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this point and find the difference between it and formation level, which will be the depth of cutting or height of bank at that point. From this depth or height recalculate the slope width, which is as before half formation width plus the slope due to the height or depth as given by the level. If this agrees with the actual width from the centre line to the point where the staff was held, then that point is the correct top or bottom of slope, and the peg may be there driven in. If not, the operation must be repeated until a point is found which is correct both as to level and corresponding distance out from the centre line. Much depends on the judgment of the engineer, but with a little practice the first or second trial will usually be sufficiently close unless in the very roughest description of ground. Scaling off the cross sections may be of assistance for the first trial point.

Setting out Bridge and Culvert Foundations.-The following is the best practical method of setting out the bridge foundations shown in Fig. 177. Drive in a peg at o, the centre of the bridge, and setting up the theodolite over this peg, range in chaining arrows at as many points as necessary on the line OA, ob, so that a cord may be stretched between them. The lines aob, COD are the centre lines of the road or stream and of the railway, respectively. On rough ground several pins will be required to enable the cord to be stretched out in the straight line. Set off OC, OD at right angles to OA, OB, and similarly range in chaining pins and stretch a cord on this line. The engineer should be provided with a tracing of the plan of the foundations of the bridge, as shown in Fig. 177, and should have drawn on it the co-ordinates of each of the angular points a, b, c, d, e, f, g, h, referred to the centre lines AOB and COD. Thus the co-ordinates of the point a are 9a and 1a; of c, 4c and 7c, and so on. The distances 07, 08, 09, 010, 011, 012, 013, 014, are to be marked on the tracing, and these are measured along the cord line COD, and chaining pins are inserted at the points 7, 8, 9, 10, 11, 12, 13, 14. Similarly the distances 01, 02, 03, 04, 05, 06, along the line AOB are marked on the tracing, and these are measured and chaining pins inserted at the points 1, 2, 3, 4, 5, 6. Now take two tapes, and putting the rings together, direct the chainman to pull them tight while the distances 9a and 1a are read on the tapes held at 9 on the line cOD and at 1 on the line AOB re

spectively by the engineer and his assistant. fixes the point a, and a peg is then driven in. are all similarly fixed by their co-ordinates, and each point.

A

The cord lines should now be taken up and passed round the periphery of the figure cabdfhgec, and a rut dug along this line. The other abutment is treated similarly, the pegs being of course

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The other points pegs driven in at

S

9

Fig. 177.-Setting out Bridge Foundations.

put in at the same time as those for the first abutment, and before taking up the cord lines. The outline of the excavation for the foundations of the bridge is thus clearly defined on the ground, so that the foreman can make no mistake in his digging.

On one of the pegs at the end of each wing as c and d, a level should be taken, and a note of the depths from the pegs to the bottom of the excavation should be given to the foreman. Levels on the ground at all the pegs should also be taken for the sake of

getting the correct quantity of excavation. When the bridge is on the skew the procedure is the same, the lines AOB, COD being set out at the proper angle with each other.

Setting out Culverts.--Usually only type drawings of culverts are furnished, showing the cross section of the culvert and the plan and elevation of the front and wing walls, and it devolves upon the engineer to calculate the length of the culvert and make a correct plan of it on the basis of the type drawing.

The first thing to do is to take levels in the bed of the stream for which the culvert is intended, and decide upon the invert level at each end. From the invert level we get the level of the top of the outside of the arch (the extrados at the crown), and the difference between this and the formation level of the railway is the height of bank above the culvert. Half the length of the culvert is then equal to half formation width plus the slope as calculated from the height of bank above the culvert, plus any allowance for face walls, &c., according to the type drawing. When the culvert is on the square, a rough sketch may usually be made from these figures sufficient to set it out, but when on the skew it may be necessary to draw out a plan of it. Having got out the plan, the process of setting out is exactly similar to that described for setting out a bridge.

Setting out Bridge Abutments. When the foundations of a bridge have been laid in, the corners of the abutments should be accurately set out with steel tapes. This is done by co-ordinates in the same manner as already described for setting out the foundations. When one abutment has been built or set out the corners of the second abutment should be set out by measuring from one centre line only, the span as measured from the first abutment being used as the other co-ordinate. This will ensure the span being correct.

Other Setting out: Centre Lines and Levels in Cuttings and Banks. During the progress of construction the engineer will continually be called upon to give centre lines and levels in the cuttings and on the banks as the work proceeds. Having all his tangent points transferred, he will usually have no difficulty in doing this, the work simply consisting in setting up the theodolite and ranging in a few pegs on a straight or turning

off a few angles on a curve. The levels required are put in by levelling from the nearest bench mark; see also To Locate a given Level, page 253.

"Boning Rods."-When a cutting is started the foreman is given a note of the depths at each peg, but when the cutting is deep and there are lifts or benches in the face he is unable to measure down these depths after the cutting has gone in a little way. The engineer must therefore give him two pegs, I or 2 chains apart, driven in so that their tops are at formation level. The foreman places his boning rods (see page 161) on these pegs, and bones in his levels in front as the work proceeds. Level pegs must be put in as often as necessary to keep them within a chain or two of the face of the excavation.

"Crossheads."-In the case of embankments the foreman is given a note of the heights at each chain peg, and in this case he puts up what are called "crossheads" to indicate the height of the bank. A "crosshead" is simply a post with a crosspiece about 2 ft. long nailed across the top of it, and at right angles to it. The " crosshead" is erected so that this crosspiece is at the given height above the chain peg plus 3 ft., the height of the boning rod. Having erected two or three crossheads, by using his boning rod, which is 3 ft. long, and sighting on to them, the foreman is enabled to keep his bank to the correct level.

Allowance for Shrinkage and Settlement. In banks tipped with soft material an allowance of about 1 in. per foot of height must be added to the height of the bank to allow for shrinkage and settlement. In high banks this allowance becomes excessive, and a smaller percentage must be added. The allowance to be made of course depends on the nature of the material, and is a matter of experience.

Bench Marks and Checking through Levels. Before doing any setting out on a railway for construction purposes the levels should be carefully checked through from end to end of the railway, and bench marks established not more than mile apart and at each bridge or other important work.

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To Locate a given Level.-In order to put in a peg or mark a point at any given reduced level it is simply necessary to

deduct this reduced level from the height of the instrument, and the difference is the "staff reading." The peg has then to be driven in until this reading is observed, when the staff is held on it. In the case of making a mark at any given reduced level on a wall, &c., the staff is to be placed against the wall and moved up or down until the correct "staff reading" is observed. The mark is then made at the foot of the staff.

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