Page 64 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.14 THE WORK

Check Runs. When all the necessary points have been taken, the accuracy of the work may be
checked by taking levels back to the starting point. This is usually a faster operation than the out-
ward trip, as it is only necessary to take transfer points and benchmarks. If frequent benchmarks
have not been placed, it is advisable to use the same turning points, or to take readings on a few
of the grade points, so that if an error is present, it may be localized. It is not necessary or desir-
able to set up the instrument on the same points for the return trip.
The two elevations found for each point should agree, but a difference, varying with the care
with which the work is done, generally exists. Benchmark runs should be held to within a few
hundredths of a foot, even in rough work where a difference of several inches on a grade point
might be allowable. If any considerable amount of cut or fill is needed, even benchmarks may be
left as approximations, until skill or time is available for a more careful run. Any discrepancies
found in the check run should be listed in the notes.
If benchmarks are set at the beginning and end of a run, and check properly on the return trip, it
will not be necessary to back-check any later run on which these two elevations show correctly.
However, if benchmarks have been set by other parties in some previous survey, they should be
checked the first time they are used, as they may be wrong or their description misunderstood.
Grade Stakes. Centerline road stakes are set by instrument and measurements from a prepared
baseline. Shoulder, slope, and other side stakes are set from the centerline.
Grading information, that is, the cut or fill necessary at each stake, may be determined in sev-
eral ways. The preferred method is to take the ground-level elevation at each stake by instrument
and rod to the nearest hundredth of a foot or eighth of an inch, figure the difference from the
desired elevation, and mark the difference on the stake.
Ground is often so rough that some dirt has to be patted down flat at the foot of the stake to pro-
vide a recognizable base for the rod. It may be advisable to put a crayon mark on the stake at ground
level, in case anything should change it. This mark, usually a horizontal line, will be needed when
grades are marked on it.
Many surveyors prefer to use the top of the stake rather than the ground level. In this way they
have a firm base for the rod, and they do not have to be concerned with the possibility that dirt
might be kicked away or added. However, this usually requires measuring the stake height to give
ground level for yardage calculations.
A horizontal crayon line on the stake may be used instead of the top or ground.
The elevations observed at the stakes are subtracted from those required by the road plans. Plus
numbers indicate that fill is needed; minus numbers that the ground must be cut or removed. The
symbols written on the stakes are F for fill and C for cut. Each stake must show location.
Another method is to supply the surveying crew with the required grade elevation for each
stake. These are subtracted from the H.I. (height of instrument) to show the reading on a rod held
with its bottom at correct grade at that location.
If the rod can be held at an elevation so that the reading is seen at the instrument crosshair, the
stake is marked with a horizontal line at the base of the rod. This line is marked SG or G. The indi-
cation is that ground level must be raised to this line.
If the top of the stake is below the grade, the rod is placed on its top. The calculated reading is
subtracted from the actual reading, and the difference written on the stake, following the symbol
F. A crayon line is made at the top of the stake, and/or an arrow is drawn from the figures to the
top. See Fig. 2.13A.
Such a measurement for fill can also be made from a line drawn at any convenient height on
the stake. It might be positioned an even distance below the grade, as 2 feet.
If the rod shows less than the calculated reading when it is resting on the ground, a cut is indi-
cated. A line is put on the stake at ground level, or at some other convenient height, and a read-
ing is taken of the elevation of the line. The actual reading is subtracted from the calculated
reading, and the difference written on the stake, following the symbol C, as shown in Fig. 2.13B.
Stakes placed for the guidance of earth-moving crews should indicate cuts and fills to the grade
they are working to produce. This is usually a subgrade, symbol SG, that is lower than the pave-
ment surface or theoretical grade, G. It is very important to make clear which elevation is meant.
Road stakes are discussed in Chap. 8.