are secured by condition, so they cannot be dropped once the remains have been excavated.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.