320. Shafts for tunnels, mines, or other purposes, when sunk through soft or loose strata, require to be lined to prevent the soil from being disturbed or the sides from being forced in.

A shaft is said to be "timbered" when it is lined with wood. Brick-lined shafts are always circular on plan, but timbered shafts are usually made rectangular, and they vary in size according to the purpose for which they are intended. 4 feet square gives the smallest area that men can conveniently work in. A shaft of this size is sometimes used for wells, trials, or ventilation; 6 feet by 4 feet is, however, a usual and more convenient size. Winding or working shafts require to be larger; 6 feet by 9 feet to 10 feet square is a usual size for them, and they are sometimes made larger, particularly when they are to be used for the purpose of ventilation as well, in which case they are divided into "upcast" and "downcast" compartments by air-tight divisions called " brattices." The lining, or " cleading," of a shaft consists of boards, as A Figs. 97 and 98, from 6 inches to 9 inches wide, which are placed against the sides and retained in their places by horizontal frames of timber B, B, called " settings."

Fig. 97.

Fig. 93.

The distance apart of these frames is regulated by the nature of the ground passed through, and their scantling will depend on the size of the shaft and on the nature of the ground, Very soft ground requires stronger frames than merely loose or friable ground, which does not exert so much pressure against the sides of the shaft.

Three feet is the least distance apart the frames are likely to be required even in the worst soils, but it should never exceed 6 feet.

For very small shafts in average ground the scantling of the frames should not be less than 4 inches square. In large shafts 7, 8, and 9 inches are usual sizes. Cleats are used to keep the sides of the frame together, as shown by Figs. 97 and 98. The thickness of the cleading A depends on the distance apart at which the frames are set: when they are about 3 feet apart, 1 to 1 1/2 inch is a usual thickness, and when about 6 feet the cleading requires to be 3 inches thick. The former is, however, the safest to adopt in treacherous ground.

The frames are kept apart at the regulated distance by vertical props E, E, Fig. 97, varying according to the weight to be supported, and the length of the props, from 4 to 6 inches in diameter.

To prevent the tendency which the timbering has to slide down the shafts, strips of timber, F, F, Figs. 97 and 98, called " stringers," are spiked to all the frames from top to bottom in succession, and the whole secured to two balks of timber, C, C, Fig. 98, laid on the surface of the ground, one on each side of the shaft. During the sinking of the shaft the last frame put in is supported by raking props, as G, G, Fig. 97.

The method of executing the timbering of a shaft 10 feet square for a coal mine is thus described by Mr. Greenwell: - " Two strong balks of timber, say 12 or 13 inches square, are laid upon the site of the shaft. These balks serve for a purpose shortly to be mentioned. The ground is then dug out to the depth of 3 feet, of such size as to allow the next frame to be laid. After this is done, the inch deals, 6 feet long are set up behind the frame, the bottom ends of the deals passing down half its thickness, and these deals will rise perpendicularly 3 feet above the surface of the ground, when a light frame, placed within them at the top, will keep them in their places. After this has been done, 6 feet more in depth of ground is taken out and another frame laid, and another 6 feet length of deals put in, also descending half way down this third frame, and of course half-way up the second frame put in, and meeting the bottom of the first length of deals. Then another frame is placed midway between these, and a row of props placed between each set of frames keeps them all level and in their proper places.

"When this has proceeded some depth, and the ground for a still further length is necessary to be taken out, there becomes a tendency for this timbering to slide down, and the balks first named come into use. Planks, called stringing planks, are then spiked to these balks, and also to all the frames, from top to bottom, thus hanging the whole. This may advantageously be done with chains when the whole pressure is downwards. Sometimes, however, it is upwards, although in all cases where this either occurs, or is in the least expected to occur, circular timber is almost indispensable. The entire system to the "stonehead" is a mere continuation of the above.

"The reason why, in the first length, 3 feet of it rise above the surface, is to allow of tip room for the rubbish down to the stonehead."*

Mr. Greenwell further observes that where the surface strata, when of clay, etc, is wet, the above mode of timbering will not be found sufficiently strong, but would be almost certain to collapse at the sides with the pressure. In this case he recommends circular cribs to be substituted for the frames before mentioned.

The internal diameter of the cribs for an 8-feet finished

* 'Lectures delivered at the Bristol Mining School in 1857.' pit should be 9 feet, and the size of the timber in the cribs being, say, 6 inches square, would allow about an extra foot all round for the walling, with which permanent shafts are frequently lined, the timber being taken out as the masonry is put in.

The mode of putting in these cribs is the same as that of the square frames. The cribs are prepared as follows: -

An accurate floor of wood is prepared, 11 feet in diameter (for a shaft of 8 feet diameter in the clear), in the centre of which is fixed a pivot, on which a radius of 6 feet moves freely, and at 4 1/2 feet from the pivot is fixed an iron point, the use of which is to describe upon the floor a circle 9 feet in diameter.

At short intervals upon this circle spikes are driven in, leaving an inch of their upper part projecting upwards.

The object of this is to form a rest to the inside of the segments during the operation of forming their joints.

The timber from which the cribs are to be cut is divided by being sawn into planking, 6 inches in thickness, and then, by means of a template set to the circle of the shaft or pit, each plank is as economically as possible cut out into as many segments as it will yield. Although it is evident that the inside of the ring, by being the segment of a circle of 9 feet in diameter, will not correspond with the outside, which is that of a circle 10 feet in diameter, yet it is sufficiently near, and thus the same cut that serves for the inside of each segment serves for the outside of another one, or, in fact, the same template will make all the segments. Several segments having been sawn out in lengths varying from 2 to 4 feet, one of them is laid upon the floor, with its inner side resting against the nails inserted as above mentioned, and being pressed close to them two or three other nails are driven into the floor, on its outside, so as to keep the piece of wood firmly in its place. The radius rod is then applied to the piece, and as large a segment marked out of it with the point of a nail as will leave a 6-inch joint at each end. The end pieces are then sawn off. Another piece of wood is similarly treated and placed end to end with the first; then a third, and so on, until the circle is completed.

Before putting in the last or closing piece, however, it is better to pass a saw through each joint in the line of the radius of the circle, so as to make them more true, and to drive up the pieces as close as possible.

Each circle of cribbing may be sent down the pit in three; parts, so many of the short segments as are necessary being nailed together by means of top and bottom cleats of inch-deal, as will make a segment of about one-third the size of the crib. Each of these thirds to have at one end a top-cleat, and at the other end a bottom-cleat, nailed one-half upon it, and the other half projecting, with the nail holes already bored in them so as to allow of their being nailed to the adjoining segment without any delay.

There are some descriptions of soil where even these cribs would be crushed, such as quicksand, which can only be got through safely by iron tubing or strong brick steining.

Fig. 99.

Fig. 100.

321. Figs. 99 and 100 show the method of timbering a " heading," or gallery, which is usually performed with rough timber. Sole pieces C, C, from 4 1/2 to 6 inches in diameter, are laid on the ground or floor of the heading. Short pieces of rough boards A, A, called "poling boards," about 1 or 1 1/2 inch thick, are placed against the roof of the heading, underneath which, so as to take the ends of the poling boards, balks of timber B, B, of the same diameter as the solo pieces, are placed' and firmly wedged up by the props D, D. To prevent the props from being knocked away or slipping off the sole pieces, spikes, called "brobs," are driven into the sole pieces around the ends of the props.

When the sides of the heading are likely to fall in, poling boards, similar to those on the top, are fixed against the sides, as shown by Fig. 99.

322. Fig. 101 shows the method usually adopted for timbering the sides of trenches and narrow excavations. Poling boards A, A, about 3 feet in length, 1 to 1 1/2 inch thick, and from 7 to 9 inches wide, are placed against the sides of the excavation as each successive depth of 3 feet is excavated. An ordinary deal plank B, 3 inches thick and 9 inches wide, called a "waling," is placed longitudinally against the middle of each row of poling boards. The planks on opposite sides of the trench are kept in their places by struts C, C, laid across and tightly wedged against them. The struts vary in size from 4 inches square to 6 or 8 inches square, according to their length and the pressure they have to sustain.

This mode of timbering can be executed as the work proceeds, and the timbers can be readily taken out in parts when required.

323. When the soil is so loose that the sides of the excavation cannot be upheld until a sufficient depth is dug to admit of the insertion of the 3 feet poling boards, long planks, called "runners," usually of deal 3 inches thick and 7 or 9 inches wide, pointed and sometimes shod with iron at one end, are placed behind the walings instead of the poling boards. The walings in this case require to be of stout timber, frequently as much as 12 inches by 6 inches, and the struts should be proportionally strong.

Fig. 101.

The runners should be constantly driven as the work proceeds, and their points should always be kept about 12 or 18 inches in the ground below the bottom of the trench, to prevent the sides from falling in and to permit the excavation to be carried to the required depth. These runners are, however, difficult to draw out afterwards, and they have sometimes to be left in, which of course adds to the expense.