Use of Global Navigation Satellite System technology allows excavating contractor to keep costs down on new suburban Chicago police station

Construction technology is getting so advanced that once-novel aspects of projects such as state-of-the-art measurement precision and fast-track scheduling are becoming the rule rather than the exception. A case in point is a new $28 million police station for the Village of Hoffman Estates, Ill., located about 30 miles northwest of Chicago, which is scheduled for a June 2010 completion. Construction of the two-story, 79,000-square-foot facility’s foundation began during the second week of October 2008 and was scheduled for completion by the end of the year—an 11-week timeframe. Critical scheduling adherence meant that the latest grading technology wasn’t just a luxury for the excavating contractor but a necessity.

The suburb’s 105-member police force reportedly outgrew its old station, which had served as the Hoffman Estates Village Hall previously. A unique aspect of the 32,000-square-foot basement level of the facility that replaces the old one is its multiple functionalities. About three-fourths, or 24,000 square feet, of the basement will be used for 44 parking spaces and the remaining 8,000 square feet are devoted to a shooting range and emergency operations center. This layout makes its grading and excavation a bit more challenging than most commercial projects, Pat Wood, project manager for MTI Construction Services, LLC, Elgin, Ill.—which focuses on municipal, commercial, industrial and institutional projects—noted in early November 2008.

“The overall project is on a traditional track as I would call it, but if we don’t get [the foundation] in by December 31, we’re going to be impacted by winter weather and there would be significant cost issues,” he said. “The critical milestone is to have the footings and the foundation in prior to December 31, prior to the onset of severe winter weather. If we end up having the bulk of the foundation/excavation work going through the heart of the winter, the January-February months—we would incur significant costs on the owner’s part. If we weren’t to incur those costs and were to hold back and not start this excavation until February or March [2009], it would be monumental to achieve completion by June of 2010. So we’ve expedited the footing and foundation schedule to avoid the associated winter concreting costs.”

Chris Aspegren, project manager for Berger Excavating, Wauconda, Ill., knew how time-sensitive the foundation work was. “[Falling behind] would delay occupancy of the building and it would just multiply the associated costs, especially with concrete,” Aspegren said. “It’s not so much my costs, but my work depends on the concrete guy [Builder’s Concrete Services, LLC, Naperville, Ill.] getting in behind me and his costs would go up exponentially with winter conditions—air entrainment of the concrete, different admixtures and the inability to actually work. The objective, before the end of this year, is to get the basement walls established so that they can continue working throughout the winter.” The next phase involved the erection of structural steel and precast hollowcore concrete floor decking. Cast-in-place concrete work was to resume in spring 2009.

Critical parking area grading

Berger started by excavating 20,000 cubic yards of soil from a 32,000-square-foot excavation, a task that was completed in only eight days from the contract award, Aspegren reports. The entire project requires a total of 30,000 cubic yards to be excavated, a total that includes a retention pond of 45,000 square feet, or just over an acre.

The most challenging part of the site-preparation work, from a grading standpoint, was the underground parking area, which has two levels. “The unusual thing about this particular building is the parking garage in the basement,” Aspegren pointed out. “It’s going to have multilevel slabs, if you will, in the basement at different grades and then we have different grades for all the footings and the piers and everything else.” The grading of the parking area floors had to be both fast and precise; the floors are designed with trench drains down the middle and pitch toward the center at roughly a 2 percent grade. “It’s important that my work’s on, because the guy who follows me up to install the drain and everything else. If I’m not right, then I have to go back and rework those areas while trying to work around him,” Aspegren said.

A Global Navigation Satellite System (GNSS) gave Berger a significant productivity boost in grading the parking area and basement and the entire five-acre-plus site. The company is an early adopter of this technology, having purchased its first GNSS in 1997. Currently, Berger is using  Topcon equipment purchased from and serviced by Positioning Solutions Company, a dealer of grading, surveying and construction equipment with locations in Illinois, Indiana, Michigan, Missouri and Wisconsin.

On this project, Berger had a Caterpillar D6 dozer equipped with a 3DMC dual antenna that receives signals from positioning satellites in order to position the dozer’s blade at the proper level to provide grades in adherence to the topographical survey. The antenna also received signals from a HiPer Lite+ base station antenna that captured positioning signals directly from satellites.

GNSS technology is automating grading and excavation. Increasingly, contractors like Berger are having machine control systems installed on earthmoving, grading and paving equipment such as scrapers, dozers and motor graders, plus milling machines and asphalt and concrete pavers.

A GNSS machine-control system uses a “rover”: a rugged antenna mounted to a shock-absorbing, vibration-damping pole and a receiver box mounted in a secure location on the machine. Satellites send positioning data to another antenna/receiver combination at a stationary base station. Positioning data is also sent to the rover on the machine. This data or RTK correction is transmitted by a stationary base GNSS receiver.  The base receiver and mobile rover work together to provide real-time kinetic (RTK) position information, revealing the machine’s three-dimensional location on the site.

Software compares the machine’s position to the design grade at a given location. The design grade information was compiled via the use of the GNSS—including “roving” of the site. The data files are loaded into a machine-mounted control box via a compact “flash card.” The control box updates positioning data and sends signals to the hydraulic valves. The blade is automatically positioned for elevation and slope. Other sensors inform the control box of certain machine conditions; for example, dozers used by contractors such as Berger are equipped with a slope (tilt) sensor on the blade to measure the cross-slope of the cutting edge. “Indicate systems” like Topcon’s 3D systems provide visual guidance for machine operators, who manually control the machine to cut or fill to the desired grade.

Berger’s dozer-mounted dual antenna has a functional advantage over a single antenna: it captures the tilt and side-to-side angles of the blade in addition to the elevation and location of the blade that the single antenna captures. This functionality is designed to provide both grading accuracy and productivity on projects in which precise grading of slopes is required. Berger also uses Topcon’s 3DMC software, which helps an operator determine where material is located on the jobsite and where it needs to be moved with the aid of a real-time, color-coded cut/fill mapping feature. The operator can select different colors—which can be displayed in darker colors to show large topographical variations—to indicate cut, fill and on grade areas on the control box. The software also allows the graphical layout to be saved as an as-built survey. Berger also uses Topcon 3-D Office software that interfaces with the in-cab control box and Carlson Takeoff software to build the three-dimensional topographical model that utilizes data acquired from site roving and loaded into the 3DMC software, as well as used to create as-built surveys.

The reference display in the cab uses a three-dimensional model that is developed as the contractor “roves” the jobsite using the rover antenna. During the roving process, the contractor first refers to the official site survey prepared by a professional surveyor to set control points. Using these control points and satellite signals, the contractor then develops the topographical model, which becomes a dynamic simulation during grading work.

Recently, these systems have become even more reliable and accurate as they have added compatibility with the Russian GLONASS satellite constellation as well as the U.S. Global Positioning System constellation. This dual-constellation capability roughly doubles the number of signals available to the GNSS antenna/receivers and provides a high degree of positioning accuracy.

Excavation floor grading complete

With the basement excavation dug, Berger began fine-grading the foundation floor subgrade in early November while crews from Builder’s Concrete set up wall formwork in one corner of the foundation and concrete mixer trucks fed a concrete pump that placed concrete into the forms. Mike Clark, Berger’s site foreman, viewed the GNSS monitor inside the cab while spreading 3-inch Illinois State CA-1 Classification Recycled Crushed Concrete with the dozer blade. The final step in fine-grading the floor would be adding a layer of Illinois State Classification CA-6 crushed ¾-inch stone. “Essentially, we’re done in the bottom until we backfill the walls,” said Aspegren. In about three weeks, he added, Builder’s Concrete would be done constructing the foundation walls and Berger would then backfill the walls from spoil piles from the foundation excavation.

Aspegren then retrieved his pole-mounted GNSS rover unit and displayed the two-dimensional model of the site on his field controller to show the footings and columns. A series of intersecting lines showed the footing and column lines. He checked the location and depth of the footings and columns, and he could see where cuts or fills were needed anywhere on the foundation floor. Aspegren reports that the surveyor came out to the site once, after the full excavation was made but prior to footing excavation to verify footing locations, and determined that the footings and columns were located within hundredths of an inch of their locations according to the official survey. Aspegren pointed out that the grade-checking was critical because any necessary adjustments would have been required prior to footing excavations.

What would grading this foundation be like without these high-tech tools, using stakes and stringlines? “We would have had a surveyor on site every day, staking and verifying the grades,” Aspegren said. “Because of the nature of the cut, this cut had to be done in two lifts in order to make it productive.” He points out that productivity on a site of this nature depends on the speed with which Berger can load the trucks and get them on the road to the dump site. If the hole had been cut to a depth of 18 feet in one pass, the time required for the backhoe to reach to the full depth would have been more than double the time required to reach 9 feet. So the hole was cut first to a depth of 9 feet and then cut another 9 feet, to the total depth of 18 feet. “As we did the first cut across the site, we would have had to set all the points and then, when we came back to do the second cut, all those points would have had to be reset as we moved through. We would have had to have a surveyor onsite almost full time. But basically, he set the control on the site, we took it from there, and when we were done, he came in and verified the readings.”

Aspegren estimates the cost savings provided by this technology in the tens of thousands of dollars. “Certainly, it’s helped us be competitive and be awarded the job,” he said. “I can tell you that the surveyor costs would probably be in excess of $10,000, but the savings in not having to do rework could be $50,000 if you added in the cost of the stone.” He added that examples of rework include refilling cuts that are too deep and making new corrective cuts.

“The main benefit of the GNSS for Berger on this project is not having to do a large amount of staking and not having a surveyor out there,” added Tim Matthews, Berger’s sales representative at Positioning Solutions. “Considering the fact that it’s a confined area and given all of the elevation changes with the multiple parking area levels, the productivity that the equipment made possible eventually saved the owner a substantial amount of money on materials that would have been spent on winter concreting.”

Wood was also impressed with the accuracy and productivity yielded by Berger’s GNSS. “It’s faster and fewer people are involved; you don’t have to have the surveyor out here on multiple occasions,” he said. “Berger has one operator taking care of a task with no one along with him. There’s also less overall project congestion when they’re utilizing that device. It’s five-plus acres, but with all the development that’s going on on the site, it’s a rather tight site and the efficiency factor of that equipment leads to less congestion. Of course, less congestion impacts productivity, safety and so forth.”

Don Talend of Write Results, West Dundee, Ill., is a publicity and communications project manager specializing in construction, innovation and technology.