Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

Fig. 262.

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Assuming that the normals have been put in, set up the instrument at any intermediate peg in such a position that the telescope when set to the angle of the slope shall line with the top of the peg, as per sketch, Fig. 262. Then release the telescope from the vertical arc and direct it to one or other of the nearest normals, adjusting the cross web to cut the top of the peg by turning the instrument on its axis. Clamp in this position and the telescope will revolve in the plane of the slope, and any point on the intermediate surface intersected by the cross web is the outcrop of the slope and the position of the peg. When all the pegs on the one side of the instrument are put in, turn the telescope to the other side to cut the next normal and proceed in the same manner. When all the pegs have been put in for this half mile distance, the instrument may be moved to the next half mile normal and the operation repeated, until the whole cutting or embankment is completed, the last normal point being in all cases the formation width at the ends.

In speaking of half-mile distances we are assuming the most favourable conditions of surface and application of the method, but in practice where the surface is undulating the positions of the normals should be at the most elevated points from which a considerable range of sight may be obtained.

In fixing the points for the slope pegs, a rod should be held in an inclined position and be brought to line exactly with the cross web of the telescope, the pegs should then be driven level with the ground surface where the foot of the rod has rested.

598.—To set out Slope Pegs when the Line is on a Curve.—The operation is similar to that described above, except as explained for taking cross sections on a curve. A variation of the tangential angle must be made for each peg, and if the centre peg shown on the diagram accompanying that explanation be taken as one of the slope pegs, it will also serve the purpose of illustrating the present one, and a brief recapitulation of the manipulation of the instrument to bring it into adjustment for the operation is all that will be required.

The normal slope pegs having been set out and the instrument set up at an intermediate one, as before explained, instead of directing the telescope in the first instance to intersect one of the next normals, set the angle repeater to double the tangential angle for the first chain in the curve, and the horizontal arc to the tangential angle for the distance in chains that the normal is from the instrument. Then turn the telescope to cut the normal peg and clamp the lower limb. Now bring the horizontal arc round to an angle of 90° from zero and clamp. Release the telescope from the vertical arc and turn it at a right angle thereto in the direction of the zero line AD, and by working the back adjusting screw tilt the instrument until the cross web cuts the normal peg again. Adjust the lateral bubble on the instrument to a level condition and it is in adjustment for the operation.

To put in the first peg from the normal, make one movement of the angle repeater and adjust the bubble. To put in the second one, make another movement of the repeater and adjust the bubble, and so on until the whole is completed. It will thus be seen that by a simple mechanical operation a vast amount of work can be done in an incredibly short space of time as compared with the levelling method, and that with little or no effort on the part of the operator.

599.—Alternative method of setting out the Normal Pegs.—Let the diagram, Fig. 263, represent the section of a cutting at the point opposite which the normal has to be set out, when the section depth may be assumed to be 16 feet, the formation width 30 feet, and the slope 1½ to 1, or at an inclination of 56° 18′. The distance bc for a 1½ to 1 slope is one-third the formation width, or 10 feet. The data required for the operation is the distance ad from the centre peg to the plane of the slope, which is found by multiplying ac by the natural sine of the slope angle 56° 18′, thus: 26 × ·831 = 21·60 feet.