Page 76 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.26 THE WORK

The IR lasers are used for heavy construction equipment in the short to medium range of 500 to
1000-foot radius from the laser. There is a laser receiver mounted on the excavator, backhoe, and
the like, for elevation indication. With a slope control laser the beam can produce a high-powered
IR or barely visible beam for a projection of 1500 or 2500 feet, or more.
The laser tube or cylinder is mounted in an enclosure which may be mounted on a tripod, in a man-
hole, in a pipe, or in other ways. The design of the instrument and mountings provides threaded adjust-
ments of great delicacy and accuracy, which permit precise regulation of direction (line) and grade.
There are usually two adjustments for vertical alignment. One levels the instrument. The other
adjusts the laser tube inside the case for percent of grade.
This percent or gradient is zero for level work. In pipe laying, it is set for whatever plus or
minus gradient is required. The setting is indicated by a dial gauge, or by a digital readout.
Power. The power source is usually a detachable battery pack or a rechargeable 12-volt truck or
automobile battery. It is important to use correct polarity, as the laser will not operate if connected
to the wrong terminals. Most models will blow a fuse, some suffer serious damage. Others have a
built-in electronic safeguard.
The laser beam is generally harmless and is usually invisible when viewed from the side or the
rear. It should be visible and harmless when seen in or through a plastic target. OSHA regulations
require workers exposed to it to wear goggles designed specifically for protection against the
wavelengths and intensity encountered.

Significance. The laser supplies a visible line-of-sight indicator that can be fixed, or set to move
in a pattern, without an operator. It can be used by anyone in its path, by means of simple equip-
ment, to determine grade and/or direction, with a high degree of accuracy. It makes otherwise
imaginary reference lines real to workers.
The laser beam is sometimes compared to a taut, no-sag stringline supported at one end only.
A transit or other sight instrument is needed in job layout, and perhaps each time a laser is set
up. In addition, there should be a program of frequent checks with a transit to check work being
done according to laser guidance.

Maintaining Accuracy. Once set up accurately, a laser remains accurate unless disturbed. It
must be, and usually is, protected against violent disturbance. But a person or object might brush
against it or strike it, ground may yield or a support may move, so that it goes off line or grade
without the change being noticed.
Devices are available that will shut off a laser if it is disturbed. It can then be turned back on,
but must be checked before further use.
With or without such a protective attachment, it is wise to recheck the setting of a laser frequently
during precise work, and occasionally in rough work.

PIPE LAYING

For guidance in laying pipe, a laser is set in a manhole or at an angle point in the line, and the beam
is directed along the inside of the pipe. Depending on the system being used, the beam may be
centered in the pipe, or aligned just above the center of the invert (inside bottom).
The laser location is usually determined by setting a transit directly above the angle point, which
(for purposes of this description) is assumed to be approximately centered in a newly built manhole.
The transit may be mounted on a rod based at the bottom of the excavation and supported by an
adjustable cross frame, by rods based on a clamp or clamps on the manhole edges, or on a batter
board. See Fig. 2.23. It is located exactly over the angle or location point by reference to baseline
and hub points.
Measurement is made from the transit down to the grade at which the laser is to be set. A mea-
suring rod support is used directly, being checked with a carpenters’ level to ensure that it is vertical.
Otherwise, a steel tape and plumb bob are hung from the center of the transit.