Poor soils require award-winning diverging diamond freeway interchange design

soils.pngSalt Lake City – Stanley Consultants, a worldwide provider of engineering, environmental and construction services, and its joint venture partner team was awarded the Engineering News-Record Mountain States 2020 Best Project in the highway and bridge category.

Projects were judged on design and construction quality, contribution to the community and the industry and how they overcame unusual challenges through teamwork and innovation, the magazine said.

As part of its ongoing response to increased development of Salt Lake City’s suburban growth, the Utah Department of Transportation targeted the intersection between Interstate 215 and Redwood Road (SR-68) in North Salt Lake, particularly because the northbound exit to Redwood Road would back all the way up to I-215. The 2-mile area around the interchange ranked as a D-level of service.

To solve the problem, UDOT wanted to widen Redwood Road and use the existing two-lane bridge over I-215 as part of a diverging diamond interchange that would span the highway. In addition, it would resurface about 2.5 miles of the highway with new concrete, and mill and fill 3 miles of asphalt on Redwood Road. UDOT called upon Stanley Consultants for the 100 percent design of the $30 million (construction cost) project.

“This project was a combination of three projects,” said Brad Humphreys, project manager for Stanley Consultants. “The scope changed during the process. We kept one bridge, added another bridge and turned the whole thing into a diverging diamond.”

Design engineers wanted to prevent soil settlement around the new bridge from affecting other structures. They were able to add enough space to keep the two bridges separated. Humphreys added that he drew on Stanley Consultants engineering expertise from the Denver and Phoenix offices in addition to from the Salt Lake City office.

Diverging diamond interchanges (DDI) over a busy freeway aren’t unique – UDOT built two others farther south – but they aren’t the norm. The Redwood Road DDI, would require an unusual configuration because of local soil conditions.

Usually twin bridges of a DDI are built fairly close to one another, about 50 feet. However, the alluvial, sandy and porous soils under the valley floor alongside the Wasatch Mountains are less than ideal for supporting the huge foundations of a modern highway bridge. If the new bridge required to complete the DDI were located next to the existing bridge, it would create a drag on the old bridge, which could cause it to sink to the point of failure.

To address the challenge, engineers located the second bridge 100 feet to the east of the existing bridge. They also maintained the vertical profile to match into existing roads, ramps and driveways. The final design reduced the new bridge’s skew, or angle that it crosses over the freeway, saving on construction cost because the design shortened the bridge. The distance between the existing structure and the new structure improves the new bridges constructability by giving the contractor access to both sides of the new bridge.

Stanley Consultants also documented an EPA categorical exclusion, a wetlands delineation, and cultural resource, historical and socio-economic reviews. Other pre-design services included hydraulics analysis, geotechnical analysis and design, sub-surface utility investigation and coordination, right-of-way acquisition and air quality and noise studies.

About Stanley Consultants

Stanley Consultants provides program management, planning, engineering, environmental and construction services worldwide. Recognized for its commitment to client service and a passion to make a difference, Stanley Consultants brings global knowledge, experience and capabilities to serve federal, municipal and industrial clients in the energy and infrastructure sectors. Since 1913, Stanley Consultants has successfully completed more than 25,000 engagements in all 50 states, U.S. territories, and in 110 countries. For more information on Stanley Consultants, please visit http://www.stanleyconsultants.com.

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