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Trial trenching is a rapid and inexpensive method of archaeological evaluation used to estimate the archaeological potential of a site.
Trenches are located at intervals across a site leaving the rest untouched. A mechanical excavator is used to dig down to archaeological features or natural geological deposits and any archaeology is recorded. No further excavation takes place at this stage. The results of the trial trenching are used to inform any future stage of work which may extend to full excavation of the rest of the site if the evaluation reveals significant finds.
There is some argument over the sampling strategy to be employed in trial trenching, especially in evaluating sites that are intended for development. Issues such as the effectiveness of certain trench layouts or the percentage of the site to be dug (normally around 5% at present) are widely discussed. Whether an effective picture of past human activity on a site can be truly estimated through this methods is widely debated. Development can destroy buried archaeology forever and a reliable evaluation methodology is very important. Whilst it is difficult to quantify the number of false negative results there have certainly been examples of evaluations suggesting a relatively limited amount of past activity which has had to be upwardly revised during the excavation.
In the UK the results of the trial trenching will be used to inform the decision on the need for any further archaeological work to be undertaken prior to development. This process is enshrined in PPG 16 and English Heritage's Management of Archaeological Projects colloquially known as MAP 2.