An Accurate Point of Reference
All photos by Don Talend
At the “Big Rock Interchange” in Little Rock, Ark., where Interstates 430 and 630 intersect, cars and trucks weave through a mass of partially built concrete and steel structures and numerous cranes and dirt-moving machines. Orange pylons and barrels help guide drivers into different lanes and around detours. Venture into this construction jungle, and the complexity of meshing construction processes with traffic flow becomes apparent.
Take the process of erecting massive steel girders for four new or reconstructed ramps that are being added to the existing cloverleaf interchange. Telescopic truck-mounted cranes operated by Barnhart Crane & Rigging Co., Memphis, Tenn., picked up these girders--which vary in length from 50 feet to more than 200 feet and weigh 15 to 60 tons--at two points and placed them onto concrete columns or jacks, depending on the section. For obvious public safety reasons, traffic was diverted when girders were erected above existing lanes, and the work was scheduled outside of rush hour whenever possible.
For one girder erection sequence that constructed the flyover ramp from southbound I-430 to eastbound I-630, the work was scheduled to be completed in 20 days. Workers were onsite around the clock in three nine-hour shifts. In late July, as the girder sections were being joined from both ends, traffic was alternately rerouted to a service road in either direction so that the work could be done above the existing northbound and southbound I-430 lanes.
The tight schedule meant that the cranes had to be located at an optimal relative position to reach the girders’ officially surveyed X, Y and Z locations. Additionally, the girders had to be placed at strategically plotted locations to allow the cranes to lift within their rated capacity. The task of marking points for the cranes’ locations fell to Phillip King, surveyor for Manhattan Road & Bridge Co. of Tulsa, Okla., who used a Topcon Positioning Systems GPT-7500 total station for the job.
Jim Jolly, general superintendent for general contractor Weaver-Bailey Contractors, El Paso, Ark., notes the importance of crane positioning in the context of the overall project. “That’s the biggest issue with the girder erection--making sure that the cranes are in the right spot so they don’t have to be moved around,” he says. “What Phil has been doing is very important, in my mind. The cranes have to reach out, and they can’t get overloaded. If they get too close, they can’t hit it. It’s got to be perfect.”
The girders were fabricated offsite at AFCO Steel’s Van Buren, Ark., plant and delivered to the site in a just-in-time process and staged near a crane. Then it was time for the critical lifting process. Van Thompson, project engineer with Barnhart, explains how each girder was erected. “The important thing from our end is weight and how far away the girder is from the crane’s center pin,” Thompson says. “The precision comes in knowing where the crane is going to be positioned onsite and where the girders are going to be staged, and knowing how far that staging spot is from the center pin of our crane.”
Another surveying task for which King was responsible was locating the ramp column and column cap elevations, and marking the locations of formliners that yielded pine cone engravings on the columns. The elevations had major implications for girder splicing.
King identified reference points for the cranes by setting up his total station in locations within 2,000 feet of the staging area, where he could localize the instrument relative to control points he had set per the official survey and obtain line of sight to the girder staging area.
“What we’ve done is use Phil’s survey marks that he’s spaced out, and pulled radius lines off of that,” Thompson says. “Where the radius lines intersect, that’s where we know to set up our cranes.”
Thompson says the precision obtained from King’s work with the GPT-7500 is critical. “It’s giving us reference points on the ground that we can take back to our model. And, based on distances from common points--his survey points are common points--we know where to set the cranes, we know where to put the girders and we know the path that the girders are going to take as they’re lifted up, swung and set.”
On days when Barnhart erected girders, King received a reference sheet with the cranes’ safe lifting radiuses. The Arkansas State Highway and Transportation Department had provided control points for each of the ramp areas, and King had verified their locations earlier. For the flyover ramp from southbound I-430 to eastbound I-630, he verified six control points on the shoulder. “Ideally, you want two control points and you set everything off of them, but that’s not feasible on a job like this because you have to maintain a line of sight,” King says.
Accurate points allowed Barnhart to determine suitable pick points on the girders. “We have a picking point for one crane and for the other crane,” King says. “They have it engineered to determine what they can pick up at a certain angle for the boom locations.”
King verified the accuracy of the points he provided using the GPT-7500. “We did a traverse on my points and came within sixteen-thousandths of an inch--you don’t get much closer than that,” he says.
Due to the accuracy and precision of the survey work, the girder erection work was able to proceed on schedule. “Taking field measurements prior to the job, when traffic was not diverted, was never an option,” Thompson says. Having accurate survey marks allowed us to plan the job without venturing near oncoming traffic. One of the most dangerous aspects of this job is working close to traffic.”
King, who has done surveying work for more than 20 years, says that the ramp work was error-free, but not by accident. “I think the key to success on any project is to check and double-check,” he says. “If you work with something every day, you know when something is not right. A lot of times I’ll have to lay out half of the column and cap from one point, and then I’ll have to move over to another point because there might be a crane in the way of my line of sight--it’s all got to match up. When you get comfortable with the equipment you’re working with, that’s half the challenge.
“That’s why construction surveying is so critical--it’s the cost,” he continues. “You’re set on a timeframe, but the cost of redoing something is just tremendous; you can eat up your profit. You can’t take a tape measure out there and check yourself. You’ve got to have faith in yourself and your calculations.”
Big Improvements on the Big Rock InterchangeThe I-430/I-630 interchange improvements in Little Rock, Ark., are upgrading an existing cloverleaf intersection and providing better access to nearby streets and collector-distributor roads from westbound I-630, which ends at Shackleford Road, to the next north-south road located west of I-430.
The Arkansas State Highway and Transportation Department wants the $124 million project--the biggest publicly funded project in state history--to be completed by the end of 2014.
The roughly $88 million Phase III of the project includes a new overpass for westbound I-630 over Shackleford Road, a rebuilt I-430 overpass to accommodate the new I-630 overpass and two flyover ramps: one going from I-430 south to I-630 east and one from I-630 west to I-430 south.
The improvements to the original interchange from the early 1970s were deemed necessary because it now carries more than 180,000 vehicles per day, considerably more than it was designed to handle. The interchange is a key transportation hub for the west side of Little Rock, one of the fastest-growing areas in the region. In fact, I-430 is the only vehicular crossing over the Arkansas River west of downtown Little Rock for the metropolitan area. I-630 is the primary highway near Little Rock providing access to major employers including the University of Arkansas Medical Sciences and the University of Arkansas at Little Rock.
The junction is known as the “Big Rock Interchange” because of a large rock formation that was uncovered in the area during construction. Rather than removing it, the decision was made to leave the rock in place and build the interchange around it.