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Mainstreaming surveying, layout work

Mainstreaming surveying, layout work

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    National contractor Walsh Construction bought Trimble Tablets to go paperless while placing concrete for runway expansion at Chicago's O'Hare International Airport. Photos: Joe Nasvik

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    3D laser scanners. Able to shoot millions of points in every scan, the most expensive and technically sophisticated surveying tool. Imagine using a conventional total station or theodolite to lay out 10 or 15 points for a project. Now imagine setting up a 3D laser scanner, which looks very much like a robotic total station, and recording a million points in a few minutes. With this data, you can generate an infinite amount of information: locate any number of points needed, record information on a site to create CAD drawings, create as-built drawings, or generate accurate records of construction progress to use when job conflicts arise. Several large general contractors, as well as a few subcontractors, own these.

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    Once regarded as leading edge, robotic total stations are becoming increasingly common because unlike their predecessor — the total station — only one person is needed to plot points. An operator can stand at the instrument to shoot points and elevations for developing site information to help generate CAD drawings, saving time and obtaining information that might otherwise be dangerous to collect. These instruments also direct excavators and concrete placing equipment that require 3D contours. Total stations cost much less, but require two people to operate: one at the instrument and one at the rod.

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    Digital levels can be extremely accurate across long distances. They gather very precise elevations over long, level routes. Surveyors often use a rod with barcode lines that the level reads to determine elevation. Sometimes the rods used have barcodes instead of numbers, so the instrument reads the barcode to determine elevation.

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    Laser levels remain the construction industry's workhorse; just about every contractor uses them to set elevations or to guide equipment. Newer models automatically level themselves, are easier and faster to use, and are less expensive than earlier models.

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    Especially good for inside buildings or low-light conditions where the laser beam can easily be seen, small laser level systems establish level automatically and provide colored laser beams to help users lay out level, plumb, and square. Add a remote sensor to lock on the beam in bright light or outside conditions, and you also can easily plot points accurately up to about 100 feet. Inexpensive compared to other instruments, they're used for laying out floor tile, ceiling panels, or smaller forming applications.


Establishing control points

Lindblad Construction Company of Joliet Inc. in Joliet, Ill., sometimes employs a professional surveying firm to provide at least two control points before beginning a project when no established points exist on a site. A few years ago, the general contractor purchased its first total station unit followed by a robotic total station the next year, and is considering buying another one.

“We were skeptical at first so we rented one and tested its accuracy by conventional means and quickly discovered the benefits, and then bought one,” says Vice President Mark Stadalsky.

Three employees are trained to use the layout instruments to set points for structures, piers, footings, and locate anchor bolts — a major challenge in their business. Lindblad markets itself on its ability to precisely locate anchor bolts for machines and equipment. The general contractor also often sets control points for subcontractors as well.

Working with a licensed surveyor

It's expensive to buy a 3D laser scanner and provide the necessary staff training. Contractors that can't afford this hire a company like SEC Group Inc., an HR Green Co. , in McHenry, Ill.

SEC Group uses 3D laser scanners, robotic total stations, laser levels, and GPS systems to establish control points, calculate excavation volumes, stake out points for mass grading, plot points for underground structures, lay out curbs and pavements, locate columns, perform anchor bolt surveys, and provide spot foundation surveys required by some municipalities. The firm also collects and interprets data for studies or record drawings and, depending on state law governing surveying and construction layout, performs some survey services that require licensed surveyors.

Mike Fischer, group leader of surveying, says the firm makes it a point to respond to client calls within 48 hours.

Although the relationship between contractors and surveyors is changing because of the influx of easier-to-use equipment, some functions are best handled by professional surveying consultants. A word of warning: Don't automatically assume that contractors' surveying divisions, which rarely employ licensed surveyors, can completely replace the industry's more experienced counterparts.

— Nasvik is senior editor of CONCRETE CONSTRUCTIONS, a sister magazine of PUBLIC WORKS. A longer version of this article is available here.