Salt and the salt industry

by Albert Frederick Calvert

In the brine-shafts employed in the case of the old rock-salt mines, in which the brine was met with at a much higher pressure than in the rock-head brine-shafts, the tapping operation was attended with extraordinary difficulties. The brine in the old workings rose to a height corresponding with that attained by the brine found at the rock-head, and as it had to be tapped through a pillar near the bottom of the old workings, the pressure was proportionately higher. When the holeing was first effected into the brine in the old bottom-bed workings, the rush of the incoming brine was so strong that it passed through the two 5 in. bore holes and rose up a 4½ ft. shaft to a height of 67 yds. in eight minutes. The shaft for tapping this supply of brine was sunk in a pillar of rock-salt, and a drift, fitted with two 5 in. bore-holes, was worked through the intervening face and into the brine. When these bore-holes were knocked through, the brine entered with the report of a cannon, and the engineer and his assistant, leaving their tools behind them, leapt into the bucket and were hastily drawn up the shaft, closely pursued by the rising brine.

An improved process for tapping the brine, which entirely removed the danger attending the operation, was subsequently introduced. This was effected by boring the last part of the main bore-hole through a stuffing-box at the other end—an innovation which prevented brine from escaping during the boring. A drift, with the usual ⅝ in. bore-hole in advance, was driven 61 yds. into the barrier, until the small bore-hole showed that only 10 yds. remained between the face and the brine that was known to be present in the old workings. Into this remaining 10 yds. of barrier a hole 11 in. in diameter was bored until nearly through, and a closely-fitted pipe was inserted into the hole for a distance of 7 ft. The pipe was 10 ft. long, but at 7 ft. from the inner end was a disc 3 ft. in diameter to rest against the face of the drift, leaving the remaining 3 ft. of pipe in the drift. About midway between the disc and the outer end of the pipe, were placed two strong iron uprights, let into a trench cut 1 ft. deep in solid rock-salt in the roof and floor to secure the pipe against the pressure. These two uprights were placed close together at the top and bottom, but in the middle they were curved so as to form a circle for the pipe to pass between them. The face of the drift against which the disc had to rest, having been carefully dressed, and a disc of india-rubber covered with red lead having been placed between the iron disc and the dressed face of rock-salt, the iron disc was secured up tight against the face by means of six set screws. A stop-valve was then fitted to the outer end of the pipe, and to this, for the temporary purpose only of completing the bore-hole, was attached an end piece with a stuffing box and a hole in it large enough for the bore rod to be worked through. The bore rock was then withdrawn and, the valve being closed, the stuffing box and the temporary end piece were removed. A range of pipes was attached to the stop-valve and, in this range, the brine was taken through the old workings and up one of the shafts to the surface.

Many geologists have subscribed to the theory that the Cheshire meres were formed by subsidences which occurred in pre-historic times, but the evidence based on the phenomena attending the modern subsidences proves that the latter were the result of artificial and readily-identified causes. Leland, in 1533, reports a sinking near Combermere and the formation of a pit containing salt-water; in 1657 a small sinking occurred at Bickley, near Malpas; and a third took place in 1713 at Weaver Hall, to the south of Winsford. No traces of any of these subsidences now remain, but, from the descriptions handed down to us, these sinkings belonged to the class of funnel-shaped holes and were of limited diameter and no great depth.

Of the modern subsidences, which are of three kinds, we have no documentary evidence prior to 1777, and the earliest distinct record belongs to the year 1790. From that time to the present day this class of sinking has continued to increase in extent year by year. In 1790 the sinking portion along the Witton Brook was recorded as being 130 yds. long by 90 yds. wide. In 1837, the subsidence had obtained an area of 1,230 yds. long by 130 yds. wide. In 1811, about 20 statute acres in Witton commenced sinking, and in the ensuing thirty-three years some portions of this area had sunk 24 ft. In 1880, the piece of water called the Top of the Brook had subsided over an area of 4,370 ft. by 1,470 ft., and in the same year it was estimated that no less than 2,700 acres of land in Northwich and Winsford were inundated.

These modern subsidences usually consist of funnel-shaped holes caused by the falling-in of top-rock mines, and of trough-shaped hollows which cannot be connected with rock-salt mining, and are frequently found in places far removed from the localities of the old workings. Of two dozen subsidences, two are nearly four miles distant from the nearest old workings or from the brine shafts, fifteen are upwards of two miles, and only one is less than a mile from either a mine or a pumping station. The subsidences could not be caused by volcanic action or the shock of earthquakes, as nothing of the kind has occurred in the districts, and it is impossible to explain them by the action of natural brine springs running to waste in the brooks or rivers, because it is known that no such springs now exist, while evidence accumulated from all parts of the world confirms the conclusion that where brine springs escape into the streams, no subsidence has ever occurred. Yet it is evident in Cheshire that some subterranean denudation must be taking place which is removing portions of the lower strata and allowing the super-incumbent earths to sink into the excavations thus made. Many theories have been advanced to explain the phenomena, but even those people whose interests have caused them to seek for alternative causes must realize that it can only be attributed to the simple and most obvious agency.

When the number of brine pits was multiplied and the natural springs of a weak solution of salt decreased in volume, it was necessary to sink down to the rock-head brine, which was a highly-saturated solution consisting of one part salt to three parts water. When this supply is pumped up, its place is taken by fresh water, which, flowing over the rock-beds, takes up its quota of salt on its way to the pumping shafts, and is raised to the surface in the form of brine.

It is not the presence of water over the beds of salt or in the old salt workings which causes the damage, because when such water has taken up salt to the extent of a fourth of its bulk, it remains inactive and makes no further ravages upon the mineral earths with which it is in contact. But when the saturated brine is pumped up and its place is taken with a new supply of water which collects its tribute from the salt strata, and that water, in its turn, is raised, to be replaced by more, and when it is known that each 100 tons of water that traverses the salt-bed to the pumps carries away with it 25 tons of solid earth, the work of destruction that is continually going on is explained.

It may be convenient to explain at this point that the subsidences caused by this simple operation of removing rock-salt from the earth in the form of brine are divided into three classes, viz.—

1. Shallow troughs, with sides not terraced or broken up.

2. Very shallow depressions extending over considerable areas.