A major interchange is undergoing a role reversal in Arizona's second- largest metropolitan area. Designed to solve traffic headaches and a dire safety problem, the Interstate 10 interchange project in Tucson is "flipping" the grade of Prince Road so that it will pass over, instead of under, I-10.
The revamp is part of a $76.4-million effort to widen I-10 along a two-mile stretch to eight lanes from six. Crews have to contend with six lanes of traffic at all times, a major railroad route and multiple utilities—all while keeping an eye out for potential archaeological finds.
Since last fall, nearly all of the project's first 180 days have been devoted to relocating 15 separate utility lines that criss-cross the construction zone. Utilities include high-pressure petroleum and gas lines, sewer and water lines, electric power and telecommunications. "It was basically an underground utility interchange," says Jody Rodriguez, utility coordinator with URS Corp., a design subconsultant.
Crews relocated 72-in.-dia and 78-in.-dia sewer lines with flows of around 22 million gallons per day. The contractor advanced four jack-and-bore operations, including a 450-ft-long, 60-in.-dia casing under I-10 to reroute sewer flow.
As is typical, crews found utility lines that didn't match the positions indicated on the as-builts. Surprisingly, some of the existing joint trenches were encased in concrete and had to be carefully demolished—in one case, while avoiding damage to a major electrical line, says Edie Griffith-Mettey, senior project manager with prime consultant AECOM.
Nevertheless, "we've been able to accomplish most all the relocations at this point without a delay to the overall project," says Todd Emery, district engineer with the Arizona Dept. of Transportation.
Besides allowing for better access and maintenance, designers had to figure out a way to protect the utilities from the weight of huge embankments and retaining walls required to elevate Prince Road. Thus, they corralled the utilities into a single utility corridor spanned by a 135-ft-long bridge. "Initially, we were going to put in a protection slab. But when we started doing the cost analysis, it was just as expensive to build a bridge," Rodriguez says.
A 223-ft-long bridge is being constructed to carry Prince Road over I-10, and a third bridge will take the road over a Union Pacific Railroad track. The existing configuration crosses the railroad at grade, which has led to significant traffic snarls and accidents, Emery says. Forty to 60 trains use the line a day, but this could double due to a planned Union Pacific expansion.
Phoenix-based Pulice Construction is building cast-in-place concrete bridges, removing earth embankments currently elevating I-10 over Prince Road and constructing temporary detours to divert all traffic in phase one to one side of I-10, says Aaron Insco, Pulice project manager. Existing frontage roads will carry half the traffic as crews complete half the bridges and roadways, then traffic will shift to the new half.
Most of the excavation from the I-10 embankments will be re-used to elevate the frontage roads that will tie into the top of Prince Road, but some 180,000 cu yd will be hauled away. Soft soil conditions have required nearly 400 drilled shafts as deep as 70 ft and 5 ft in diameter.
As crews slice away the embankments that used to elevate I-10, they will confront undisturbed earth that could contain archaeological artifacts—in fact, some artifacts already have been found during utility construction, Emery says. Construction activities will occur in two steps, pausing once crews reach virgin earth so that the archaeological team can survey and catalog the area. Meanwhile, Pulice will continue work in a pre-cleared area.
The project team also is looking out for the living. Because the area sees a lot of pedestrian activity that will be disrupted for the two-year-plus construction cycle, ADOT has set up a shuttle service to bring employees of nearby businesses to and from work on each side of the freeway. "This is the first time we've done that," Emery says. "There are a lot of firsts for us on this project."