Another useful device for controlling pipeline excavations and laying pipe is the laser. So many applications are being found for the laser that it may eventually be the only tool needed for the layout and control of construction projects. It can be quickly, accurately, and economically used for purposes such as distance measurement, alignment for tunnel borings, setting of pipes with desired grades, and setting of line and grade for many types of construction.

The laser is an intense light beam that can be concentrated into a narrow ray, containing only one color (red) or wavelength of light. The resulting beam can be projected for short or long distances and is clearly visible as an illuminated spot on a target. It is not disturbed by wind or rain, but it will not penetrate fog. A laser can be set up on a bracket or even attached to a transit telescope. The beam is aligned in the proper direction at the desired grade and can be left relatively unattended.

Today, instead of using batter boards and strings, lasers can be used to control the alignment for excavat-ing trench and setting a pipe. The laser can be set so that it shines on the boom of a backhoe so that the equipment operator can clearly see the illuminated spot. By its position, the operator can closely control the depth of digging. For laying the pipe, the laser is set in the proper direction at the desired distance above the pipe invert. With the aid of the L-shaped pole or templet, as shown in figure 10-21, the workmen can control the invert elevation. It may also be possible to direct the laser beam from the inside of manholes through the pipes being laid and to control the grade without any interfer-ence from the backill operations. This can be done even if the pipes are too small for human access.

The stakeout for an underground power line is similar to that for a sewer. For the ducts, cuts are

measured to the elevation prescribed for the bottom of the duct, plus the thickness of the concrete encase-ment, if any. In an underground power system, the bottom of the manhole is usually about 2 feet below the bottoms of the incoming and outgoing ducts. Power and communications manholes are often com-bined; figure 10-22 shows plan and section views of a combination power and communications manhole. Conduit and cable connections to buildings, street-lighting systems, traffic light systems, and the like, are low-voltage secondary lines. Duct connections from main-line manholes run to small-subsurface openings called handholes on the sec-ondary line. The handhole contains connections for takeoff to the consumer outlet. Figure 10-23 shows plan and section views of a handhole.