MINERAL SUBSIDENCE.
The alarming subsidence which took place some time ago in Scotland, on the North British Railway near Prestonpans, and which was fortunately unattended with any accident, has doubtless added a fresh source of fear to the nervous railway passenger. That the permanent way of a railway for a distance of about fifty yards should suddenly sink to the extent of two feet is almost incomprehensible at first; and had this subsidence occurred whilst the train was passing, instead of immediately afterwards, the consequences might have been disastrous. It is the case, however, though it may not be generally known, that subsidences—fortunately only gradual, and comparatively inappreciable—are taking place over many of our railway lines, and that ‘minerals’ are actually being extracted from underneath nearly every line of railway under which there is any mineral to get.
The damage done to the line at Prestonpans was reported to have been caused by coal-workings which were there long before the railway was laid; but if it was caused by them at all, it was on account of their being influenced by the working of a seam of coal below them, which was going on at the time the subsidence occurred. It is the fact, however, that when a Railway Company acquires ground under its parliamentary powers, the minerals underneath the ground do not pass along with it. This may seem a little surprising at first; but it is not so when it is considered that very frequently the proprietor of the surface of the ground and the proprietor of the minerals underneath it are different persons. Of course the proprietor of an estate under no reservations is proprietor as high as he can get and as deep as he cares to go; but he may sell or lease the minerals and retain the surface, or vice versâ. Thus it is that a Railway Company has only, as it were, a right of passage over the surface; and that its right goes no deeper, except for the construction or up-keep of its lines. By Act of Parliament, however, the proprietor of minerals below any railway line, before proceeding to work them, must give notice to the Railway Company of his intention to do so, so as to give the Company an opportunity of buying him off, should it feel disposed. If it does not declare its option to purchase the minerals, the workings proceed, and the railway has to take its chance. The mineral owner will, however, be held liable, should any damage occur owing to improper working.
The subsidence of a railway line underneath which the minerals have been worked is as a rule very gradual, and extends over some length of time. Many railway passengers must have noticed the walls of waiting-rooms disfigured by ungainly cracks, the stone lintels displaced, the hearthstones awry, and many other signs, which are caused by the working of minerals underneath. Some station-masters can show you on the stone face of the platform the number of inches the line has sunk. As a matter of fact, were it not for the gangs of surfacemen the Railway Companies employ to watch any irregularities in their lines, in a very short space of time the permanent way would in many places probably represent something like the proposed line of the Undulating Railway, a fantastical scheme of long ago. The railway in Ayrshire which runs over the old workings of the famous Wishaw coal-seam, especially suffers in the way of subsidence; and some parts of the railway in the west of Fife are known to have gradually sunk to an extent of over ten feet.
But railway lines are not the only parts of the surface which are subsiding owing to the working of minerals. The whole surface of the land surrounding the many pits and mines which are continually belching forth their wagon-loads of coal or other mineral, is gradually subsiding as the extraction of the mineral proceeds; and damage amounting to thousands of pounds is annually being done to the surface and the buildings on it owing to mineral workings. As the period and extent of the subsidence and the damage following on it depend greatly on the method employed in working the coal, a word or two here on this subject may not be out of place.
There are two recognised methods of working out coal. The old method is what is known as the ‘stoop-and-room’ or ‘pillar-and-room’ system; and the method introduced into Scotland about the beginning of the present century is known as the ‘longwall’ or ‘Shropshire’ system of working. The first system explains itself by its name. After the bed of coal is struck, ‘rooms’ are worked out, leaving ‘pillars’ or ‘stoops’ to support the superincumbent strata. The object to be attained in this system, as practised in the olden times, was to have as large a room worked out, and as small a stoop or pillar of the coal itself left, as was consistent with the safety of the mine and the support of the surface, while the mine was open. But this system entailed the entire loss of the pillars so left. To obviate this loss, the method now generally adopted is to drive narrow rooms or passages, seldom exceeding fourteen feet, through the seam, leaving large pillars—about seventy-five per cent. of the mineral—until the extremity of the available coal is reached. When, however, no regard is to be had for the surface, and the coal has been thus worked out as far as can be done, the miner commences to work backwards, taking out the stoops or pillars as he goes. The whole roof of the mine then comes down; and this is the most dangerous kind of subsidence. It does not only take effect immediately above the place where ‘stooping’ has been going on, but it also ‘draws’ round about it.
The ‘longwall’ or Shropshire method of working is what is known as the system of complete excavation; that is, the miner takes out the whole coal as he proceeds, leaving only perhaps a foot on the roof, should the overlying strata be soft, and props up a passage with wooden supports as he proceeds, to enable him to keep an open way to the face of the coal. The portions worked out are packed on each side of the ‘road’ with the waste material taken out with the coal. This method of working, though it necessarily implies subsidence, is on the whole the safest for the surface, and is generally the one adopted. In fact, as mineral landlords are paid, in lieu of rent, a royalty or lordship on every ton of coal or other mineral brought to the surface, and as the tenant can more quickly extract the mineral by the wooden props method, he is generally bound in his lease to work in this manner, when practicable.
Should the coal be worked on the stoop-and-room system, and pillars of coal of sufficient size be left in, the surface will not be injured to any appreciable extent, at least not for many years. As is often the case, however, seams of coal are worked out one below the other; and when the lower one gives way, the pillars above may fall like a pack of cards. There is no saying where the subsidence would reach in such a case. If the pillars do not break, the way in which the ‘rooms’ close up, if the floor is soft, is rather peculiar. The roof does not all fall in, as would be expected; but the enormous weight of the superincumbent strata pressing on the pillars causes the floor between them to rise up or ‘creep,’ and the room becomes closed. On the other hand, if the stoops of coal are taken out, the roof comes down with a crash, and the effects on the surface may be disastrous; but of course it sometimes pays better to get out all the coal and let the surface go, than to allow the workings to get closed up and the coal in the pillars to be lost for ever.
The subsidence following on a ‘longwall’ working is gradual, but sure. The surface is not broken to any great extent, but comes down in one sheet, and not irregularly, as in stoop-and-room workings. The strata generally come to rest in about three or four years. A row of houses which have been cracked through and through on the subsidence reaching the surface, have been known to close up again when the strata have settled.
The damage done by pillar-and-room workings is irregular both as regards effect and time. It may commence, stop, and commence again. Houses are literally wrecked by it. So palpable is it, indeed, that actually the sound of the crushing and subsiding of the house can be distinctly heard. The slates are twisted off the roof, the chimneys hang in all directions, the walls are rent asunder, the foundations give way, and the house is rendered uninhabitable. An instance of this is to be found in the salt-workings of Cheshire, in the neighbourhood of which, houses are constantly being wrecked. Thousands of pounds are paid every year by mine-owners for damages done to surface proprietors, farmers, and others; and there is no more fruitful source of litigation than surface-damage.
Even under public roads, we find the minerals being worked. The public have only a right of passage, the minerals underneath belonging to the adjoining proprietor, and it is not an uncommon circumstance in mining districts for a road to suddenly sink several feet.
In the case of a proprietor of minerals in lands adjoining the sea, his right, as a general rule, extends only to high-water mark. Below that line, underneath the foreshore and the sea itself, the minerals belong to the Crown. The Crown, of course, can lease the minerals, and they are very frequently worked under the sea itself. In such a case, great precautions have to be observed in the workings, to prevent any chance of the sea breaking in, though, when the stratum above is rock, the mineral is sometimes worked out within a very few feet of the bottom of the sea! In some mines, the roar of the ocean above can be distinctly heard.
This, however, is a digression from the subject of subsidence. Subsidence of the surface above our almost inexhaustible beds of coal has been going on, and will go on more or less, until that day in the dim futurity which has been foretold, when our coal-seams will have become exhausted, but when, let us hope, the inventive genius of posterity will have discovered another fuel, or done away with the necessity of fuel altogether.