Introduction. — Hamsterley Colliery is situated on the southern slope of the Derwent Valley, about 10 miles south-west of Newcastle-upon-Tyne, and on the Consett and Blackhill branch of the North-Eastern Railway.

The royalties, which lie chiefly to the east, have a slight dip in that direction, and are heavily watered. Most of the coal in the upper seams lies above the drainage level of the district, and has been drained by a water-level drift driven from the level of the River Derwent; but, as a large area of coal in the lower seams is below this level, it was decided to sink a shaft at the lowest point in the royalty, and fit up a pumping-station in order to drain this area.

The most suitable site was on the alluvial flat near the River Derwent, close to and on the dip side of a large fault which intersects the royalties. A bore-hole was put down on the site of the shaft, and proved the existence of a bed of sand and gravel extending from the surface to a depth of 25¾ feet, and of very soft and loose ground to a depth of 54 feet 8 inches. The following is a section of the strata sunk through :—

	Ft.	Ins.	Ft.	Ins.
Soil	1	0
Sand, dry	2	9
Sand, with water	9	6
Sand, with gravel, heavily watered	12	6
			25	9
Blue clay, with boulders	6	9
Large boulders	2	0
Dark blue to black shale, very soft	11	8
Seggar-clay	3	0
White post, very soft	5	6
			28	11
Stronger and darker post	30	0
			30	0
Total			84	8

It was at first proposed to sink by means of piles and afterwards to tub the shaft, but as the site was close to the river, and a heavy flow of water was expected, it was decided to adopt the following method of underhanging or suspended tubbing, in order that the feeders might be at least partly tubbed of as met with.

Sinking Plant. — The finished shaft is 10 feet in diameter, and the whole of the plant in connexion with the sinking was driven by electrical power, which was generated by the colliery company at their power-station, about 1¼ miles distant, the current being three-phase at 550 volts and 60 cycles.

Fig. 5 is a general view of the sinking plant, which consists of :—

(1) Two electrically-driven winches, — for raising and lowering the pump, and the other for dealing with the kibble, etc. Both winches and motors are exactly alike, but the rope from the crab (or pump) winch passes over a pulley in the headgear, round one of the pulleys on the pump, then up and over a second pulley in the headgear, down and round a second pulley on the pump, and finally up, and is made fast to the headgear. This gives a 4 to 1 ratio of reduction, so that the greater weight of the pump and rising-main (full) can be raised at one-quarter of the speed of the kibble with the same power of motor. Each winch is driven through double reduction gear, and the motor-shaft is extended with square end, to which a handle can be fitted, so that both men and pump can be wound out of the pit by hand in the event of the current failing. For starting and stopping, a three-phase reversing controller is provided for each winch.

(2) The sinking pump (Fig. 6) consists of a vertical-spindle single-stage centrifugal pump of the turbine type, with gunmetal impeller, and is capable of delivering 500 gallons per minute. It is driven direct from a vertical-spindle three-phase motor. The motor is protected, but ventilated, a fan for forcing air through being provided on the rotor. The weight of the rotor-shaft is taken on ball-bearings, and as the impeller of the pump is coupled direct to the rotor-shaft, these bearings have also to take the weight of the revolving parts of the pump. The delivery of the pump is taken out on two branches which pass up, as shown in Fig. 6, one at each side of the motor, into an inverted Y-piece on the top, and through a non-return valve into the rising-main, this non-return valve being also provided with a bye-pass for priming the pump. The pump-casing is bolted on to the distance-piece of the motor, and the impeller shaft on to the rotor spindle as already described. Under the Y-piece is fixed a block carrying two pulleys to take the wire rope which supports the pump, motor, rising-main, and column, as before mentioned. By this arrangement, as the sinking proceeds and the level of the water goes down, it is only necessary to add lengths of piping at the top and to lower the pump down by the ropes. In the engine-house at bank is placed the switch-box with 3-pole switch, fuzes, and ammeter for controlling the pump circuit, and also an oil-filled compensator with change-over switch for starting the motor. The handle of the pump compensator is placed in a convenient position within reach of the motor attendant while sitting at either of the winch-motors, so that the whole of the electrical installation is easily controlled by one man.

The Sinking. — A commencement was made by excavating an annular trench in the sand to a depth of 4 feet, with an outside diameter of 19 feet. An annular concrete foundation-ring, having an internal diameter of 10 feet 10 inches, and an outside diameter of 19 feet, and 8 feet deep, was then laid, the top of the concrete being 1 foot below ground level. The concrete block was strengthened by scrap-iron bars and rails laced across each other. The concrete was composed of equal parts of cement, sand, and broken bricks. Great care was taken to level the top of the concrete block, as this formed the bed for the cast-iron hanging-ring, from which the whole of the tubbing was suspended, and upon this the vertical position of the shaft depended.

The hanging-ring or curb (Figs. 2 and 3, Plate VIII.) was of cast-iron 11 inches deep and 19¾ inches wide in the bed. It was in six segments, bolted together to form a circle 10 feet in diameter inside the flanges. Three out of these six segments had holes, A, sloping downwards (Figs. 2 and 3, Plate VIII.), through which the liquid cement was poured for grouting the course of tubbing below. All the joints were machined, and lead-sheeting 1/8-inch thick was placed between them to form a watertight joint.

The hanging-ring was laid on the foundation with the inner flanges projecting over the edge of the concrete, the top of the ring being flush with the surface of the ground. As a further precaution against slipping, and in order further to spread the weight, two baulks of timber, 20 feet long and 12 inches square, were laid across, one at each side of the pit, and bolted to the hanging-ring.

The sand was then taken out of the centre of the pit for a depth of 5 feet. This was done without difficulty, as there was a concrete lining for a depth of 3 feet. A course of tubbing was then put in, and bolted to the bottom flange of the hanging-ring, strips of lead-sheeting being inserted between all joints, both vertical and horizontal, before the bolts were put in.

The bolts being thoroughly tightened up (Fig. 7), the space at the bottom of the tubbing all round the shaft-bottom was rammed tight with well-puddled clay, and liquid cement was poured into the grouting holes of the hanging-ring above, so as to fill up the space behind the tubbing. Long tapered plugs were driven into the bottom grouting holes, these having a twofold purpose — first, to prevent the liquid from being wasted, and, second, , to preserve the holes for grouting the next course below.

Fig. 8 shows the method of grouting, the cement and sand being mixed in the requisite proportions at the surface and sent down dry in a box to the pit-bottom. Pails of water from the sump were filled up with the cement, and stirred well before being poured into the grouting holes by means of the funnel, as shown in the illustration (Fig. 8).

The mixture of grouting was varied, as required near the surface, where the sand was fairly dry, the mixture was 2 of sand to 1 of cement but in the worst ground, where it was required to set quickly, pure cement was put in.

Little or no difficulty was experienced in putting in the first course of tubbing, as the water was just showing in the pit-bottom ; in fact, the above work was all done by ordinary surface labourers, with a mechanic to direct operations. The only point that need be mentioned was the manner of getting the closing segment of the circle into place while the adjoining ones were in position. This was done on this and subsequent occasions by excavating the shaft-side behind the last segment, thus making room for it to swing endways in behind the adjoining segment, and then bringing it forward into position.

The tubbing (Figs. 2 and 3. Plate VIII.) is of cast-iron, 1 inch thick, with six segments to the circle. It is in two sizes, namely, eight courses of segments, C, 2 feet 5½ inches deep, for going through the sand and worst ground, and eight courses of segments, B, 4 feet 11 inches deep (Fig. 3, Plate VIII). The object of the deeper rings was to save joints, as these, being machined, are costly. Every alternate segment in the circle is provided with a grouting hole, A, just above the bottom flange (Fig. 2, Plate VIII.), which slopes downwards as in the hanging-ring. All the joints are made with strips of sheet-lead, 1/8-inch thick, accurately cut to pattern (Fig. 4, Plate VIII.), each strip projecting 1/8 inch beyond the face of the joint to allow for caulking, which was done after the tubbing was completed. The bolts were all turned 1 5/16 inches in diameter, and to prevent the threads from being damaged, 1/8 inch of it was turned off at the end of each bolt. In order to make the bolt-holes as well as the joints watertight, a conical lead washer (Fig. 4) was put on next the flange at each end of the bolt, and a metal washer (Fig. 4, Plate VIII.), having an inverted cone to fit over the lead one, was put on next to it. When the nuts were tightened up, the soft-lead washer was squeezed between the cast-iron of the flange and the conical space in the metal washer, and formed a perfectly watertight joint.

It will be seen from Fig. 2 (Plate VIII.) that small projecting horizontal ribs were cast on the outside of the tubbing. These formed supports for the tubbing in the cement grouting.

On Monday, July 27th, 1908, the actual sinking was commenced below the first course of tubbing, with four men in the pit-bottom, who began by digging out the sand at one side of the pit, wide enough to get in one segment of tubbing. This segment was made good and bolted to the course above, sand being next taken out for the adjoining segment, which was also bolted up, and so on, until the circle was completed, when the bottom was rammed with clay as before, and cement-grouting poured in through the holes in the course of tubbing above. While excavating for this course, the sand was somewhat difficult to keep back until the segments were got in, especially as the men were all new to the work, but it was finished without mishap by 6 a .m., two days later.

For the next and subsequent courses short slabs of wood, 2½ feet long, were used as piles. These were driven in a sloping direction under the tubbing, so as to keep back the sand until a sufficient depth was obtained to insert a segment, continuing round the shaft until the circle was complete. A great quantity of brushwood was also used for the same purpose, being especially necessary at the open ends of the segment until the next one was joined up against it. When sinking for the fifth course of tubbing, the sand was so soft that the men had to have short planks to stand on to prevent them from sinking into it.

For the first three courses the quantity of water was not great, and was kept down by a hand-pump worked by two men. At a depth of 12 feet a small electric centrifugal pump, having a capacity of 150 gallons per minute, was put in. A sump was prepared for it by having in the centre of the pit a frame, 3 feet square and 18 inches deep, made of battens. This was driven down into the sand as the pit-bottom advanced, and the suction of the pump kept working out of it. It was only found necessary to keep this pump going intermittently, as the quantity of water varied most remarkably. It was noticed regularly, while the men were sinking and putting in the segments, the shaft sides being then more or less open, that the quantity of water increased considerably, and the pump was kept going at its fullest capacity to enable work to be carried on in the pit-bottom; but, after each course was completed and the bolts tightened up, the quantity was greatly reduced, especially after the grouting was put in. This was most particularly noticeable when the Boulder-clay was reached at a depth of about 30 feet, as when this course of tubbing was completed the pit was quite dry, and the pump was temporarily taken out. It was, in fact, so dry that water had to be brought from the river for the purpose of mixing the cement for the grouting. Although the large sinking pump was at hand on the works, it was not found necessary to use it until the tubbing was finished and the pit was being sunk through the stone below in the ordinary manner.

Fig. 1 (Plate VIII.) gives a section of the shaft as tubbed. It will be seen that at the surface a course of deep segments was put in, as the ground was fairly easy, and it was deemed advisable to keep the shallow segments for the worst ground. The eight shallow courses were next inserted, the remainder of the deep segments being put in last. Before the sixteenth course was put in the sinking had reached a good firm stone ; this last course was therefore not quite so necessary, and was not inserted until later. The sinking was carried down for over 30 feet into the post stone, when a walling bed was formed, and the shaft was lined with walling lumps 9 inches thick. This walling was carried up to within about 6 feet of the bottom of the tubbing, and a wooden crib of elm was made to suit and laid on the top of it, being wedged firmly from behind. The last course of tubbing, before mentioned, was then put in, and the joint between it and the crib wedged and made tight.

While the sinking and walling was being carried out below it, the whole length of tubbing remained suspended from the hanging-ring at the surface, entirely without support at the bottom (except so far as it was self-clinging in the grouting) until the walling was joined up below it. The total weight of tubbing with bolts, etc., in the shaft was over 56 tons.

The time occupied in sinking the shaft, putting in grouting, and completing the tubbing to a depth of 55 feet from the surface, was 28 working days, including the time occupied in laying the hanging-ring, but not including a stoppage of 6 days after the tenth course was finished in the Boulder-clay, due to a consignment of bolts being delayed in transit.

Cost. — The total cost of sinking, tubbing and finishing the pit for the above-mentioned depth was as follows :— Materials, £627 2s. 10d. ; labour, £141 0s. 3d. ; total, £768 3s. ld., or £13 19s. 4d. per foot.

The whole of the work was executed without an accident of any kind to the workmen ; and, in the opinion of the writer, the economy and safety of this method of putting in tubbing should make its adoption general in all cases where ordinary wedged tubbing can be used.

The initial cost of the machined tubbing with bolts, etc., is no doubt greater than ordinary tubbing, but this is more than compensated for by inserting the permanent lining as the work progresses, thus saving the cost of both materials and labour for putting in any temporary lining, and also affording greater protection to the workmen below from falls of side. The actual cost of putting it in will also be less than for wedged tubbing.

The only objections to its general use, so far as the writer can see, are :— (1) The absence of a good foundation from which to suspend the tubbing (2) the difficulty of getting in the grouting against high pressure and (3) the difficulty of securing a tight joint at the bottom of the tubbing course to hold the grouting when the pressure is great.