Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/507

 June 14, 1872. THE BUILDING NEWS. 485 NOTES ON EARTHWORK.—VIII. LOCOMOTIVE engine on rails cannot draw its due load up steep inclines, and in many parts of this country a long line of railway cannot be made without cutting deeply through hills. When the depth much exceeds 100ft., a tunnel can generally be made at a less cost than an open cutting, especially when the ground is such that it willnot stand with steep slopes. Tt is calculated that in the United Kingdom about 80 miles of railway tunnelling have been already made, the greatest length of any one tunnel being alittle more than three miles. It isnot the length, however, that is the greatest difficulty to be overcome. Very long tunnels are generally through hard rock, to get through which requires certainly more time, but less ingenuity, and calls for less risk of life or limb than is necessary in some other kinds of ground. Considering the size of the excavation required to be made in bad ground—nearly 30ft. in height, and about the same in width—to make such an excavation through clay or loose ground, great exertions must be made, and many precautions taken, to ensure success, and especially where much water is met with. Much water, however, is perhaps more often met with in the rock tunnels. Foul air, chiefly carbonic acid gas, is met with in passing through coal-beds and the contiguous strata, and in old coal hollows, from which the coal has been removed. Otherwise, the foul airthat causes the greatest inconvenience in tunnelling is that produced by exploding powder in excavating the tunnel. ‘The shafts are generally sufficiently numerous to give good ventilation after the heading has been driven through, and until then a circulation of fresh air is kept up by blowing it down the shafts with fans, through sheet iron pipes or wooden trunks. As the number of tipping places that can be arranged at the head of an embankment governs the time required for the completion of an open cutting, so does the number of shafts that can be sunk govern the time in which a tunnel can be driven. Shafts are usually from 100 to 200 yards apart. In ordi- nary ground they should not exceed 100 yards apart if the work is to be completed in twelve months, but more often the distance is 200 yards apart, and sometimes more, owing to the great cost of sinking deep shafts, the use of which, moreover, for the most part, ceases when the tunnel has been opened. For the sake of opening the line for traffic at the earliest possible moment, however, a few extra thousands spent in shaft-sinking may be justifiable enough. It is only a question whether the traflic will repay the outlay, and as the presumption is that the traffic will repay the main outlay, it would seem to be true policy to ensure that the tunnels shall be completed by the time the other works are completed, at whatever cost almost, although it has often happened that the opening of a line otherwise ready has been delayed because a tunnel was not so. The line of a tunnel is sometimes curved, but is usually straight. The line of a straight tunnel is set out with the transit instrument, the telescope of which is mounted on high standards, so that it can be turned completely round on its axis. For accuracy, it is neces- sary to have the instrument firmly fixed ona solid bed of masonry, and as there are usually obstructions to the line of sight, caused by erections at the mouth of the shaft, the transit instrument is elevated on a pier of masonry, sometimes 30ft. or more, which is surrounded by an outer casing or weather shell of poles and boarding, entirely separated from the masonry tower, so that the vibration caused by wind cannot be communicated to the in- strument. ‘The use of these erections ceasing before the completion of the tunnel the ma- terials are used up in the works. When the tunnel is curved, however, a portable theodo- tangential angles. The curve is not difficult to set out on the surface, but below it is so. There are difficulties also with the centreing of the arches; but, perhaps, the most im- portant objection to a curved railway tunnel is that the engine drivers cannot see the ends of the tunnel in passing through it. Some of the shafts of a tunnel are usually left open for ventilation, but the number re- quired for this purpose is seldom so many as are required for the due expedition of the work, and working shafts, in addition to the permanent shafts, are therefore sunk for the latter purpose and filled in again. The per- manent shafts require to be lined with masonry—steined, but the sides of the work- ing shafts, except in rock, are sufficiently supported by timbering. About 9ft. square is a usual dimension, but the proper size depends on the size of the skips intended to FIG.9 FILAG N i” * “yh (GCG . SS NY ll LAL LZ LLL LE YY: LL SECTION Fc LONG* SECTION CRISS_SECTION be used, and this, again, upon the power to be employed in winding; for the shaft should be large enough to allow one skip to be drawn up at the same time that another is being lowered, and to pass each other with a good clearance. 9ft. allows clearance for skips 2ft. Gin. square at the top. Where the ground in which the tunnel is driven throughout is so hard that but small progress can be made in a given time the skips need not be so large, for there will be a comparatively small quantity of material to be brought up daily, and the size of the shafts can be diminished accordingly ; but, on the other hand, the rate of progress may be such as to oblige a large quantity of stuff to be sent up the shaft at each turn, and then 9ft. might not be large enough. But 8ft. square is not an unusual dimension. Fig. 9 represents the vertical section of a timbered shaft, and Fig. 10 lite is necessary with which to set out the } the plan. After the shaft has been sunk to a depth of about 7ft., horizontal bars are laid along the sides, the ends of two of them being notched to receive the ends of the other two, in such a manner as to bring the pressure of the earth behind one pair to bear longitudi- nally against the other pair. The dimensions must be greater or less according to the nature of the ground. Whole timbers 12in. or 13in. square are in some cases necessary, while in others a dimension of 6in. or 8in. is sufficient ; but round timber does very well; larch of 8in. or 9in, diameter being often used. Behind the horizontal bars a sheeting of battens or deals is placed against the sides of the shaft. The battens or deals are cut to a length of about 6ft., and their tops are secured by inserting another set of horizontal bars similar to the first. To support these, props are set up at the angles, resting upon the first set of bars. To secure the head and foot of each prop, 6in. spikes, made with a head turned in one direction only, are driven into the struts. They are thus capable of being drawn out by inserting the end of a small crow-bar under the head. To hold up this first set of timbering while another is being put in below, raking struts are inserted under the lower set of bars, by cutting a trench across the bottom, sloping down to the centre from each side, the depth of which at the centre should be sufficient to allow the excavation to be carried down far enough to get in another set of horizontal bars, say 6ft. down. When the raking struts are got in the remainder of the earth can be excavated down to the level of the centre hole, and another set of bars put in, behind which and the bars above another set of sheeting can be inserted, and props set up and secured as before. In rock that will stand without timbering the shaft is sunk circular, and both sorts of sinking sometimes occur in the same shaft. The head frame which supports the pulleys over which the winding ropes are carried should be of considerable height above the ground in order to allow a bank to be formed around the mouth of the shaft, for, consider- ing the large quantity of earth that must be dis- posed of, itisadvantageous to have it delivered at such a height above the surface that it can be easily taken away. For sinking the shafts and driving the headings, however, a common windlass is often used, the horse-gins being set up only on the commencement of the tunnel itself, except when large quantities of water have to be drawn out. Water barrels are hung from the bale rods a little below the centre, and secured to the bale rods by a catch at the top, so that by releasing the catch they are easily turned over and emptied. The centre line being set out on the sur- face a mark is made on each side of the shaft in that line, and the corresponding points at the bottom found by plumb lines and marked on the roof of the heading. From these marks candles are suspended from time to time, and the workmen in each heading guide themselves in the true centre line of the tunnel by bringing another candle in line with the two suspended at the bottom of the shaft. After the work has proceeded some distance other permanent marks are correctly aligned from the shaft forwards, which are used in a similar manner. Before the tunnel is excavated to the full dimensions a small heading 5ft. or 6ft. high and 4ft. or 5ft. wide is pushed forward each way from the shaft to get the line, and from which to commence the widening out of the excavation to its full dimensions. This widening out is done in short lengths—9ft. or 10ft. in bad ground and 15ft. or 20ft. in good ground, The tunnel being thus constructed in “lengths” different parts of it take appro- priate names. ‘The length immediately under the shaft is the shaftlength. The first length on each side of the shaft is the side length, and each succeeding length the leading length for the time being, until another can be got in,