The most commonly cited benefits of Global Navigation Satellite System (GNSS)-enabled machine control are grading and excavation productivity, and the cost savings that result from a reduction in manual tasks such as excavation depth verification using conventional surveying methods. The use of this technology on recent underground utility work for a state vehicle maintenance facility outside of Phoenix demonstrates that it has something in common with the Construction Management at Risk (CM at Risk) project delivery method, in which cost control is a major priority.
In early 2009, Haydon Building Corp., Phoenix, constructed a 24,000 square foot, $9.9 million field maintenance shop for the Arizona Department of Emergency and Military Affairs (DEMA) in Florence, AZ, located about 60 miles southeast of Phoenix. It was one of the first projects in which Haydon used GNSS to control an excavator digging for underground potable water pipe, sewer force main, gravity sewer and storm drain pipe servicing the new facility, which is equipped with work bays and a lube bay for DEMA vehicles, as well as administrative areas. All told, Haydon installed 3,535 feet of 10 inch PVC pipe for potable water, 2,000 feet of 4 inch high density polyethylene (HDPE) sewer force main, 500 feet of 8 inch PVC gravity sewer and 100 feet of 12 inch HDPE storm drain. All but the potable pipe was new and not tied into an existing system.
Under the CM at Risk method, the construction manager serves as part of a team with the owner for the design and construction phases of a project. During the construction phase, the construction manager’s role converts to the legal equivalent of a general contractor once a guaranteed maximum price is established for the construction work. The construction manager is responsible for ensuring that the project stays on schedule and conforms to the contract details and, therefore, takes on more of the risk that other project stakeholders otherwise would assume. Benefits of this approach include flexibility, quality control and cost control; rather than managing multiple contracts, the construction manager can bid and subcontract a portion of the work at any time and sign the construction contracts, even when the design of an unrelated portion is not complete.
In the spirit of construction cost control, Haydon has also incorporated GNSS grade control technology into its day to day operations over the past several years and purchased two X63 systems from Topcon Positioning Systems in 2008. The company may purchase several more X63 systems in the future as the technology becomes an even bigger part of day to day operations, according to Jeff Fields, Haydon’s GPS manager.
GNSS impacts dirtmoving
A GNSS machine control system uses 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 machine. The stationary base and machine work together to provide real time kinetic (RTK) position information, to reveal 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 built from site plans.
The data files are loaded into a machine mounted control box via a USB flash drive. 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 are equipped with a slope (tilt) sensor on the blade to measure the cross slope of the cutting edge. “Indicate systems” like Topcon’s 3 D systems provide visual guidance for machine operators, who manually control the machine to cut or fill to the desired grade.
During the past several years, these systems have evolved in terms of flexibility and reliability. An alternative to a base station – that Haydon uses – is subscribing to a network that provides positioning corrections using cellular technology. Where available – such as through Branco Machinery of Gilbert, which also sold and services Haydon’s Topcon Positioning System’s GNSS equipment – this option provides flexibility. The contractor does not have to set up a separate base station on each job site.
“With a base station, you’re setting on a known point on that job. On that known point, that base station transmits to the machine or rover corrections of where it is,” said Todd Hermsen, Haydon’s Topcon sales representative. “When you move to a cellular communication with a network, the machine is actually dialing up to that network via cellular communication and receiving its corrections.”
He added that Branco’s TAZNet (Topcon Arizona Network), which went online in mid 2007, reaches the Phoenix and Tucson metropolitan areas. “What it allows [Haydon] to do is take their base and rovers that they may already have and instead of tying up that base station on that job, gives them the flexibility of taking that base station and converting it to a rover. Someone like Jeff is able to use it for his survey work and not have that base station on the job at all times.” Once Branco installs cables and sensors on the machine and activates the modem, the contractor accesses the network via a wireless telecommunications service provider.
GNSS has become even more reliable and accurate in recent years by adding 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.
Like CM at Risk project delivery, this equipment can provide flexibility to the grading or excavation portions of a project. “Things change so quickly and if we have our 3 D model set up and we want to pull off one section of the job and go to the next, we don’t have to get the surveyors to come out and re-stake it,” says Fields. “This technology gives us the ability to do that. That’s one of the big reasons we decided to go with the 3 D system.” Fields, who has a surveying background, learned conventional surveying skills while working for his grandfather starting at age 14 and eventually surveying in several Midwestern and Western states over the years. He now uses the GNSS equipment to verify takeoff estimates and collect location data used to build digital site models using Carlson software. Fields also troubleshoots the system with the help of Branco and Hermsen, noting that a phone call gets the system up and running again quickly.
Excavating with precision
In early March 2009, with crews staking curb work for a winding entry road and masons constructing brick walls on the new maintenance facility, Fields walked over the buried pipe that had been installed three months earlier. Haydon had used a relatively new method of exposing the existing communications and water utilities, which would all be kept in service, before locating them with the X63. A Ditch Witch FX60 Vacuum Excavator was used for subsurface utility engineering, a nondestructive method of locating buried underground utilities that also improves worker safety. The vacuum excavator located utilities by forming clean cut, targeted potholes above the utilities without adversely affecting the surrounding soil stability.
After locating the utilities, Haydon had Dirt Pro LLC of Higley, AZ, build 3 D site models and loaded the models into the X63 system control box via a flash drive. A Caterpillar 330 CL excavator was equipped with Topcon’s X63 grade control system, which is specifically designed for excavators. The system consists of four temperature compensated, 360 degree CAN based tilt sensors that measure angles from the cab, boom, stick and bucket; a GX 60 color touch screen control box; two GPS+ antennae; and a GPS+ receiver.
While digging trenches, the excavator operator ensured the proper digging depth by viewing on the GX 60 the machine’s exact position on the site, in addition to the bucket’s constant position. The system eliminated the need for a grade checker to continuously monitor excavation depth. The depth for the potable water pipe and sewer force main was about 4 feet and the width about 2 feet. The depth for the gravity sewer was about 8 feet and the trench width about 3 feet.
“Basically, [GNSS] eliminates the need for a guy manually checking grade, whether he’s shooting it with an eye level or just simply putting a tape down or even a grade stake,” said Manny Tarazon, pipe superintendent for Haydon’s Heavy Civil Division. “You can also check the elevation on your pipe – if you put that bucket right on top of that pipe, you can actually take a shot. We were sloping the sides of the trenches, too, so the system even helped us to make sure we had our proper slope, which was 2:1.”
FOR MORE INFORMATION:
Topcon Positioning Systems Inc., (925) 245-8300, www.topconpositioning.com
ABOUT THE AUTHOR: Don Talend of Write Results, West Dundee, IL, is a publicity and communications project manager specializing in construction, innovation and technology.