Archaeological evaluation by trial trenching
The planning application stage
Like geophysical survey, trial trenching is a common form of archaeological evaluation required generally, though not exclusively, at the planning application stage of a development.
The purpose of evaluation by archaeological trial trenching is to confirm the presence or absence of buried archaeological remains within a proposed development site and to establish their significance (in line with NPPF), date, state of preservation and character.
Very often trial trenching is used to corroborate the results of an earlier geophysical survey which has detected the presence of potential archaeological remains in the form of geophysical anomalies.
Determining the scope of the investigation
The results of trial trenching are used by Local Planning Authorities to determine whether further archaeological investigation will be required in the form of a pre-commencement condition and in addition, what the scope and extent of that further investigation should be. The most usual forms of a pre-commencement archaeological condition are either open area excavation or strip, map and record.
Alternatively, should the trial trenching have indicated low potential for archaeological remains within a development site a condition for archaeological monitoring (a watching brief) during groundworks could be applied to the proposed development site.
The scale of trial trenching
Local Planning Authorities normally require that a percentage of the total size of the development area is tested by trial trenching. Sample sizes are normally between 1% and 5% of the area to be impacted by development.
The dimensions of a trial trench are usually 50m x 1.84 or 30m x 1.84, the width of 1.84 being commensurate with width of the bucket on a mechanical excavator. The depth that a trial trench extends to depends upon the depth at which the archaeological horizon is located.
In a greenfield development this could be as little as 0.30m below ground surface whilst in an urban context the trench could exceed 2m or 3m in depth and so for health and safety reasons require shoring or stepping.
Once the mechanical excavator, using a toothless ditching bucket to ensure a clean, level surface, has removed overburden under the direction of an archaeologist and exposed archaeological remains, a team of field archaeologists carries out excavation by hand of the archaeological features usually by sampling a selection of the features exposed rather than all of the features.
The aim is to obtain dating evidence, for example in the form of pottery sherds and environmental evidence from the fills of the archaeological features. This helps to establish the date and function of the remains which have been uncovered. Those archaeological remains may be judged of local, regional, national or in rare cases international significance.
Based upon this evidence the Local Planning Authority will decide what further archaeological investigation will be required to mitigate the impacts of the proposed development on the discovered archaeological resource.
Topics for developers’ guide
- desk-based research
- aerial photography and lidar
- cartographic analysis
- review of geotechnical data and deposit modelling
- and more…