An Artistic Overpass

By K. Robert Wendel

Beautiful scenery, relatively cheap land and retiring baby boomers are creating a thriving community in Sahuarita, about 30 mi. south of Tucson on Interstate 19.

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The growth is also creating a lot of traffic.

Founded in 1994, the town has grown from just 1,900 residents to more than 10,000 today. Getting in and out of the town has been difficult with the existing narrow bridge.

"That old bridge is a huge bottleneck," said Sahuarita town manager Jim Stahle. "There's so much traffic going off that narrow bridge and the geometry is so bad it's hard to get off the freeway and take a left in either direction."

To remedy that, the Tucson office of Granite Construction is teaming with the Arizona Department of Transportation to build a new and wider overpass on Duval Mine Road spanning the heavily traveled interstate at Sahuarita. It won't be just another gray concrete structure. Designers incorporated playful forms and a unique radius on the central piers, creating an overpass with a western motif. Forms of horses are also incorporated into the retaining walls.

"This is one of the hardest projects I have done," said Granite project manager Dave Darling. "There's a lot of public art incorporated into the structural design, so it makes it interesting."

Granite Construction is building the $12.6 million project under an A + B contract, which means the company will face liquidated damages of $6,000 per day for every day beyond the 300-day contract time frame that ends in October.

Contractors are moving a massive amount of dirt to create new, longer and wider approaches to the new overpass. Tucson-based Gravel Express is moving more than 250,000 cu. yds. of borrow material from a nearby pit, with another 97,000 cu. yds of excavation for the roadway and drainage.

Although the existing overpass is in good shape, engineers made the decision to tear it down after examining the approaches.

"Structurally, the overpass was in decent shape, but it was just too narrow," said ADOT senior resident engineer Ken Damgaard. "We designed the entire configuration and interchange to make a tight diamond interchange so the ramp comes up to the intersection as close to the bridge as possible."

Crews will place more than 15,000 cu. yds of portland cement concrete paving for the eight ramp approaches. The 9-in.- thick PCCP sits on top of 4-in. of asphaltic cement base course.

Intersections get a 5-in. layer of asphalt in two lifts, sitting on 6 -in. of asphaltic base course. The entire project is topped off with a friction course of rubberized asphalt.

The abutments retaining the ramps are poured- in-place concrete parapet walls that are on a radius.

The abutments sit on 87-ft.- long, 6-ft.- diameter. drilled piers and were installed by the Tucson office of Becho Inc.

"For this location, it seemed cast-in-place worked a whole lot better," said transportation project manager J.D. Taylor of Tucson's HDR Engineering.

"The contractor was able to build the abutments quicker, faster and cheaper than a mechanically stabilized earth wall."

HDR was the engineer of record on the project

The bridges, which span 240 -ft. over I-19, are precast type 5 modified girders in 120-ft. sections. The spans rest on the radius pier in the middle of I-19.

Once the first bridge section is built, traffic will be switched over to it. The old bridge will then be demolished and contractors will start on the second phase of the project.

When complete, the new bridges will be six lanes, with two in each direction along with turning lanes in both directions. Both intersections are signalized and also feature pedestrian sidewalks.

The project should alleviate some of the congestion caused by due to a large retail center on Duval Mine Road on the east side of I-19.

Because of the record amounts of rain this year in Arizona, crews are having a tough time staying on track. They're working eight to 10 hours a day, six days a week, when the weather permits.

"We'll get it done in time if we don't get too much more rain," said Granite superintendent Gary Bowing.