I-90 Quinn Creek Structures

Design Highlights: Quinn Creek Structures

In 2020, MDT hired Morrison-Maierle to design replacements for the three-span structures. The design’s main highlights include:

Accelerated timeline

Since the Quinn Creek Bridges are on the Bozeman Pass, we had a narrow construction window because of the higher elevation and potential for an early winter. Traditional construction would have resulted in two full seasons, which would have been frustrating for the traveling public, contractor, and MDT inspectors. Because of this, we worked on finding a solution that could be completed in one summer.

We focused on creating a positive experience for the traveling public during design. As a key travel corridor between Livingston and Bozeman, this section of I-90 sees more than 17,000 vehicles daily. We wanted to find a solution that would reduce delays, shorten construction time, and provide a smoother experience for commuters and travelers.

Using the bridge slide, we reduced construction to just one season rather than two, cutting down the time with crossover head-on traffic. We also avoided active construction during shoulder seasons, which come with an increased risk for weather-related challenges like temperatures too cold for paving, rain, snow, and ice.

A tight and efficient construction schedule was also crucial for the homes and businesses in the Quinn Creek area, as they only had one way to get to the frontage road under these bridges. Additionally, our ability to reduce construction time helped with the labor shortages caused by the Bozeman area’s affordable housing shortage.

Slide-into-place design

Slide-into-place design involves building the new bridge deck to the side of the original and then sliding the new structure into place once the bridge is complete. This is advantageous because it shortens construction time and reduces the time travelers use crossovers and deal with two-way traffic and reduced speeds.

For Quinn Creek, we used the slide design for one structure and completed the other using more traditional methods. This allowed us to complete both bridges over the course of one summer, even with the wet spring of 2022 that caused construction delays.

Low-density cellular concrete

The original embankment had settled more than a foot since its initial construction because of the poor soil conditions at the ends of each bridge. We opted to use low-density cellular concrete to minimize settlement from the weight of traditional backfill materials. Cellular concrete is a lightweight air-bubble-filled material, and at 38 lbs per cubic foot, it weighs about 25% of regular concrete and is far lighter than traditional bridge end backfill at 130-140 lbs per cubic foot.

Shortened structure

We shortened the structures by placing the new abutments in front of the existing abutments. This allowed us to use just one span instead of the original three while maintaining vertical clearance.

Other advantages of the shortened structure included the ability to reduce the size of the deck area, which will help with its attendant future maintenance expenses and more easily facilitate the bridge slide.

Effective Design Now and in the Future

Several techniques we incorporated on the Quinn Creek project will contribute to how similar engineering projects occur in the future. For example, this marked the first time we used slide-in-place construction on an interstate project. Due to the conditions, the need to expedite this project, and public safety concerns, it was an excellent trial for this type of design. Pairing the slide design with more traditional methods demonstrated how to flex future designs to include multiple routes to the end goal.

While beneficial, slide-in-place construction contributed several complicating factors to the project. The slide was associated with several challenges.

We had to design the structure in a way that allowed it to be lifted (similar to jacking up a car) and pulled into the correct place without twisting, dropping, or torquing, all of which could lead to cracking the new concrete deck. The tolerance consideration was just 1/8 inch, heightening the stakes.

Finally, because the bridge sits on a 5.3% longitudinal grade, there’s a 6-foot difference in height between one side of the bridge and the other. The steep grade puts the bridge at further risk of sliding down the slope while being pulled laterally into position.

Thanks to the new structures, travelers between Bozeman and Billings can count on safe and reliable transportation over Quinn Creek.