It’s time to apply to the Roads & Bridges Top 10 Awards, our annual list of the best road and bridge projects completed this year.
A lot of folks wait until the last minute to apply, and I get that. I procrastinate all the time. Just ask my mom, who requested I paint my old bedroom “at some point this summer.” And I plan on completing the task this weekend, just a few days shy of Sept. 21, which, though not official socially, is the actual end of summer.
I don’t want you to be like me, though. And to inspire you to apply our end of the year awards, I’m going to tell you about one project that has already applied: The I-10/Houghton Road traffic interchange reconstruction project, 15 miles southeast of Tucson, Arizona.
Traffic at this 50-year-old interchange was projected to rise by 50% by 2045, so Arizona’s Department of Transportation worked with Stanley Consultants to build a new diverging diamond interchange. The $24.4 million project began in the summer of 2020 and finished this spring.
This is the second full diverging diamond interchange in Arizona, and it increases the capacity from 8,000 vehicles per day to 50,000.
The full diverging diamond interchange (DDI) is considered much safer than others because there are fewer conflict points, particularly in left turn movements. Furthermore, there are lower construction costs, lower right-of-way impacts, the highest traffic accommodation and positive public support.
As part of the original design concept study, Stanley Consultants engineers researched similar DDI projects in other states to design Houghton Road, which was custom in many ways. For example, rather than using bridge abutments, the design called for full height retaining walls behind piers, a concept Stanley Consultants refined with its own adjustments on the mammoth South Mountain Freeway program, the largest in ADOT history. The design refinements are being copied on other projects in Arizona.
These pier-style exposed abutments are like a regular pier except for a bridge span on one side and an approach slab span and a mechanically stabilized earth (MSE) retaining wall on the other. The retaining walls simplify analysis, design, and detailing. The abutment primarily carries the vertical load while the retaining wall primarily carries horizontal forces. The result is construction simplicity, constructability, flexible work sequencing and a competitive cost.
The Stanley Consultants-led team resolved the question of proper setback distance from the abutment and pier to the MSE wall. It also developed details to keep the MSE wall compacted, behind the wall and prevent it from migrating outside. Another innovation was to integrate the MSE front wall with MSE wing walls in various configurations.
An additional challenge faced by the design team was that two nearby projects tied into the Houghton I-10 intersection. The city of Tucson was designing a project to the north while the county was designing south of the interchange. Both had to be coordinated with the interchange’s construction to ensure a seamless design between various roadways and maintaining traffic during construction.
Because it was a busy intersection and close to state fairgrounds, maintenance of traffic (MOT) was managed carefully. Construction of the new piers and bridge occurred beside the old bridge, which carried traffic while the new structures were built. The interchange was seldom closed for construction, except for some night and weekend disruptions.
The full DDI layout allows local traffic to shift to the left when the traffic signal is green, allowing motorists to turn directly more safely left onto entrance ramps without stopping at an additional traffic signal or having to wait for oncoming traffic to clear. Other features of the Houghton design include:
· A new 7-lane bridge designed by a subconsultant with pedestrian and bike lanes and full shoulders.
· Green pavement markings were painted in turn lanes to highlight where motorists are supposed to drive. Special signage and pavement markings and lighting at all ramps, crossroads and the interchange guide both motorists, pedestrians, and cyclists.
· An aesthetic design included piers shaped to look like cacti, with desert colors throughout.
· Various turning movements throughout DDI accommodate large trucks.
· Crossover intersections accommodate special over height vehicles.
· Space to accommodate I-10 widening.
Pretty cool project, right? Projects like this are why we hold these annual awards. It’s not a finalist. Not yet, anyway. That might depend on you and whether you apply to Roads & Bridges Top 10 Awards. Our deadline of Oct. 1 is rapidly approaching.
So, please, click here and apply. And remember: the more details you give the better. We love learning about every aspect of these projects. It’s part of what makes our job so fascinating.
Thank you for your time and your participation in these annual awards.