“It’s a quiet little town.”
That’s how Gannett Fleming Project Manager Karen Hobbs describes the town of Flagstaff. And she’s not wrong. A small metropolis of 70,000 souls, Flagstaff is couched in the mountainous, arboreal landscape of northern Arizona, a region that sees more than 11,000 freeze-thaw cycles each year. Roadbuilding in this region is a distinct challenge—but when it is done right, the results are awe-inspiring. Case in point being the reconstruction of I-40 through Kaibab National Forest.
This section of I-40, between mileposts 156 and 161, was eating up more than $300,000 of the area’s maintenance budget addressing potholes. Truck traffic was 24% ADT. Tire blowouts and vehicular damage were the norm. The Arizona DOT wanted a long-term solution, and as a result, portland cement concrete pavement (PCC) was employed as a wearing course for the first time in this region.
“We get a lot of snow up in this area; this is high country,” Hobbs told Roads & Bridges. “In some years this area gets over 100 in. of snow. In summer, it’s a monsoon. From June through September we can get repeated storms dropping up to 1.5 in. of rain at a time, over and over again. The reason the road failed over time is that it sits in a saddle between mountain ridges, so the water sits there with no way to escape. Typically our roads are asphalt, due to the prevalence of freeze-thaw and ease of maintenance, but on this road ADOT went with concrete without even an asphalt friction course over it. We can’t use rubber up north, of course, because it freezes. But ADOT was looking for better longevity on this road.”
Over the course of two seasons, the project was brought in. The eastbound section was done in 2017 as a full reconstruct. The entire pavement section was taken off, processed and relaid as a rougher 8- to 9-in.-thick subgrade, to allow water to drain through to pipes installed to aid dispersal. Then a new 14-in. doweled PCC was set on top. In 2018, crews have address the westbound side—which was treated differently.
“Because of the poor subgrade and saturated soils, the existing PCC was left in place and the new PCC was built over the top of it,” Hobbs said. “There was about 7-9 in. of asphalt concrete there at varying depths, because this was originally built over old Rte. 66. They removed the AC, left the PCC in place, then put a new AC base of variable depth on top of the existing PCC to correct the superelevation to meet standards (variable thickness = 2 in. to 1.5 ft), then the new PCC was laid on top of that at 14 in. thickness.”
The final result is a road that can handle the damning climate and keep the Flagstaff area on the move.
Location: Coconino County, Ariz., outside Flagstaff
Owner: Arizona Department of Transportation (Northcentral District)
Designer: Gannett Fleming Inc.
Contractor: FANN Contracting Inc.
Cost: $34 million
Length: 61,600 ft
Completion Date: Aug. 1, 2018