Grades From the Sky

When Guinn Construction Company of Bakersfield, CA, won a large contract for clay placement and grading for final closure of a central California landfill, some strict requirements came with the job. A specific amount of clay had to be placed on top of the landfill: no less than 2 ft. and no more than 2.2 ft. The project also required Guinn to accept existing subgrade, which had variations of up to 1 ft.Completing the project using a laser-based grade control system would have required finding the high point for every slope and setting the laser from that point to grade. However, the plane grade would have created an overfill of clay, exceeding the 2.2-ft. maximum. An alternative was the traditional, labor-intensive method of establishing a grid of trade stakes and straight-grading between them. Although this method would have allowed the contractor to precisely meet the project’s requirements, the extra time needed for grade setting would have been at Guinn’s expense.Working with two companies that provided surveying and technical expertise, Guinn opted for a different approach: a grade control system based on global positioning system (GPS) technology. GPS works by obtaining position information from satellites. Mounted on the contractor’s equipment, the grade control system receives differential corrections from a base receiver. Each position is referenced to a digital terrain model of the design surface to determine cuts, fills, and position of equipment on-site. Worldwide Surveying of Pacheco, CA, and Total Control Construction Systems of Temecula, CA, joined with Guinn to turn this tricky project into a demonstration of a new GPS-based technology, SiteVision, produced by Trimble of Sunnyvale, CA. The client for the landfill project had prepared a traditional preconstruction grade control plan, which called for a surveyor to conduct a topographic survey of the subgrade to verify base elevation quantities. A 50-ft. grid of elevations was to be used to define all the critical grade breaks of the subgrade. Once the contractor determined that final grade had been achieved, the surveyor would be called back to observe the grid points and certify final grade. The GPS-based system allowed the project to be carried out differently. With Worldwide Surveying’s involvement, project control and topographic information were used to create a digital terrain model for the existing subgrade and an AutoCAD background file of the critical grades and contours. These data were then used to develop the graphics on the SiteVision display, which an operator can view on a computer monitor inside the cab. On the monitor, an operator can see a site map with the machine’s position indicated or various text views conveying position information. Three light bars mounted within easy view guide the operator to cut or fill along alignments, indicating how to move the corners of the cutting edge to match design alignment.

“I no longer have to wait on a grade checker,” says Steve Smith, heavy-equipment operator for Guinn. “I just look at my display and light bars and go to work.”With the SiteVision system installed on Guinn’s Caterpillar D4 bulldozer, the operator could move to any location and get almost instantaneous position and grade information without the services of a grade checker. The operator could control the placement of the clay by directing the scrapers to the exact spot where material was needed. Project specifications required the 2 ft. of final cover to be placed in four 6-in. compacted lifts, and the system’s vertical offset display function allowed the operator to cut grade at every lift. After final grade was achieved, the client’s surveyor conducted an as-built topographic survey of the site and found that specification compliance was 100%, finished surfaces required no reworking, and there was no need to chase cuts and fills to finish the job. “This is like getting my grades from the sky,” remarks Guinn Project Superintendent Matt Morrow. “It allows me to go anywhere on the project and graphically see what cuts or fills are needed without waiting for my grade setters.” For this project, Worldwide Surveying established a base receiver location and completed a site calibration of the project control. One base station can control many pieces of equipment on-site, even without being in the line of sight of the equipment. As new technology continues to change the roles of contractors and surveyors, a typical future scenario for a subdivision, for example, might see the contractor receiving a digital terrain model and site calibration and beginning work immediately, with none of the traditional grubbing, rough grade, and finish grade stakes. Surveyors who have been intimately involved with GPS for the past 10 years realize that this technology will change the roles of the contractor as well as that of the surveyor. For example, on a typical subdivision, the contractor will receive a digital terrain model, site calibration, and an as-built survey for pad and slope certification. Traditional methods would include clearing and grubbing stakes, rough grade stakes, finish grade stakes, and as-built survey for slope and pad certification. As soon as the contractors have the digital terrain model and the site calibration from the surveyor, they can start to work. A contractor will no longer be concerned about shutting down a job for lack of stakes.