Construction Business Owner, June 2009

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, AZ, 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., constructed a 24,000-square-foot, $9.9 million Arizona Department of Emergency and Military Affairs (DEMA) field maintenance shop in Florence, AZ, located about sixty miles southeast of Phoenix. It was one of the first projects in which Haydon used GNSS to control an excavator digging excavations 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 linear feet of 10-inch PVC pipe for potable water, 2,000 linear feet of 4-inch high-density polyethylene (HDPE) sewer force main, 500 linear feet of 8-inch PVC gravity sewer and 100 linear 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 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, 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 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" provide in-cab 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-one that Haydon uses-is subscribing to a network that provides positioning corrections using cellular technology. The contractor does not have to set up a separate base station on each jobsite.