TOP 10 ROADS: Full frontage

October 06, 2011

Not only did the I-19 East Frontage Road have a 1.5-mile gap in it, between the Canoa Road and Continental Road interchanges, but the existing road was 30 years old and did not meet Arizona Department of Transportation (ADOT) standards. The two-way frontage road did not have shoulders for disabled vehicles. The I-19 northbound ramp at the Continental Road interchange was so close to the frontage road that there was no room for traffic signals. As a consequence of the less-than-ideal situation, the Continental Road interchange operated with significant delays.

After considering seven alternatives, the design team for ADOT and the Pima County DOT decided on an innovative braided ramp that combined the northbound on- and off-ramps and the frontage road into a single intersection with Continental Road. This is the first time this solution has been used on Arizona’s interstate highways, and the state plans to consider it again for problem interchanges with two-way frontage roads and growing traffic volumes.

The project included constructing three new bridges, widening an existing bridge and widening Continental Road under I-19. They widened Continental Road under the bridge from 80 to 114 ft without adversely affecting the existing bridges. The widening was critical to the success of the project. Without it, the level of service at the Continental Road interchange would not have improved significantly.

Five different structural alternatives were analyzed, and grouted micropiles selected. The micropiles were placed using continuously threaded hollow core rods 50 ft in depth and grouted from the bottom up. They were placed 3 ft apart and staggered using a standard soil nail drill rig. Each abutment has 156 micropiles, creating a rigid soil mass and allowing the removal of the sloped embankment with the placement of a retaining wall.

Using micropiles also allowed construction to be phased with lane closures and permitted the bridge to remain open to traffic throughout construction.

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