Feature: Improving the Grade

GNSS helps contractors grade and excavate the construction site for a high-tech high school.
by Don Talend

A new high school to open in Sandy, Oregon, in September 2012 will be a 21st-century facility in every way. Therefore it’s reasonable that the grading and excavation contractor is using technology on the site that’s entering the 21st century mainstream—global navigation satellite systems (GNSS) for grade checking and automated grade control.

The new $110 million Sandy High School will serve up to 1,800 students and cover about 310,000 square feet, nearly twice that of the existing school’s footprint. Financed by a $114.9 million bond, it will feature a new library that will include a main library, a drop-in computer lab, and computer research stations and will be nearly three-times larger than the existing school’s library. Fully equipped science labs will accommodate more lab experiences. Athletic facilities will include an indoor track and provide seating for 1,800 in the main gymnasium. A performing arts center will have stadium seating for about 500 people and will include an orchestra pit. The facility will be fully wireless, accommodating student handheld-learning devices, and will have a guest network for parental access. Architectural features include clerestories above the gymnasium for natural lighting, a displacement air system for air quality, an underground cistern to capture rainwater for non-potable use, and green roofs.

Sandy High School is slated for a Gold certification from the U.S. Green Building Council’s Leadership in Energy & Environmental Design (LEED) system. The site of the school itself, which covers more than 80 acres, is designed to suit outdoor learning experiences with ponds, trails, and wildlife. Given the rolling terrain of the Portland area, Konell Construction and Demolition Corp. faced many challenges in ensuring that final grading met specifications when numerous cuts and fills were completed.

By the time site preparation was complete and pavements were constructed, the general contractor, Hoffman Construction Co. of Portland, had begun constructing foundations and slabs on grade, which were to be completed by February 2011. By January, work was to begin on the building structure and skin, and interior rough-ins and finishes would start in April and are scheduled for completion by November.

Hilly Terrain

In addition to the rolling terrain, Konell was challenged by red clay in the soils and the need for a lot of dewatering that resulted from the presence of several nearby springs, according to Andy Webber, vice president. To deal with the problem, about 4,000 linear feet of underslab drains were installed, as well as 400 linear feet of footing drain. These areas are waterproofed under the slabs in order to prevent water from pushing up into the locker rooms, orchestra pit, and one wing of the building. Webber added that these measures are permanent construction fixes. Additionally, Konell used a total of 20 pumps for dewatering, some of which were installed in sumps (to excavate trenches) and in footings. Konell also added swales to redirect water through the site.

Another challenge was building structures to protect wetlands from runoff on a gravel road near the new school’s soccer field. Webber reported that all stormwater trenches and three detention ponds were constructed in summer 2010 to address these issues. Also, Konell constructed all critical storm lines before winter arrived. In addition to all of these permanent measures, the contractor armored additional areas to ensure sediment filtration. Even with these measures, Konell dealt with runoff on a daily basis.

Many of the areas where slabs on grade were constructed also needed to be cut. On projects like this, the costs of grade checking can become a major factor. In an effort to cut costs, project owners often pressure the contractor to handle grade-checking functions with their GNSS systems, a task traditionally handled by surveyors. On this project Konell used a base and rover HiPer Lite+ RTK system for grade checking, location of underground structures, and as-built checks. “We use the [GNSS] on all of our cleanouts,” said Webber. “All laterals for future construction are checked with it. We also locate stakeout points set by the surveyor in case they are disturbed during construction.”

With an excavator and dozer moving dirt nearby, Konell’s Jake Thornburg checked the elevation of the subgrade for a slab with the RTK system. He placed the pole on the red clay, checking the elevation display on a pole-mounted FC-100 data collector running Topcon’s Pocket 3D software. Thornburg used the system and the same process for a road that winds its way around the perimeter of the site. Since Konell started working on the site in September 2010, Thornburg reported, the surveyor on the project confirmed that the grades were within one-tenth of a foot.

Using GNSS receivers to check grades is nothing new to Konell, which first purchased a system about four years ago from surveying and construction supply company, PPI Group, Inc., which has offices in Portland and Seattle. It does not take long for Konell’s workers to learn how to use the system, according to Webber. “The learning curve with using the rover has been half of what I expected to get the field guys up to speed,” he said. “Jake picked it up right away and didn’t have any issues, and we’ve got another guy who’s really familiar with it. They picked it up really fast—it was a short learning curve.”

Thornburg added that RTK allows them to easily perform as-built checks and pointed out that the software allowed him to determine at a glance where cuts and fills were made and how much dirt was moved at any given time. Touching the FC-100 screen with a stylus, he said, “as I zoom out, it shows me the whole project, the contour lines with all of the grade changes. There’s the sidewalk; there’s a roadway; it’s still showing all of the contour lines. There’s the whole gymnasium building pad and parking lot.”

Automated Grade Control

One technology that was new to Konell was automated grade control for its dozer and motor grader. Before starting work on the new high school, the contractor purchased another HiPer Lite+ system, including a receiver and control box, and switched the system between a Caterpillar D-6 dozer and a Caterpillar 140G motor grader. Grading of the classroom wings was cut and filled with the D-6. The grader was used on a large parking lot, a bus parking area, the main gymnasium pad, and the loop road. The grader completed the native subgrade as well as the rock grade.

The 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. With corrections supplied from a local base, the system provides the machine’s three-dimensional location on the site in real-time. On-board software compares the machine’s position to the design grade at any given location, with the design-grade information having been built from the site plans.

The data files are loaded into the 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 used by contractors such as Konell 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.

GNSS has become even more reliable and accurate in recent years with the addition of a larger and more consistent Russian GLONASS satellite constellation to the U.S. GPS constellation. This dual-constellation capability roughly doubles the number of signals available to the GNSS antenna/receivers and provides a higher degree of positioning accuracy.

Konell had its dozer and motor grader configured for rough and fine grading, respectively. The grader is equipped with three sensors that work in conjunction to account for blade tilt and rotation as well as mainfall, i.e., the slope of such a long machine from front to back. Additionally, the motor grader is equipped with a valve kit that ties into valves on the machine and gives them the ability to make fine corrections. The dozer was equipped with a slope sensor, and the operator viewed a horizontal bar showing how close the blade put the grade to specification on the monitor in the cab.

Konell Construction and Demolition Corp., like a growing number of construction and excavation companies across the country, is finding ways to take advantage of machine control and guidance technologies to construct projects like Sandy High School with greater efficiency and confidence than ever before.

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Don Talend of Write Results Inc., West Dundee, IL, is a print and e-content developer specializing in covering technology and innovation.

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