Page 80 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.30 THE WORK

If the pipe has been carefully set to grade and alignment, and the laser is properly mounted, the
laser can project the correct centerline or flow line from one pipe through the rest of a straight run.
However, alignment should be checked on a transit-set pole or target 15 to 25 feet ahead.
Refraction. All light beams, including lasers, may be bent out of a straight line if they pass
through air of varying density and/or temperature.
If sun-warmed pipe is set in a cold trench, or cold pipe is put in a warm trench, or if hot seal-
ing compound is used, a mixture of masses of air of different temperatures may occur inside a
pipe, causing the laser beam to vary from a straight line.
This situation can be (and must be) corrected by using a blower to direct a steady current of one-
temperature air through the pipe. Blowers using the same 12-volt current as the laser are usually
supplied with them, and their use is a routine precaution that often avoids costly confusion and
mistakes.

Targets. Targets for use in pipes are usually translucent plastic, or metal with a plastic window.
They may be premarked with centering or other information lines, or such indicators may be put
on with erasable markers on the job.
One size of target may fit several or all sizes of pipe, or there may be a separate size for each
pipe. Some are snap-fitted into the pipe, others have fairly elaborate holding and extension
devices, and may include a level vial to check the installation.
Ordinarily, a pipe target is inserted in each pipe at the end away from the laser, and the pipe is
shifted until the beam is centered on indicators, usually cross lines, with or without bull’s-eye cir-
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cles. If the laser is set correctly, and the target properly used, accuracy to within ⁄ 8 or ⁄ 16 inch is
readily obtained. The target is removed as soon as the pipe has been secured in place, and put in
the next pipe when it is ready.
Other targets may be range poles or surveyors’ rods, with a surface on which the beam can be
seen readily, even in brightly lighted areas. There are also electronic sensors that can register the
beam even when it cannot be seen.
Changes. From any one setting, an inside-pipe laser can be used for only one straight section.
If the direction or gradient changes, the instrument must be reset at the angle point between the
two lines.
If such changes are frequent, it may be better to put the laser on the surface, as described in the
next section. It can then be used for gradient only, with direction determined by transit.
A curved pipe must be set from surface instruments.
Ditch Depth. The laser may be used to regulate ditch depth in between setting pipe sections.
Readings may be taken on the bucket of a hoe, the shoe of a ditcher, or a rod held on the ditch
floor close to the excavator.
This continual guidance greatly reduces the need for hand grading while laying the pipe.

LASERS ABOVE GROUND

Potential. The potential of lasers in construction work above ground is very great. A general
simplification and improvement in handling a variety of leveling and grading problems may be
expected.
Here we will deal with two applications: grade reference within a specific construction area,
and guidance of excavating machines.
Tracking Level. The grade laser tracking level is mounted on a substantial tripod, is leveled,
and delivers a rotating beam of laser light. When set for automatic operation, its beam passes over
the whole area of a construction site at 6 to 40 revolutions per second (rps).