Page 92 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.42 THE WORK

A machine may need two stringlines, one at each side. However, if it has an automatic cross-
slope control, only one line is required.
Accuracy in locating such stringlines is of the utmost importance, as line and grade produced
in the automatic operation are absolutely dependent on it.
There are various ways of setting up a stringline. The following description follows a booklet,
Stringline Manual, published by CMI Corporation.
Apparatus. The string holder is made up of a metal stake (standard length 42 inches), a sliding
bracket that can be secured to the stake with a setscrew, and a horizontal rod that slides in the bracket,
and can be locked in it with another screw. One end of the rod is notched to hold the string.
The string is usually a special yellow polyethylene selected for strength, stretch characteris-
tics, visibility, and rot resistance. Regular strong white cord may be substituted in an emergency.
The string is carried between jobs on storage reels. When in position, it is tightened by means
of take-up reels, one at each end. See Fig. 2.39.
An engineers’ steel rule is used to measure the distance from string down to the top of the
grade stake (tack line). This distance has been previously determined, and it is the same for each
grade stake in a section.
A plumb bob may or may not be used for horizontal alignment.

Location. The stringline is placed outside the strip or lane being graded. In highway work, this
usually means in the shoulder, 2 or more feet beyond the edge of the lane. It is preferably 18 to
30 inches above grade, although some machines can use higher and lower settings. It should be at
least 5 inches above the ground.
It may be possible to use one setting for up to three processes—subgrade finishing, laying
selected subbase, and paving. Clearance from lane edge must be sufficient to accommodate
windrowing and removing of waste material, passage of machine tracks if outside the lane, and
minimum tracing arm lengths.
Stakes are driven about 1 foot outside of the line proposed for string, alongside marking hubs
or grade stakes.

Hubs (Grade Stakes). Hubs are usually 2 2 wood stakes, placed by surveyors along the line of
the string, parallel to the roadway centerline. They are driven to grade (bluetops) or to some fixed

FIGURE 2.39 Stringline and take-up reel. (Courtesy of CMI Corp.)