a stretch of route location for a road or railway. In order to control the construction involved, the pegs and profile boards shown must be set out at intervals of 10 to 30 m along the whole stretch of construction.
The first pegs located would be those defining the centre-line of the route (peg E), and the methods of locating these on curves have been dealt .The straights would be aligned between adjacent tangent points.
The shoulder pegs C and D, defining the road/railway width, can be set out by appropriate offsets at right angles to the centre-line chords. Pegs A and B, which define the toe of the embankment (fill) or top edge of the cutting, are called slope stakes. The side widths from the centre-line are frequently calculated and shown on the design drawings or
Pegging out a route
computer print-outs of setting-out data. This information should be used only as a rough check or guide; the actual location of the slope stake pegs should always be carried out in the field, due to the probable change in ground levels since the information was initially compiled. These pegs are established along with the centre-line pegs and are necessary to define the area of top-soil strip.
Setting-out slopes stakes:
pointsAand B denote the positions of pegs known as slope stakes which define the points of intersection of the actual ground and the proposed side slopes of an embankment or cutting.
The method of establishing the positions of the stakes is as follows:
(1) Set up the level in a convenient position which will facilitate the setting out of the maximum number of points therefrom.
(2) Obtain the height of the plane of collimation (HPC) of the instrument by backsighting on to the nearest TBM.
(3) Foresight onto the staff held where it is thought point A may be and obtain the ground level there.
(4) Subtract ‘ground level’ from ‘formation level’ and multiply the difference by N to give horizontal distance x.
(5) Now tape the horizontal distance (x b) from the centre-line to the staff. If the measured distance to the staff equals the calculated distance (x b), then the staff position is the slope stake position. If not move the staff to the calculated distance (x b), level again and re-compute the calculated distance. Repeat as necessary.
The above ‘trial-and-error’approach should always be used on site to avoid errors of scaling the positions from a plan, or accepting, without checking, a computer print-out of the dimensions. For example, if the side slopes of the proposed embankment are to be 1 vertical to 2 horizontal, the formation level 100 m OD and the ground level at A, say, 90.5 m OD, then x = 2(100 − 90.5) = 19 m, and if the formation width=20 m, then b = 10 m and (x b) = 29 m. Had the staff been held at A1 (which had exactly the same ground level as A) then obviously the calculated distance (x b) would not agree with the measured distance from centre-line to A1. They would agree only when the staff arrived at the slope stake position A, as x is dependent upon the level at the toe of the embankment, or top of the cutting.