Trial trenching

An evaluation is required where it has been determined that a site has the potential for the recovery of archaeological remains and these could be impacted/destroyed through development.


However, this potential cannot be confirmed until it has been 'ground-truthed' through field evaluation. The most common form of evaluation is the excavation of trial trenches, the aim of which is to ascertain the presence of any archaeological features/deposits and characterise any remains. Evaluation is almost always carried out at the pre-application stage in accordance with the National Planning Policy Framework. 


The amount of trial trenching is commonly calculated on a percantage of the application site, and this is often around 4%. This is broken down into a series of trenches to provide optimum coverage of the site. Trenches are often linear in form and standard lengths are 10m, 20m and 50m. The width is often determined by a toothless machine bucket width which is commonly 1.6m or 1.8m. Trenches are machine excavated down to the archaeological horizon or natural geology whichever is first, with any archaeological remains being hand-sampled and recorded. 


For sites of reasonable size, a combination of evaluation techniques such as geophysical survey and trial trenching can often be the most efficient and cost-effective evaluation solution. In this scenario, the geophysical survey is carried out first, the results of which will show whether there are any geophysical anomalies that could represent archaeological remains. These are then targeted for trial trenching with some trenches placed in 'blank' areas to serve as controls. As geophysical survey is a recognised evaluation technique itself, the ratio of trial trenching is often reduced to around 2%. Once the evaluation has been completed and the results have been assessed, these are presented in an illustrated client report.


Trial trenching is intended to be a rapid form of archaeological investigation and only sufficient to determine the character of any remains so that the Local planning Authority can take an informed decision as to the significance of any heritage assets on the site when considering an application for development.


In the scenario where no archaeological evidence is encountered or where it is superficial, then it is often the case that there will be no requirement for any further archaeological investigation and it will not feature as a material consideration when determining a planning application. Conversely, if there is archaeological activity on the site then, depending on its significance, the county Historic Environment Service (HES) will advise the Local planning Authority to permit the application (with respect to archaeology) as long as a condition is placed on the granting of planning permission for an archaeological programme of works prior to or during development groundworks. This commonly ranges from monitoring and recording during groundworks to full site excavation. In situations where the archaeology on site is considered to be of national importance, the HES can object to the application. 


We are routinely involved in trial trenching evaluations and have a varied portfolio of this type of investigation across the South West. From evaluating house extensions to developments across many hectares, we offer a complete solution that ensures that our clients have all the information they need to progress their development plans.