GPS machine-control technology is increasingly being used in earthmoving applications. A Phoenix, Ariz.-based firm recently used machine control to eliminate the need for surveyors to check the depth and slope of utility excavations at a military field maintenance shop. The cost savings boosted cost-minimization efforts under 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-ft2, $9.9 million Arizona Department of Emergency and Military Affairs (DEMA) field maintenance shop in Florence, Ariz., about 60-mi southeast of Phoenix. It was one of the first projects in which the global navigation satellite system (GNSS) was used 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. Haydon installed 3,535-ft of 10-in PVC pipe for potable water, 2,000-ft of 4-in high-density polyethylene (HDPE) sewer force main, 500-ft of 8-in PVC gravity sewer and 100-ft of 12-in HDPE storm drain. All but the potable pipe was new and not tied into an existing system.
Haydon used the CM-at-Risk delivery method, in which 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. The construction manager is responsible for ensuring that the project stays on schedule and conforms to contract details, including a guaranteed maximum price for the construction work, and takes on more of the risk than other project stakeholders otherwise would assume. Benefits of the 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.
How It Works
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 are also sent to the machine. The stationary base and machine work together to provide real-time kinetic (RTK) position information, revealing the machine’s 3D location on the site. Software compares the machine’s position to the design grade at a given location.
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 visual guidance for machine operators, who manually control the machine to cut or fill to the desired grade.
During the past several years, the systems have evolved in terms of flexibility and reliability. An alternative to a base station is subscribing to a network that provides positioning corrections using cellular technology. Where available this option provides flexibility. The contractor does not have to set up a separate base station on each jobsite.
Because an excavator has so many moving parts working in conjunction, equipping excavators with GNSS grading technology, as on this project, requires different configurations from dozers and graders.
A GNSS-equipped dozer has a single-slope (tilt) sensor on the blade to measure the cross-slope of the cutting edge. Like a dozer, a grader has a slope sensor for blade tilt in addition to a rotation sensor at the rotation swivel. A third sensor, known as a mainfall sensor, is mounted on the mainframe of a grader and provides slope measurement in the direction of machine travel for adjustment of the blade up or down. The sensors work together to maintain cross-slope while allowing for the rotation of the grader blade.
The excavator’s bucket can operate towards and away from the operator cab as well as up and down. It has a swinging body and a two-piece articulating element between the machine body and the earthmoving attachment, giving it a wider range of motion and requiring more sensors to account for swing, cross-slope and long slope as well as to pinpoint the bucket location. A typical excavator control system must make four measurements: cab to boom; boom to stick; stick to bucket hinge; and bucket hinge to bucket teeth. The GNSS must account for boom-and-stick articulation (and thus the distance between the bucket and cab at any given moment), bucket tilt and machine body levelness. Tilt sensors on the stick, boom, bucket and machine body continuously calculate the distance from the cab to the bucket teeth.
Topcon’s 2D systems for excavators comprise these non-GNSS sensors and can detect various physical references, such as the existing surface, a hub, a previous cut, or a rotating laser. The operator can choose a reference and enter a cut or slope depth. The systems allow the operator to create multiple elevation or slope designs and cut to the design without the need to stop and reestablish a reference. For 3D grade control, the GNSS components on the machine serve as the machine’s 3D GNSS sensors and pinpoint the location of the bucket teeth relative to the slope.
After locating the utilities in late 2008, Haydon loaded 3D site models into the 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° 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-ft and the width about 2-ft. The depth for the gravity sewer was about 8-ft and the trench width about 3-ft.
“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,” notes Manny Tarazon, Pipe Superintendent for Haydon’s Heavy Civil Division. “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.”