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In view of comments received regarding the experiment the following has been prepared as an addendum to the original paper.
There can be little doubt that any intervention within the plough soil will affect its properties on reinstatement. Whether this change in properties is sufficient to dramatically to alter the results of the experiment is another matter. The size of the trenches was deliberately chosen to strike a balance between covering an area large enough to capture any possible truncation and small enough not to bias the effects that may result from reinstatement of the ploughsoil.
In the paper our comments regarding "softness" of the ploughsoil were a record of statements made by the farmer who had noticed the significant affect that the large evaluation trenches had had on his subsequent ploughing regime. The trench size employed in the evaluation of the site varied between 50m x 4m (minimum width) to 50m x 8m (maximum width). The direction of ploughing varied across the site so that in some instances the tractor was running across previous evaluation trenches (between 4m and 8m wide) and in other instance was running along the trenches (between 50m and 100m in length). The noticeable areas of soft ground which the farmer commented on were in respect to the larger of these trenches and coincided directly with areas where both the tractor and plough were traversing the trench backfill at the same time. In this instance the combined weight of the machine and plough dropped together into a backfilled trench and ran along it. It was not clear whether the affects of the soft spots were noticed more in wet weather than in dry.
In the current experiment, due to the size of the trench the plough and tractor were not over the backfilled trench at the same time. Consequently the weight of the tractor did not overcompensate for the level of the plough as the tractor would have been on firmer ground as the plough passed over the pit. Additionally the ploughing was carried out in a period of dry weather. Over larger areas and in wet weather I would argue that the effect of this softer ground is more significant.
It should be remembered that in addition to the excavated material a depth of 4cm of glass chippings were deposited in a shallow cut in the base of each pit (this extra material accounts for at least an additional 12% volume to that excavated). When the monitoring pits were backfilled they were level with the surrounding ploughsoil. In this respect, unlike the case of the evaluation trenches, a certain amount of manual compaction was used when backfilling in order to reduce the additional volume to fit the backfilled pit. Consequently the material put into the monitoring pits once the marker artefacts had been deposited was more compact than that in the earlier evaluation trenches. In some ways this would have been more akin to the soil that was excavated.
The experiment was designed to minimise any possible affects that reinstatement may have had.
Producing an identically compacted backfill to the surrounding ploughsoil is another issue. Having had extensive personal experience in reinstating trenches by hand it is amazing how much volume can be lost through compacting the soil by hand. In some instances this has been up to 35%. Compaction in itself may not be the only factor that affects the density of the soil. Moisture content is also very important. Compacting wet material often results in a 'jelly affect', which does not share the same qualities as a well turned over and established ploughsoil.
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