Vertical Section. 134 the first in a similar manner, and the process repeated until the top of the water-bearing, strata is reached, the vertical joints being broken in each course, as in building masonry (Figs. 127 and 128). The spaces between the plates and the sides of the excavation are filled in Horizontal Section. Elevation of Back. JL Fig. 126. ட with soil or concrete packing. A wedging curb will be placed on the top if it is found that the water rises above the level of the last line of plates. All the horizontal and vertical joints are then carefully wedged, as long as the grain of the wood between the joints can be opened with a chisel, commencing at the bottom and proceeding upwards, attacking each ring in order, and plugging up the hole through the centre of each segment at the same time. If this operation is carefully performed it will be found that the length tubbed will be quite dry. A o -B Half Section on AP Figs. 127 and 128. In many instances much time and money is saved by not waiting until the bottom of the waterbearing beds is reached, but putting in wedging curbs at intermediate places and building tubbing up from one to the other, successive feeders of water met with being thus kept out of the shaft. Of course, for the success of this operation, it is necessary that the nature of the beds met with is such as affords foundation for the curbs, but although each wedging curb may not be water-tight during the time of sinking, yet when the pressure of the lower length of tubbing is brought up against it such leakage may be altogether or nearly stopped, and, although each foundation may be bad by itself, yet when they are brought to bear in support of each other, the water may be stopped back. In the Seaham winning* ten successive lengths of tubbing were thus put in, and, although the total quantity of water which the engineers had to contend with at different periods of the operation was 6240 gallons, yet never more than 540 gallons per minute was actually in the pit bottom, this being the maximum amount, the average quantity being 136 gallons. The total amount of water tubbed back was 4880 gallons per minute, which would have been the quantity required to have been raised or pumped to the surface if intermediate wedging curbs had not been inserted. After reaching an excellent foundation in the coal measures, three main wedging curbs were put in as the base of the iron tubbing, and the sinking through the coal measures commenced without a drop of water in the bottom. Messrs. J. J. Atkinson and W. Coulson† were the first to point out the curious accidents which happen to tubbing fixed between an upper and lower wedging curb through the confinement of water and air. It has never been satisfactorily explained how air and gas confined behind tubbing can have a greater pressure than that due to the hydrostatic head, but it is a fact that such is so, and unless some escape is provided, no matter how thick the tubbing is, the inevitable result will be that it becomes cracked or displaced from its seating. To prevent such occurrences, either the water behind each lift is connected with the water behind the other lifts by means of small pipes, and thus, in effect, rendering the whole of the tubbing open-topped through the medium of the uppermost lift, or a pipe is carried up from behind the tubbing to the height necessary to balance the pressure of -a water. As this takes up a large quantity of pipes a short length is sometimes inserted through the tubbing near the top of the lift, and only extended a small distance up the shaft, but a loaded valve is provided at the top, where all the pressure of the water is. This valve discharges the air and prevents the pressure getting higher than is due to the water alone. The more general practice is to place a valve (a, Fig. 129) in the wedging curb, and to carry a length of pipes, b, behind the tubbing to the next wedging curb. After the tubbing has been wedged and plugged the water rises and drives out all the air. When water has been running through the pipe for some hours the valve a is closed. Fig. 129. Strength of Tubbing. The thickness of cast-iron tubbing varies directly with the pressure it has to support and the diameter of the shaft. As the pressure also varies as the depth, if the diameter and the depth are both doubled, the thickness of the tubbing will have to be increased four times. Mr. J. J. Atkinson gives a complete reasoning for the following formula, from which the thickness at any depth can be found: : where t equals thickness in inches, d equals the diameter in feet, p equals the pressure in tons per square inch due to depth, m equals the working load or resistance to crushing of the material employed. Remembering that a cubic foot of water weighs 625 lbs., 12 cubic inches will weigh o434 lb., so that for every foot of depth a pressure of 0434 lb. per square inch is exerted. To obtain, therefore, the pressure per square inch due to any head of water, the depth from the surface in feet is multiplied by 0434. The resistance of cast iron to crushing (average of various qualities) is about 90,000 lbs. per square inch, but to be on the safe side, one-sixth of this amount (15,000 lbs.) is taken as the working load, and should be substituted as the value of m in the formula given above. To the thickness so found, inch should be added to allow for corrosion of metal, and wear and tear. In shafts of large diameters the thickness of the upper segments should never be less than inch, or they are liable to be fractured by blows. In the above formula notice is not taken of the strength imparted by flanges and ribs, which will give additional security. Theoretically, each segment should be different in thickness to the others, but as this would involve considerable expense in casting, the thickness is varied about every 8 or 10 yards. Corrosion. Certain substances contained in solution in water have a very injurious effect on iron, saline matters and chlorides being especially destructive. No satisfactory means have yet been devised for stopping such action, the best preventive, probably, being a coating of a hard varnish applied before the tubbing is seated. The front of the segments in upcast pits, where furnace ventilation is employed, is also attacked by the gases generated by the combustion of the coal. Sulphurous acid is produced, and mixing with water forms sulphuric acid, which rapidly eats away the iron to such an extent that in a few years its nature is completely destroyed, and it gets so soft that it can be cut with a knife. The best and generally used preservative is a lining of fire-brick, a seating for it being made by fixing one of the wedging curbs so that it projects from 3 to 6 inches into the shaft. The great objection to this procedure is that by covering up the face of the tubbing the detection of leaks is made difficult, but of the two evils the lesser is chosen. Cost of Tubbing.-Mr. G. C. Greenwell* gives the following statement of the actual cost of putting in metal tubbing in a shaft 14 feet 9 inches diameter : Cost of wedging curb:— Dressing and preparing bed for curb, and laying same ready Wedging (stone very hard), Wedges (5435 used) and sheeting (material and manufacture), 2 lbs., at 6s. 9d. per cwt.), * Mine Engineering, pp. 166-169. £74 16 8 Cost per yard of tubbing: IO segments to circle, each 18 inches high by 1 inch thick, weighing 4 cwts. I qr. 12 lbs. 6s. 9d. per cwt., = Painting, tubbing, sheeting wedges* (4428 used), and backing with soil, marl, &c., . Putting in and wedging tubbing Putting in, Wedging (twice in going up and once in going down), £28 16 6 416 O 10 9 I I 9 £34 10 6 Shireoaks shafts have more tubbing in them than any others in England-viz., 170 yards put in in eleven lengths, and weighing about 600 tons in each shaft. The internal diameter is 12 feet, and the pressure at the bottom is about 196 lbs. per square inch. Mr. John Jones, the present underviewer, who put in the tubbing, states that the cost per yard of the lower and stronger part, which has a thickness of 1 inch in the body, was as follows: 126 cwts. cast iron, at 78., Fixing and wedging, Wedging curbs and laying (each about 10 yards apart), SINKING BY BORING.-Kind-Chaudron Method.-Looking at the ease with which bore-holes are put down through waterbearing rocks, the idea occurred to engineers that supposing the tools and implements employed were made large enough, it might be possible to bore shafts. Little difficulty was encountered with the actual boring operations, but for a long time it was found impossible to successfully dam back the feeders of water, as no means were at hand to put in a water-tight lining. Cylinders of tubbing were lowered into the pit, but it was found impossible to make a joint at the bottom impervious to water. After many failures the difficulty was surmounted by Mr. Chaudron, by the introduction at the base of the tubbing of what is known as the moss-box, and he, in conjunction with the celebrated bore-master Kind, devised a scheme by means of which numerous pits have been successfully sunk through beds containing a very large amount of water. The boring tools are similar to those ordinarily employed, modified to suit the changed conditions. First of all, a smaller shaft, 4 to 5 feet diameter, is bored, which is kept 50 or 60 feet ahead, and then the main shaft is taken out to the size required. The cutter for the smaller shaft consists of an iron framework (Fig. 130) in the base of which are fixed, in sockets, a number of steel cutting teeth, a, which can be easily replaced if anything goes wrong. This tool is fitted with two guides, b and c, which are also furnished with cutting teeth. When the shaft has been bored sufficiently deep with this tool, a larger one (Fig. 131) is inserted, this differing from the first, not only in its size, but in the fact that the teeth in it are set on an inclined plane, and that the central part is furnished with a loop or guide, a, which fits into the smaller hole already bored. Owing to the shape of the teeth the strata is cut in the form of an inverted cone, and all the * These wedges were 4 inches long by 1 inch on face by inch thick. débris produced falls down the inclined slope into the smaller shaft, in which, at the bottom, is placed an ordinary kibble, which collects the material and renders the use of a sludger unnecessary. These tools are moved up and down by an oscillating lever at the surface, just the same as in an ordinary boring apparatus. A winding engine, drums, and ropes are provided for the rapid removal (during changing) and lowering of the tools. Sinking thus proceeds until the solid foundation is reached, where the seating for the base of tubbing is found. While the shaft is still full of water a water-tight joint is made by the moss-box. This consists of two rings of tubbing (a and b, Fig. 132) which can slide over each other, and each of which has a bottom flange turned outwards and an upper flange turned inwards. These two are strung together by iron tie-rods, c, and the space between them completely filled with moss, so that when the upper Fig. 131. Fig. 132. Fig. 130. one slides down this moss is compressed. Other segments are connected above these two rings, all of which have the flanges pointing inwards. The tubbing consists of cylindrical rings, about 4 feet 6 inches high, cast in an entire piece. There are no vertical joints. A strengthening rib is cast inside each ring, and the top and bottom flanges are turned in a lathe, and bolt-holes bored in them. Before being used each ring is tested by hydraulic pressure in a specially constructed box with from two to five times the pressure it has to support. These rings are put together at the surface with of an inch of sheet lead between the joints, and the whole structure lowered bit by bit by screws and strong iron rods. The chief point upon which successful lowering depends is the |