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Kent Archaeological Review extract

Letter to the Editor --
Archaeology in depth.
by H Pearman.


Archaeology is not customarily regarded as a hazardous occupation but a series of accidents were reported last year involving collapsed excavations and prompt me to write a few words on safety precautions in digging.

There is no mathematical formula for determining how deep one can safely dig a trench before it becomes potentially dangerous. The point at which earth will move depends upon a combination of its density, composition, size and shape of individual grains, the friction between them and the amount of water present. Trenches which are safe in dry weather may become very dangerous when wet. In the building industry trenches down to four feet deep are usually unlined and, if damp, gravelly or sandy, the sides are shaped outwards. Beyond four feet some wooden support is added simply because beyond that depth a run in can be dangerous. The lining can be either horizontal or vertical planks at not less than one foot intervals held by more planks in the other direction. Cross struts at foot depth and three foot length intervals hold the sides of the trench apart.

The next four feet is dug narrower than the top four. If it were dug wider a run in could occur between the gaps. This step is lined as it is deepened and at eight feet a third step is commenced. Thus a twelve-foot deep trench would have three steps of descending widths of say 3 feet 9 inches, 3 feet, 2 feet 3 inches.

Having spent seven years in the excavation of one Roman well I can assure readers that such excavations are not lightly to be entered upon. Although the finds could be valuable, all too often the well may have been used as a rubbish pit at various stages of its life and contain several hundred feet of filling. It would be as well to check on the depths of modern wells in the vicinity to gain some idea of the depth to be expected before commencing the venture.

If any shaft is uncovered it can be tested both for depth and the presence of noxious gas by lowering a candle or other naked flame lantern on the end of a measuring tape. If the flame continues to burn there is sufficient air to breathe.

Shafts of up to 20 feet can be descended on rigid ladders, provided that the descender wears a crash helmet. If the depth is greater than this then would be adventurers should be dissuaded from experiments with knotted ropes or home made rope ladders. It would be best to contact a local pot-holing club via the Cave Research Group of Great Britain who will come equipped with proper lifelines, miniature wire ladders, and the knowledge of techniques beyond the scope of this letter to describe.

If excavation is contemplated then a hand-operated or mechanical winch will have to be installed and any person being raised or lowered on a seat must be independently life-lined from above. Debris will be winched up in seven-gallon oildrums with welded steel handles. The first object up on each trip must be the seat in case a second person has to descend in an emergency. It will be necessary to construct a wooden umbrella at the bottom standing upon a single metal post. A car tyre on top of the structure will help to absorb the impact of any bucket which parts company with the hook. The area around the head of the shaft must be boarded over and railed off and any object likely to be accidentally knocked over the edge removed. The rate of excavation is of course limited by the digging capabilities of the man at the bottom. If however the shaft is deep and the filling soft, then the time taken to raise and lower the bucket becomes the limiting factor. In either event the maximum rate likely to be achieved in a three-foot diameter shaft is eight inches an hour.

I am currently listing for publication all ancient caves, conduits, tunnels and mines in Kent (not wells) and would be pleased to hear from any of your readers who have knowledge of any site.

Yours faithfully.

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