C.—3

80

Circular shafts in ground which does not contain an excessive amount of water are generally lined with brickwork or masonry, which may be dry or set in mortar, according to circumstances, and is made of sufficient thickness to withstand any pressure likely to be brought on it. Shafts lined in this way are generally timbered temporarily until a suitable place is reached for a foundation. When this point is reached a bed is prepared for the foundation, and a curb of timber or cast iron is laid on this bed to form a level seating for the brickwork, which is built upon it to the top of the shaft. Excavation is then continued, leaving a bench of rock to support the lining until another suitable foundation is reached, when a fresh curb is put in, and brickwork is carried up on it until the previous length is reached, the bench of rock being removed in small sections and replaced by brickwork. If small quantities of water are likely to find their way into the shaft, garland curbs are used —that is to say, curbs which project from the brickwork and form annular gutters which intercept water running down the sides of the shaft, the water being taken away from the curb by a pipe leading to a tank or lodgment. If the ground is not sufficiently strong to support the different sections of brickwork or benches, the lining may be suspended by iron rods from a strong frame on the surface until a firm foundation is reached. If large quantities of water are met with a brickwork lining takes the form of " coffering"—that is to say, a brick and cement walling backed with puddled clay. In a shaft at the Plas Power Colliery, near Wrexham, North Wales, the pit was lined with a 9 in. dry-brick wall on wooden curbs as excavation was proceeded with until a suitable foundation for the coffering was reached. At this point a cast-iron wedging-curb was put in on which to carry up the coffering. This wedgingcurb consists of a flat hollow cast-iron ring formed of two plates connected together by vertical webs, and made in segments of a convenient size. Thin sheets of wood are put in between the segments, and wedges are driven in between the back of the curb and the ground until there is no room for any more. Then holes are made in this wood packing by means of iron spikes, and as many hard-wood wedges as possible are put in. Sometimes two or three of these wedging-curbs are used, one on top of another. At the Plas Power Colliery, when the wedging-curb was put in, three courses of brickwork set in Roman cement were built up on the curb and the wedges behind it, wooden plugs with holes through them being put in at intervals to let out the water from behind, and vertical wooden pipes, perforated every 3 in. down their length, were put in against the dry-brick wall to lead the water to these plug-blocks. The coffering, a section through which is shown in Fig. 3, consisted of three rings of brickwork, and was laid in hydraulic lime. At every three courses bricklaying was stopped while puddled clay was put in between the

Section of "Coffering."

coffering and the dry-brick walling, the holes in the wooden pipes being plugged as the clay rose. The two outer rings of bricks were first laid, leaving a space between them of s|in. The third ring was placed centrally in this space, and hydraulic-lime grout was run into the spaces, so that the central ring of bricks had a continuous sheet of hydraulic lime on each side of it. When the coffering had risen so high that water ceased to accumulate behind it, the wooden pipes were filled in with stones, and hydraulic lime was run in. The coffering was then completed, and the holes in the plug-blocks were filled in with wedges. In the coffering of No. 2 pit at Llanbradach, South Wales, there was no outer wall of dry brick, the coffering consisting of eight rings of brick, all breaking joint vertically, except the two inner ones. The whole was set in cement mortar, and was backed with cement instead of puddled clay. Coffering has this advantage as compared with cast-iron tubbing, that it is much cheaper; but at the same time it is subject to the disadvantages that it requires much greater care in construction, and cannot be easily repaired. Cast-iron tubbing consists of rings of cast-iron segments, smooth inside, but with external flanges and stiffening ribs, as shown in Pig. 4. The segments are usually Ift. to 3 ft. high and about 4 ft. long, and are provided with a hole in the centre to permit the escape of water until the lining is complete. The foundation consists of one or more cast-iron wedging-curbs, and the tubbing is built up on these ring by ring, thin pieces of deal being placed between the joints. Such tubbing is frequently built up in short lengths with wedging-curbs between them, and as soon as a length is completed the joints are all tightly wedged and the central hole plugged, thus—