High Precision on the Move
With up to five lanes running both north and south, Interstate 15 south of Salt Lake City is a vital transportation corridor for fast-growing Utah County as well as for the state at large. In 2009, the Utah State Legislature approved a $1.725 billion bond to move forward on a project called the Utah County I-15 Corridor Expansion (I-15 CORE), a project that was originally started in 2008 but then put on hold due to budget constraints. One of the nation’s largest construction projects, I-15 CORE will expand the freeway by two lanes in both directions from Lehi Main Street to Spanish Fork Main; extend the express lane from University Parkway in Orem to Spanish Fork; rebuild and reconfigure 10 freeway interchanges; replace and restore 55 bridges; and provide additional improvements that will meet or exceed travel demands through the year 2030 including using 40-year concrete pavement along the entire corridor.
When PRC discovered early in the construction stage that the original topographic mapping was at 1-foot contour intervals and did not contain enough detail for final paving design, the team turned to DEI Professional Services LLC, a civil engineering and land surveying firm based in Phoenix. DEI had developed a system of mobile surface “scanning” that would allow the team to create a detailed topographic map without closing I-15 to traffic. Using conventional surveying equipment, DEI’s system produces digital terrain models of existing pavement surfaces and can “scan” new road surfaces as they are built during construction. “To be confident in our design, we decided to do the mobile mapping over nearly 19,000 lineal feet across all lanes at the north end of the project,” says Scott MacKenzie, PRC’s survey manager for I-15 CORE. “We got bids from other companies for the mapping, and Jason [at DEI] gave us the highest confidence level that he could do the mapping within the time frame and accuracy that we needed.”
In March 2010, PRC awarded DEI a $1.9-million contract for the construction surveying over about 10 miles of the project, which includes 28 permanent bridges and six temporary bridges.
Regular GPS EquipmentThe survey work began on April 27. To accomplish the mobile mapping, DEI fitted each of three half-ton pickup trucks with two Topcon GR-3 receivers. The receivers, one on each side of the truck, are attached to a steel frame that extends across the truck bed above the rear axle. Inside each truck cab are two Topcon FC-200 data collectors, one for each receiver. The data collectors use a wireless Bluetooth connection to the receivers.
To perform the work, the three trucks line up behind each other and drive each lane of I-15, traveling at about 1.5 mph. A Topcon feature on the data collectors called Auto-Topo collects a shot from each side of the truck at 25-foot intervals. The mobile mapping is done at night to minimize traffic disruptions. “We’re rolling a total of six receivers at once,” says Randy Hager, DEI survey manager. “Behind the trucks, we have a highway patrol car with lights flashing, then a truck with a mobile arrow board, then behind that a crash attenuator truck. We can do some mobile mapping during the day, but not in areas of live traffic. We’re not allowed to impede traffic during the day.”
The three trucks provide redundancy of data, says Jason Kack, RLS, a DEI principal. “We create a digital terrain model with each truck’s information,” he says.
The center lanes of I-15 are being overlaid in some locations, and the new lanes are being added to the outside in a separate construction phase, so it’s important to precisely locate the existing elevation of the roadway--and the data collected by the receivers can do that. Kack says that on average, 92 percent of the collected points fall within +/- 0.04 foot of each other.
Once the points are gathered in the Topcon FC-200 data collectors, the points are downloaded into Topcon Tools, software that is located at DEI’s onsite construction trailer. “In Topcon Tools, we edit out the outlying points, the GPS points that have high residuals,” Kack says. “Those are not accurate points.”
Next, the data are exported to AutoCAD. “In AutoCAD, we make three DTM surfaces,” Kack says. “Then we have proprietary software that can average those three surfaces into one. The final DTM surface is exported to MicroStation, which UDOT and PRC utilize.” DEI also provides graphs that indicate a statistical distribution of the points collected.
For GPS correction signals, DEI uses a cellular telephone base station set up by Rocky Mountain Transit. “With the cell base, we’re able to cover all of our area,” Hager says. “It’s good because we don’t have to set up a base station and take it down every day.”
Hager likes the Topcon equipment. “The GR-3 receivers are sturdy and they’re very reliable,” he says. “We have to use them under harsh conditions. And I can take my FC-200 data collector from a GR-3 to a total station or a robot; it’s interchangeable. I can even run a digital level with it.”
A Speedy SolutionDespite the relatively slow driving speeds, Kack says the mapping can be performed quickly. “We’ll have mapped 3.5 miles of both the northbound and southbound sides in just six nights,” Kack says. “We did five passes on each side, for a total of 10 passes. From the time they told us to start until they had the end product was just two and a half weeks.”
And speed is paramount at I-15 CORE. MacKenzie says PRC could not afford to set control points and do setups with a regular prismless robotic total station, which can do scanning. “We would have needed [at least] 40 and probably 60 setups with a total station, and we didn’t have that kind of time,” MacKenzie says.
MacKenzie is impressed with the work so far. “From what I’ve seen, the mobile mapping works very well,” he says. “It’s done exactly what we hoped it would do. We analyzed what DEI has given us, and you can see that it’s useful information.”
MacKenzie says the resulting data can be used to establish pay quantities for roadway layers--crushed stone, asphalt, and concrete. But there was another factor that played a role in the decision-making process. “The big advantage to mobile mapping is safety,” he says. “That’s the big one for me. We don’t have to have anybody walking on the pavement. Your exposure of people to traffic is eliminated.”
PRC is currently considering whether to use the mobile mapping process for as-builts on other areas of I-15 CORE. For that work, Kack plans to use a Topcon 9001A1 robotic total station and an FC 2200 data collector to do mobile as-builts on the asphalt and concrete layers of the pavement. This combination would provide increased accuracy over the GPS systems. With one setup of the robot, DEI can gather data on 2,000 feet of roadway. The truck would drive each lane in succession, and then a surveyor would move the robot and repeat the process.
As part of its construction staking effort, DEI will also do the surveying for three bridges being built with accelerated bridge construction methods. These bridges are completely assembled on temporary abutments near their permanent locations and are then moved overnight into their permanent abutments. The effort requires extremely precise quality control and quality assurance over the bridges’ dimensions to make sure they fit.
“PRC is a very forward-thinking company, and they embrace the latest technology to provide the best product and value for UDOT,” Kack says. “It is very exciting to work with PRC and UDOT on such a landmark transportation project.”
For more information about the I-15 CORE project, visit www.i15core.utah.gov. More information about DEI can be found at www.deipro.com, and more details about Topcon surveying and imaging equipment can be found at www.topconpositioning.com.
Georeferencing for Easy Object IDAs DEI collects the as-built shots with the GPS rovers and develops the DTM surfaces in AutoCAD, the firm makes sure to georeference any objects that may be of interest to PRC and UDOT, such as automated traffic management signals, water valves, drainage structures and stop signs. The surveyors in the field take digital photos of the objects, and these objects are then precisely located and referenced in the deliverables through the use of appropriate symbols at the point locations. Clicking on each symbol in AutoCAD, MicroStation or Google Earth opens the corresponding digital photo.
DEI is also using AVI technology to georeference 30-second video clips and spoken narrative about certain objects. For example, Kack says if a water pipe junction is about to be buried, it can be useful to the owner to have video verification of the technician explaining that the junction had a proper thrust block in place.
“Georeferencing provides greater detail, by utilizing pictures and narrated video, than is possible with conventional as-builts,” Kack says. “Anyone can take pictures and video for a two-year span, but georeferencing adds the ability to locate the data quickly and with the understanding of the precise location of the feature being captured.” Georeferencing samples can be downloaded at www.deipro.com/html/downloads.htm.