A strong foundation for growth

November 2013 » Features » PROJECT CASE STUDY
Interstate interchange provides gateway access to support community values and regional development.
Interchange design considered the area's natural resources including wildlife, reservoir shorelines, and a wetland habitat.

A highway interchange is much more than a viable solution to link intersecting roads with safe transition zones. Often, it holds the key to effective community growth and development. This was certainly the case for the City of Fort Collins and the town of Windsor, two communities that rely on the interchange of Interstate 25 (I-25) and Colorado State Highway 392 (SH392) for their economic vitality.

Project
I-25/SH392 Interchange Reconstruction, Windsor and Fort Collins, Colo.

Participants
Colorado Department of Transportation
City of Fort Collins, Colo.
Town of Windsor, Colo.
Slaton Bros. Inc.

Product application
Two-stage retaining wall uses Tensar Grade Separation Solutions and Geopier Rammed Aggregate Pier System to accommodate consolidation of fill and variable batter angles and design geometries.

The interchange is an integral part of the regional transportation network and, for Windsor, nearly all traffic passes through the intersection of these two well-traveled routes. An increasing number of drivers also use it as an alternative access point to Fort Collins. Moreover, the interchange offers key access points to residential areas in Larimer County and also links to the Budweiser Center, an important regional entertainment destination.

From a continued growth standpoint, some models estimate that traffic will double or triple its current demand by 2030. Given current needs and projected growth, it was imperative that a well-planned and constructed interchange be delivered to the region. To see this vision come to life, reconstruction of several major highways was a critical need identified to ease existing traffic congestion and to build effectively with the future in mind.

In 2011, as work began on the new I-25/SH392 interchange, Fort Collins and Windsor partnered with the Colorado Department of Transportation (CDOT) to find solutions to the many challenges facing the construction of the new interchange.

According to Fort Collins officials, the project fixes a failed interchange and revitalizes a key gateway to the two communities. Along with enhancing safety and easing congestion, planned improvements embodied the values of the communities by protecting viewsheds, strengthening economic activity, conserving surrounding wetlands, and balancing the built and natural environment.

Project stakeholders wanted a structure that embodied the community's "can do" attitude and confidence. The result is an improved interchange with four new traffic lanes as well as left-turn lanes, 6-foot bike lanes, and 6.5-foot sidewalks along with a carpool and vanpool park – a nod to the conservationist nature of the community.

The area is comprised of both commercial and residential buildings. It was important to bring stakeholders together when it came to making key decisions and conducting cohesive planning for the high-priority project. The interchange area had the opportunity to become a remarkable gateway to the communities of Windsor and Fort Collins, but also a memorable feature to all travelers arriving in northern Colorado.

Challenging site conditions
When planning the interchange project, the city's stakeholders had several natural resources requiring extensive consideration. The area is home to a bald eagle migratory roosting habitat and also includes the shorelines of Fossil Creek Reservoir and Swede Lake, and a wetland habitat. These natural resources are vitally important to the community and its surrounding area. The Colorado Division of Wildlife ensured that the buffer standards established by the Fort Collins Land Use Code were considered and addressed throughout the planning and reconstruction phases of the project.

Another factor that the engineers took into consideration was alleviating existing traffic congestion and lengthy travel times. This project was seen as the foundation for this rapidly growing area, allowing for further commercial and residential growth along the roadway by offering multi-modal transportation options, the development of new commercial centers near transit, and the interconnection of open lands.

Through the duration of the project, the interchange remained open to traffic with the exception of a two-week time period, mid-project, when the old bridge was demolished and traffic was rerouted to the first phase of the new bridge.

The improved interchange includes sections of mechanically stabilized earth (MSE) retaining wall structures with variable batter and veneer facings of cut stone, providing a lower cost alternative to cast-in-place concrete retaining walls and other conventional solutions. Additionally, the MSE structures blend in well with the local environment while performing their essential stabilizing function.

A senior bridge engineer involved in the project stated that the project was unusual because of the variable geometry of the walls, which were constructed using welded-wire-form walls. He also mentioned that the cut stone veneer echoed the look of the surrounding boulder and stone outcroppings and that there were many variable wall batters and offsets that required installation precision.

Slaton Bros. Inc. worked with engineers from Tensar International, a provider of geosynthetic soil reinforcement, and other consultants to address the project's unique geometrical requirements. Ultimately, they selected several Tensar Grade Separation Solutions including the Mesa Retaining Wall System, Tensar Wire-Form Retaining Wall System, and Tensar Temporary Retaining Wall System. Additionally, the Geopier Rammed Aggregate Pier System (RAPS - by Geopier Foundations, a Tensar Corporation subsidiary) was selected as a component of the project's foundation system.

The interchange retaining walls feature variable batters and offsets. A cut-stone veneer echoes the surrounding boulder and stone outcroppings.

Since consolidation of the fill material was anticipated, the design called for construction of a two-stage wall. Stage one was installed with the more flexible Tensar Wire-Form System that can tolerate movement as the fill consolidates; stage two utilized the more rigid and aesthetically matched cut-stone veneer that serves as the final dressing for the two-stage wall. After the first stage (wire-form wall) of the installation was completed, shotcrete was applied to the wall face as a substrate for the second stage (cut stone veneer). The Tensar wall system was selected because it could conform to the challenging variable batter angles and design geometries of the project.

Stage one of the two-stage wall used a Tensar Wire-Form Retaining Wall System to accommodate movement as the fill consolidated.

While a two-stage wall is typical on projects where excessive consolidation is expected, the geometries associated with this project were quite unique and were a result of site boundary constraints and special limitations.

Mesa Retaining Walls Systems were installed in other areas, providing an excellent solution to a challenging project. All retaining wall structures were stabilized using Tensar high-density polyethylene (HDPE) geogrids. In the welded-wire-form structures, the geogrid provided soil reinforcement and tie-ins for the shotcrete facing and cut-stone veneer. In areas where Mesa Retaining Wall Systems were being installed over weak soils, RAPS, engineered by the Geopier Foundation Company, were installed to improve the existing foundations.

Stakeholders' involvement
Keeping the community involved and informed was also a significant priority for the project team. In all, more than 30 property owners within the Corridor Activity Center were directly affected by the interchange. To ensure effective and ongoing communication, two municipal websites provided up-to-date community information. Additionally, during design and construction of the project, the project team conducted five property owner meetings for the immediate area and two public workshops for the larger community.

The interchange project is a great example of effective collaboration. Efforts required collective agreement between Fort Collins, Windsor, CDOT, Larimer County, and the North Front Range Metropolitan Planning Organization, with the final result being a cohesive vision between all parties. It is something that everyone involved is quite proud of and, most importantly, it addresses the needs of the surrounding communities, with an eye to the future.

The completed project improves both local and regional traffic, offering access into both Windsor and southeast Fort Collins. For Windsor, the interchange is the primary access point, while it serves as the first of five access points into the Fort Collins community.

The aesthetically pleasing solution was chosen because it met the expressed needs of the project stakeholders including design performance, landscape features, development standards, unique needs of affected parties, future growth requirements, and natural resource requirement within the area. Additionally, CDOT and local municipal owners were pleased that the project was completed on budget and five months ahead of schedule.

Today, the reconstructed interchange serves as a positive reflection of Fort Collins and Windsor as places to live, work, shop, and eat, offering a variety of transportation options that improve the quality of life in these communities.

Dustin Bennetts, P.E., West District manager, Grade Separation Solutions for Tensar International Corporation, contributed to this article.

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