Page 77 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.27

FIGURE 2.23 Transit and laser.

The laser is set inside the manhole, at the level of the pipe center or other measurement level.
There are a number of ways of setting up, of which two will be described.
Vertical Rod Mounting. In this system, both the transit and the laser are mounted on the same
vertical column, which is composed of calibrated sections with lockable male-female joints.
Cross section is square.
This column may be supported toward the top by an expandable cross brace inside a manhole, or by
a batter board. At the bottom, it may be held by a tripod, or by forcing its lower end into soft dirt, or to
a support on a steel plate. On fresh concrete, it may rest on a steel pin driven down through the concrete.
By reference to rod calibrations, and/or by measuring with a tape, the laser is clamped to the rod
at the desired level, which may be at the pipe center, just above the invert, or at any convenient
point between. It faces in the direction in which the pipe is to be laid.
The laser grade indicator is set at the correct slope or gradient for the pipe (see Fig. 2.24), up or
down, then the laser is connected to its battery and turned on.
The transit is clamped to the upper part of the column, so that it can look in the direction of
pipe laying.
When the ditch has been dug 15 feet or more toward the next manhole, the laser beam should
be shining along its centerline, or at least very close to it. A stake or target is placed approximately
in the center of the ditch to make the beam visible. The transit is rotated on the column by adjusting
a knob, until its vertical crosshair is centered on the beam, as seen in Fig. 2.25.
The transit is now pointed straight down at the laser, and aligned with a reference mark on the
barrel by adjusting a different knob than the one mentioned above. The transit is alternately
aligned with the beam and with the mark, until it remains exactly centered on both. The vertical
line of sight (vertical hair) through the transit now includes the laser beam.
The transit is now aligned with the marker on the next forward manhole by rotating the verti-
cal rod (to which both transit and laser are clamped) relative to its supporting crossbar, with a fine
adjustment. Both instruments, being clamped to the rod, remain in alignment with each other.
Therefore, when the marker is bisected by the transit’s vertical crosshair, the laser is projected
accurately along the line for the pipe.