Handling the Debris. — In order to afford as much safety to the men in the shaft bottom as possible, it is common to cover up the mouth of the shaft with the exception of an opening just large enough to let the kettle pass through. One very good arrangement for banking the kettle consists of a carriage which can be run across this platform covering the shaft mouth and placed under the loaded kettle. The kettle is lowered on to this carriage and detached from the winding rope, a spare kettle standing empty upon the carriage being attached. When this empty kettle is raised by the winding gear, the carriage is drawn forward, and the loaded kettle taken away and emptied while the empty one is being lowered to the sinkers in the shaft bottom. A diagram showing this method of dealing with the debris is given in Fig. 33.

Sinking a Circular Shaft. — After laying off the shaft and providing means for lowering a central plumb-line from a fixed position on the surface, excavation is commenced and continued until firm ground is reached. During this portion of the operations it is generally necessary to secure the sides by means of temporary curbs or rings, with backing deals inserted behind, and held in position by wedges driven between them and the curbs. When the stone head is reached a level bed is prepared and a walling curb laid down; from this curb the permanent lining, usually of brickwork, is started and carried up to the surface. A space several inches wide is left between the ground and the walling, and this space is packed with sand or fine ashes, the object being to equalise the pressure of the sides upon the wall. This packing is done as the walling proceeds, while at the same time the temporary lining is removed, care being taken not to expose too much ground without support. After the first section of brickwork has been built in, sinking is resumed, a ledge of ground being left to carry the walling above. After the shaft has been sunk for a further two or three fathoms, temporary support being given to the sides if necessary, a new bed is prepared and a second section of walling started in the same way as the first. After the building has reached the ledge supporting the section above, this ledge is cut away bit by bit, and the walling closed, thus completing the second section. From this point sinking and walling is carried on alternately until the shaft has been sunk to the desired depth. The excavation is kept vertical by the central plumb-line already mentioned, but each section of brickwork is started in line with that above by means of six or eight short plumb-lines suspended round the walling and hung down to the base of the new section. At intervals of about 60 feet, water rings or garlands are used to collect the water and prevent it running down over the face of the brickwork. A section of a circular shaft showing the brick lining and a water ring is given in Fig. 34.

Special Methods of Sinking. — Sometimes the position of the shaft is such that special difficulties are met in sinking. The ground may be soft near the surface, there may be sand containing a great deal of water, or immense quantities of water may find their way into the shaft during sinking; in such cases the difficulties have to be overcome by methods specially devised to deal with them.

In sinking through very soft ground a cylinder is generally used. This cylinder may be entirely constructed of cast-iron plates bolted together, or it may be constructed with a heavy cutting edge of cast-iron or steel fitted with a light steel plate cylinder, inside of which the ordinary brick walling is carried. In such cases the soft ground is removed from the bottom, and as the cylinder sinks plates or walling are added at the top. In order to get the cylinder to sink with the excavation of the bottom it may be weighted with pig-iron placed on scaffolds fixed inside. When the addition of such weight becomes necessary it should be placed as close to the bottom of the cylinder as possible, as this tends to prevent the cylinder from being dragged out of the vertical. Any tendency on the part of the cylinder to lean over to the side is counteracted by shifting the pig-iron over to the opposite one. In this way the cylinder is kept going down in a truly vertical path. When the rock head is reached, sinking and walling is proceeded with in the ordinary way. Fig. 35 shows a composite iron and brick cylinder being sunk through soft mud at Musselburgh on the shores of the Firth of Forth. Where thick beds of running sand or other loose strata containing much water has to be sunk through, a freezing system is frequently adopted. The soft water-bearing beds are first pierced by a number of boreholes, which are lined with light steel tubes. These boreholes are placed in a circle a few feet larger than that required for the shaft, and spaced from three to four feet apart.

The bottom of each borehole is carefully plugged up, and a small tube inserted inside so as to leave an annular space between it and the outer tube lining the borehole. All these inner tubes are connected up to a common feed pipe through which a refrigerating liquid is circulated. This liquid passes down each inner tube and up the annular space between it and the outer one. All outer tubes are connected to a common return and the liquid passes back through this to the refrigerator after lowering the temperature of the ground. in the immediate vicinity of each borehole. By the continuous circulation of this cooled liquid a solid core of frozen ground is gradually formed in the position to be occupied by the shaft. As soon as this zone of ground is frozen to such an extent as will provide a sufficient thickness of ice wall to resist the external pressure of the loose ground, the shaft is sunk through the frozen strata in the ordinary way. After sinking is commenced, all but the outer ring of boreholes are disconnected from the refrigerator supply pipe, and the tubes withdrawn. When the rockhead is reached a lining of brickwork, or if much water is expected, cast-iron tubbing is carried to the surface. Usually the space between this walling or tubbing and the sides of the excavation is filled with concrete, which, on setting, fixes the walling or tubbing firmly in position. Further sinking below the soft beds is carried out by the ordinary method. The low temperature is usually produced by a Carre's refrigerator, while the liquid circulated through the pipes is a solution of Calcium Chloride cooled to a temperature of — 12 to — 15 degrees Centigrade. This solution does not freeze until a temperature of — 40 degrees Centigrade is reached. Liquefied ammonia gas is sometimes used in place of calcium chloride, and in some respects is a more suitable substance.

In some cases shafts have to pass through rocks which give off immense quantities of water, in such cases, if the underlying beds are of an impervious character, and the coal-seam 150 yards or more below the water-bearing stratum, the shaft may be bored through this stratum and the water held back by tubbing. The chief method of dealing with cases of this character is known as the KindChaudron. Here the shaft is bored out in two operations; the first generally takes about 4 feet or thereby of the complete shaft, while the second and final enlarges this to the finished diameter, which may be from 15 to 20 feet. Immense boring tools fixed to the end of heavy rods are raised by means of a steam-engine through 12 to 15 inches, and dropped back on the bottom of the shaft. From 20 to 30 blows per minute are made with the small tool or trepan, as it is usually called, while the large one is usually operated at the rate of from 10 to 15 blows per minute. The tools are rotated slightly between each blow, the whole operation being the same as percussive boring, but on a much more extensive scale.

After the shaft has been bored about 30 feet below the water-bearing bed, a special tool is lowered and a seat prepared for the tubbing. The tubbing, which must be watertight and sufficiently strong to resist the pressure of the water, is built in complete rings, and fitted with a special telescope piece on the lower end. Packing material is placed between two flanges, one on the sliding piece, the other on the lower ring of tubbing, and the whole lowered into the shaft. This telescope piece, or moss-box as it is usually called, forms a watertight joint between the tubbing and the sides of the shaft, and allows of the lining being gradually lowered into position. This is done by placing a horizontal watertight diaphragm across the bottom of the tubbing, and fitting it with a pipe leading to the surface. So long as this pipe is kept closed, the tubbing is floated by the water in the shaft; and by controlling the escape of water through this pipe, the down movement of the tubbing is controlled. Rings are added at the top as the tubbing is lowered, until it settles on the bed prepared for it, and afterwards the space between this lining and the side of the shaft is filled in with liquid hydraulic cement. After this has set, the water is pumped out of the shaft, the diaphragm removed, and sinking continued beyond the bottom of the tubbing by ordinary methods.

Water in Sinking Shafts. — Water is met with in greater or lesser quantities in all shafts, and has to be raised to allow sinking operations to proceed. For some little time at first it may be possible to raise the water with the kettle, but usually as the shaft gets deeper, the volume of water increases, and it becomes necessary to adopt more efficient means of dealing with it. Several methods are available, but a good form of turbine pump, electrically driven, gives as good results as any. The pump and its discharge pipes are slung in the shaft by strong wire cables, which are used to raise and lower it when necessary. A pump of this class is shown in Fig. 36.

Sinking Accidents. — Sinking, in spite of all precautions that may be taken to prevent accident, is dangerous work. Explosives have to be handled, outbursts of water or gas may occur, material may fall down the shaft, and kettles or scaffolds may get detached from ropes, or get upset. To prevent these accidents as far as possible, electric light should be provided, and all shots should be fired electrically, careful examination should be made for any traces of inflammable gas, and if found no naked lights should thereafter be allowed in the shaft. Proper banking arrangements should be provided at the surface, and all loose material carefully removed from the sides of the shaft to prevent anything falling down. Non-spinning wire ropes should be used, and the sinkers made to ride inside he kettle rather than on its edge. Where work has to be (lone with the open shaft below the worker, only men of known capacity and experience should be employed in doing it. The responsible man in charge of each shift should satisfy himself that everything is in good order, and only the best appliances procurable should be used. Attention to these small details will frequently prevent accident to both life and property.