“A mine is a hole in the ground with a Cornishman at the bottom,” so says the old adage, but, when is a hole a mine shaft? – and when is it something else completely? It is apparent, when reading statements in the press or hearing journalists on tv or radio, that most who comment on such holes in the ground, which mysteriously appear from time to time, always unexpectedly and often catastrophically, do not have the foggiest notion about what is there.
The first fact about holes related to Cornish mining is that they can be found almost anywhere in Cornwall. Few parishes were without some mining or mine exploration, although it is true that the overwhelming majority of mine holes are sited in a relatively few districts. Mining related holes in the ground can be caused by one of several things: a shaft, a backfilled openwork, a stope immediately beneath the surface or a “costean” pit. As there were several thousand mine shafts sunk in Cornwall, shafts tend to be the obvious culprits (for the media) when an unexpected hole appears.
A shaft is a vertical or sub-vertical hole or pit, which was sunk from surface for depths of anything from a few feet to a couple of thousand feet. Shafts can vary in size from as little as two foot square to as large as twenty foot by ten. Most shafts were probably adit (ventilation) shafts and a great many of these conform to the 18th century norm of six foot by four. One danger with adit shafts is that perhaps two-thirds of them were abandoned within a decade or so of being sunk. At the bottom of these shafts, immediately over or just to the side of the adit tunnel, timbers were placed to keep the attle (rubbish or refuse consisting of broken rock)from choking the adit and preventing the water from draining away. Most adit tunnels were driven before the end of the 18th century – over two hundred years ago – and the original timbers remain in situ.
The danger of such adit shafts collapsing is undoubtedly enhanced by the capping or plugging of other shafts on the same adit system. Almost every shaft capping exercise results in rock, earth and other debris falling down the shaft. If an adit is below, it is all too frequently blocked or partially blocked by the debris. Water backs up and old timbers are loosened and collapse. The backfill above the timber sinks and a hole appears in an unforeseen place at surface.
Much of the above scenario is repeated when old stopes are capped or partially filled, or when larger shafts are dealt with. Catastrophic collapse due to a large engine shaft or stope going down can cause extremely large holes at surface, and can only be dealt with properly by experts spending large amounts of money. Backfilled openworks usually contribute to the picture in a less dramatic fashion, as do “costean” pits. Most open-work problems result from settling or slumping of backfill rubble. This can cause loss of foundation stability, resulting in cracked walls and paths. If the openwork is connected to shafts or underground stopes, then the problem can be more spectacular.
“Costean” pits were sunk to search for tin and other metallic ores. Problems due to these are usually small-scale and result from backfill settlement. Most exploration pits are between eight and twenty feet deep and can cause little serious problem. Some “costean” pits, where the lode was found at the bottom, were developed in depth and along the strike of the lode, and these are potentially more serious. The sudden appearance of a hole in the ground may not be welcome, but it need not always be disastrous. The first thing needed is accurate information on the nature and extent of the mining business to determine the implications; practical, financial and with regard to safety. Remedying the situation may not always be cheap, but it need not necessarily be cripplingly expensive either. Of one thing we can be certain, the enthusiastic and indiscriminate capping of shafts, so popular with district councils who apparently have access to vast funds for such a purpose, is not solving the problem of holes in the ground. Such capping may well be a major contribution to a much larger and longer lasting problem in the future.