Staking The Grade With GPS

By Christina Fisher | September 28, 2010

When Hilton Head, South Carolina-based Malphrus Construction Company, Inc. won the site preparation bid for a new Target import warehouse in Pt. Wentworth, Georgia, Todd Malphrus, company vice president, quickly realized that he would have to add to and standardize his inventory of GPS equipment. Like many site preparation projects, this one would include grading the subbase, installing the utility infrastructure, constructing the building pad and parking lot, and building the roads. The difference with the Target project would be its scope — a 2-million-square foot concrete slab for the largest single building warehouse under roof in the United States.

A third-generation company founded in 1939, Malphrus Construction was one of the first contractors to start using GPS equipment and grade control systems in the coastal region of South Carolina and Georgia. Initially, Malphrus had a small mixed fleet of GPS and laser hardware and software. With such a major project on the horizon and the company's recent expansion into Charleston, Scott Dehbozorgi, Survey Department manager with Malphrus, explained that standardization of their GPS equipment became more important, as did innovation and customer service. After researching their options, Malphrus chose Trimble equipment because of the range of products available to outfit the company's fleet of dozers, motorgraders, scrapers and excavators. They went with Spectra Integrated Systems (Spectra I.S.) headquartered in Charlotte, N.C., as their dealer.

When Malphrus first arrived at the site in August 2005, clearing and grubbing was finished, so the survey department began the process of comparing the existing conditions to the site plans they received from general contractor Ryan Companies. "Normally, we would just shoot it conventionally with a robotic instrument," explained Dehbozorgi. "However, with a site this size we set up a base and we set up the Trimble rover on a four-wheeler to drive the site."

With the initial site survey completed, Todd Malphrus called Joe McNamara, vice president of Spectra I.S., to determine the best system for grading the subgrade, which also includes a 43.5-acre bowl-shaped parking lot, and placing the base stone. They chose a GCS 900 dual receiver GPS system for two dozers and a motorgrader, which was also outfitted with a laser augmentation system.

Compared to a single mast in the center of the blade with a cross-slope sensor, the GCS 900 dual receiver system features GPS antennas on both blade tips. Either configuration — single mast or dual mast — is ideal for rough grading, achieving an accuracy of about one-tenth vertical tolerance. However, to achieve the required quarter-inch vertical tolerance on the contours of the bowl-shaped parking lot, Malphrus replaced one of the GPS masts on the motorgrader with a 3-foot laser receiver and then set up a rotating laser.

"The bowl-shaped parking lot had contours that wrap around," explained Dehbozorgi. "It has grading lifts down the center in rows, then the contours — literally circles — go around each one. It is very tricky to grade. It's not straightforward like cutting roads in a subdivision or even a highway with its crown."

Using conventional staking and grading methods to grade the parking lot, an operator would have to set survey hubs and stakes around the perimeter of the parking lot and pull a string from that hub to the existing low point to which he would be grading. As the operator continued to cut, the string would be pulled to verify the grade. The GPS system eliminates most of this process, and hubs are only intermittently used to verify grades.

The dual GPS system provides elevation information for both blade tips. This ensures the blade is angled properly and in the proper position each time, even when the motorgrader is running in straight lines across the project and jumping grade breaks. Furthermore, the blade is fully automatic; the operator only needs to manage the material coming off the blade.

"This was the first time I used the system, and it made a believer out of me," said motorgrader operator Tony Pye. "The flat part I wasn't worried about, but the way the bowl is made (the contours) round. We only put out a couple of hubs and pulled string to verify, and it was right on the money. Using the GPS system took half as long as the traditional system."

"This is our first GPS unit with laser augmentation," said Todd Malphrus, "and we've been impressed with it. Our guys took to it pretty quick. They were using GPS, so this was just another step — but it's an important part of this job."

Malphrus is also using the dual GPS motorgrader system with laser augmentation on the 46-acre building pad. By doing so, Malphrus is bringing it within a quarter-inch tolerance as well and saving on surveying. "This is one of the bigger sites we have," said Dehbozorgi, "but we probably have gotten the least amount of staking requests for it because of the GPS systems we're using."

Malphrus has seen significant increases in production with the system and has essentially eliminated the need for an onsite surveying crew. The surveying team can then work elsewhere on the site verifying grades with conventional methods or a handheld GPS unit; they can also be back in the office learning to create and implement the three-dimensional software models for future projects.

Joe McNamara points to this extensive use of the technology as a key component to Malphrus' success. "With this technology, people can use 50 percent of the capability, or they can use 80 percent to 90 percent capability. Because Malphrus has someone like Scott and a surveying department to manage the data, the machines, the sites and the surveying process, they are able to use the equipment to its fullest extent.

"They have total control. Malphrus does their own site calibrations, and they also take the CAD files and create their own models to load into the machines. If a company has that capability, they can figure out value-engineering ideas when the plans come in. If a company subcontracts out the modeling, they've missed that opportunity."

As with any new piece of equipment or technology, there was a learning curve, but Scott Dehbozorgi expects that on every job. "You're always learning new tips and new tricks — ways to build a GPS file faster, ways to set up a base faster, better positioning. Things are always changing; it's dynamic by nature, but Malphrus is always willing to try new things that will make the project move faster."

Site preparation on the project is expected to be complete by the end of the year. However, with the warehouse walls only beginning to go vertical, Target is already in discussion with Malphrus to expand its facility by another 20 acres to 30 acres.


Project Statistics

Earthwork: 750,000 cubic yards

Concrete: 50,310 cubic yards

Stone: 250,000 tons

Storm pipe: 15,000 linear feet ranging in size from 15 inches to 72 inches

One of the challenges on the project has been getting aggregate to the site. Normally, aggregate is brought from Macon or Augusta, Ga., by rail to Savannah. However, the railroad companies have diverted many of their rail cars to the Gulf region to help with the recovery efforts. This has affected delivery of stone and aggregate to the Low Country region, and the situation is expected to get worse. As a result, Malphrus has exhausted much of the local aggregate supply and has had to truck in stone from Columbia, S.C., which is three hours away, or Augusta, Ga., which is over two hours away. While concrete suppliers have so far been unaffected, there are concerns that aggregate shortages will soon affect them as well.