Page images
PDF
EPUB

plumb-bob. The instrument usually employed (Fig. 80) is made

Fig. 80.

of deal boards, 1 inch thick. The foot-piece is 6 feet in length and 4 inches broad. The vertical piece is 3 feet in length, and 6 inches broad at the lower end, tapering upwards until, at the top, it is 4 inches broad. In addition to the plummet, there is a small spirit-level fixed to the foot-piece to serve as

a check. The instrument is placed on the floor of the level, and if the latter is being driven truly horizontal, the plummet will hang in the hole made in the vertical piece for its reception, and at the same time the bubble will be in the middle of its tube.

The main horse-roads in collieries should be driven with a slight inclination towards the shaft, so that water may flow from the workings to the sump. Experience has shown that an inclination of 1 in 130, or a little more than inch in the yard, gives the most advantageous effect in drawing by horse-power the loaded waggons towards the shaft, and the empty ones back.

Fig. 81.

Care must be taken that this inclination is not exceeded, as, in driving levels, there is always a tendency to rise too fast. For maintaining the required inclination, a piece of board,

inch

thick, should be screwed to the foot at the end of the instrument, which is nearer the shaft.

In order to test the inclination of an underground roadway with an inclination of 1 in 5, Mr. W. Wardle uses the instrument shown in Fig. 81. It is provided with two sole-pieces, the bottom of the lower one being at a distance of 1 foot from that of the upper one at one end, whilst at the other end it is brought to a feather edge.

An ingenious clinometer, invented by Colonel G. P. Evelyn (patent 1885, No. 1964), may be advantageously used for setting out levels at any inclination. It consists of a curved tube filled with water or dilute spirit, on which floats a small bubble of compressed air. Adjacent to the tube, and concentric with its outer periphery, is the graduated arc of a circle. When the airbubble is at the zero point of that arc, the base of the stand, in which the tube is mounted, is horizontal, and any inclination from the horizontal is shown in degrees by the position of the bubble on the graduated arc. The tube is easily filled and emptied, and the size of the bubble is regulated by a screw-cap fitting over the cork.

To drive a level straight at a given bearing, plumb-lines are suspended from points in the roof previously fixed by the dial or theodolite. These lines indicate the direction in which the level has to be driven, and should be placed 30 to 60 feet apart.

Curves for Engine Planes.-Curves may be set out underground by means of a theodolite on a short tripod, and candles or lamps instead of ranging poles.

The direction in which curves for engine planes should be set out is sometimes roughly ascertained by making a careful survey and plan of the pillars and headings, through which it is required to drive the curve. The survey is plotted on a large scale—e.g., 16 feet to an inch. The curve drawn on the plan is divided into equal distances marked by points. The latter are then connected by dotted lines. By means of a protractor, the bearing of each of these lines is determined. For greater accuracy, offsets are measured at every 6 feet on each side of the lines to the sides of the curve. In this way data are obtained from which the curve may be set out.

Setting-out Tunnels. The centre-line of the tunnel having been ranged on the surface of the ground, a series of shafts are sunk from 100 to 200 yards apart along that line. In order to transfer the ranging of the line from above to below the surface of the ground, it is necessary to have two marks, consisting of nails driven in the cross-timbers in the centre-line at the bottom of each shaft as far apart as possible, to enable the line to be pro

longed from the bottom of the shaft in both directions. To determine the positions of the marks underground, a rangingframe is erected over the shaft. It consists of three half-timbers framed as a triangle and supported at the angular point by stout props. From the frame are suspended two plumb-lines, which are ranged by a transit-instrument.

As this process cannot be satisfactorily used except in calm weather, Mr. F. W. Simms introduced the following modification:-By means of the transit-instrument, the engineer ranges two stakes in the centre-line at the surface, each being about 16 feet from the centre of the shaft, so as to be safe from disturbance while the work is in progress. To mark the exact position of the centre-line a spike (Fig. 82) is driven into the top of each stake. The hole of each spike is carefully ranged in the centre-line, a piece of white paper being held at a short distance behind it, so as to render it visible to the observer at the telescope. A string is stretched centrally across the mouth of the shaft, and its ends are passed through the holes in the spikes. It is then drawn tight and made fast. At each side of the shaft a plank is fixed at right angles to the string, and so placed that one side hangs over the shaft about 3 inches, so that a plumb-line may hang from it without coming in contact with the side of the shaft. A plumb-line being hung from each plank directly under the cord marking the centre-line, the lower ends of these plumb-lines represent two points in the centre-line at the bottom of the shaft.

Fig. 82.

The approximate ranging of the heading connecting the lower ends of the shafts is effected by means of candles, each hung from the timbering in a sort of stirrup. The upper portion of the candle-holder (Fig. 83) employed by Mr. Simms is made of thin

Fig. 83.

sheet-iron with a number of holes in it. The lower portion is of iron wire, carrying a socket for the candle. By means of the rack, the latter can be raised or lowered to the proper level, and being hung by a flat plate, it is prevented from rotating.

The accurate ranging of the centre-line, when the heading has been made, is effected by stretching a string between the marks already ranged at the bottom of the shaft, and fixing, at intervals of about 40 feet, either small perforated blocks of wood carried by cross-bars, or stakes with eyed-spikes driven into their heads, so that the holes may be ranged by the string exactly in the straight line.

This method was employed by Mr. Simms for setting-out the Blechingley and Saltwood tunnels on the South-Eastern Railway. Both these tunnels were straight from end to end, as is generally the case. Their centre-lines were ranged with a transit-instrument of 30 inches focal length with an object-glass of 23 inches aperture. In order to command a view of every shaft, the instrument, mounted on a cast-iron stand, was set up on the highest point of ground as near the middle of the tunnel as possible, and raised above the surface by the erection of a temporary observatory. This consisted of a building of larch poles, in the centre of which was a brick pier 30 feet in height for the support of the instrument.

When the length of the tunnel is not very great, the transitinstrument and temporary observatory may be dispensed with, and the 6-inch or 5-inch transit-theodolite used with advantage. This was done in setting-out the Clifton tunnel in 1871 to 1874. This tunnel is straight, on an incline of 1 in 64. It is 1737 yards in length. At a distance of 276 yards from the lower end of the tunnel, where it approaches to within 140 feet of the Blackrock Cliff, a side drift was opened from the face of that cliff to the line of the tunnel. The tunnel was driven from this drift, and from two shafts sunk 998 yards and 4.63 yards farther on.

When the tunnels are of great length, and can only be driven from the ends, the setting-out is much more difficult than when shafts can be sunk along the line. The direction of the axis of the tunnel is determined by a traverse or a triangulation connecting the two ends. In very long tunnels, such as those of the Alps, traversing is not sufficiently accurate, and recourse must be had to triangulation, as was the case at Mont Cenis and St. Gothard. The Mont Cenis tunnel, upwards of 6 miles in length, was set-out from both ends. The junction was effected without any error horizontally, and with only a foot of divergence vertically.

Remarkably accurate results in tunnel alignment were obtained in 1888 in the Croton aqueduct at New York. The points of commencements of two headings were 6,400 feet apart, the one being 270 feet and the other 353 feet below the surface. The diameter of the heading was 16.5 feet, The direction was obtained by means of two plumb-lines, 16.5 feet apart, let down each shaft. When the two headings approached each other, the final connection was made by two drills meeting in the same hole from opposite sides of the rock, and after the blast had been fired, it was found that the error in grade was 0.014 foot, and in alignment 0.09 foot.

In New South Wales, a very successful alignment was effected

by Mr. T. W. Keele* in the construction of the Nepean tunnel, a conduit for supplying Sydney with water. The tunnel is 23,507 feet long, the bases at the east and west ends being 254 feet and 212 feet respectively, situated at the bottoms of precipitous limestone gorges. There were six shafts, admitting of only 12-foot bases, the depths varying from 210-5 feet to 324 feet. The length between shafts Nos. 2 and 3 was 4,341 feet, the headings meeting at a point 3,018 feet from shaft No. 2. The error in alignment was ğ inch, and in grade inch. The tunnel is 7 feet high and 9 feet wide, and is inclined at the rate of 21 feet per mile.

Points

The line was transferred from the surface to the bottom of the shafts by plumbing. At the shafts brick pedestals were erected, one on each side, on the centre line, and about 50 feet apart, the tops being a foot above the shaft platform. were then accurately established on each, and a steel wire, 0·02 inch in diameter strained, at its utmost tension, from point to point across the shaft. The process of plumbing down the shaft was then proceeded with. An 8-inch transit theodolite on its centering legs was then set up in one of the headings, and the intersection of its cross-wires brought into coincidence with the line as given by the plummets. After the instrument had been adjusted to prolong the line into the heading, a hole was drilled in the roof and a wooden plug inserted; and on this the point was obtained by sighting on to a plummet lamp, of the type used in Pennsylvania, suspended from it. In order to give the levels in the tunnel, the value of a bench-mark at the bottom of a shaft was ascertained by measuring the calculated distance from the surface with a steel tape; and the levels were run into the headings. At intervals of 100 feet, hooks in pegs in the sides of the tunnel, and opposite to each other at right angles to the line, were so adjusted that strings stretched through them were exactly 23 feet above the grade. The plummet lamps, hanging from the centre-pegs in the roof, being then lowered until their lights were even with the horizontal strings, the axis of the tunnel was determined, and the miners were provided with both line and grade. All that they required to do was to place a candle at the face in line with the lights from the plummet lamps, and measure down 2 feet 9 inches to find the grade of the invert. Bench-marks were established at intervals

of 500 feet, and were frequently checked.

The lengths of the headings and the results of the alignment, when the junctions were effected, were as follows:

*Min. Proc. Inst. C.E., vol. xcii., 1888, p. 259.

« PreviousContinue »