The original bridge on County Road 93 stretched 400 feet with 20 spans, connecting a remote part of Logan County, Colorado. Years of use, coupled with intense river flows, had severely weakened its timber girders and undermined its foundation through scouring, leaving the structure compromised and in need of immediate removal. In its place, a temporary shoofly with corrugated metal pipe culverts was constructed by the County. This ensured traffic could continue flowing but was only a short-term fix.
The challenge was twofold: first, securing design and construction funding for the County to support a costly replacement structure; second, designing and constructing a durable, cost-effective bridge that could meet modern safety standards and withstand the intense hydraulic events of the South Platte River.
To tackle these challenges head-on, SEH embarked on a targeted approach to funding, design, and construction support that would bring a reliable, modern replacement to this essential crossing.
Drawing on expertise with the Bridge Replacement Off-System (BRO) grant program, our Team identified the County Road 93 bridge as a prime candidate for this federally funded initiative managed by the Colorado Department of Transportation (CDOT). The program supports local Colorado agencies in replacing deteriorating bridges, and SEH has guided multiple agencies statewide through planning, grant applications, design, and construction phases. For this project, the experts secured three grants: one for design, one for construction, and a supplemental grant to address construction cost increases due to COVID-19-related inflation.
With funding in place, the focus shifted to designing a structure that would not only meet CDOT standards but also address the unique site challenges. The project began with a detailed exploration of possible configurations and, after a thorough alternatives analysis, selected a 250-foot-long, three-span bridge. The optimal superstructure was determined to be a reinforced concrete deck supported by precast, prestressed concrete bulb tee girders. This design promised a long-lasting solution with minimal maintenance needs, ensuring safe and reliable passage for local traffic.
Equally critical to the project’s success was the bridge’s foundation, designed to withstand the river’s erosive flow. Guided by extensive geotechnical investigation and research on similar bridge crossings over the South Platte River, the team opted for a deep, driven high-displacement, closed-end pipe pile system to provide optimal stability in the sandy soil conditions.
The commitment to structural integrity extended well beyond the bridge itself. The surrounding environment, especially the northern tributary of the South Platte River, posed significant challenges that necessitated close collaboration between SEH's structural and hydrologic and hydraulic (H+H) engineers. Ensuring the bridge could withstand both routine and peak water flows was essential for its long-term reliability and safety. This led to a comprehensive hydraulic analysis, where the interaction between the bridge and the river was carefully examined in detail.
The hydraulic analysis included advanced two-dimensional (2D) modeling to ensure the new bridge could handle peak flows without risking erosion or undermining, and without negatively impacting the floodplain upstream or downstream. After evaluating several configurations, the team selected a two-cell overflow concrete box culvert to cost-effectively manage high-flow conditions and protect the surrounding area from erosion. The final design, a 250-foot-long, three-span bridge, was chosen to meet all CDOT bridge drainage criteria and Logan County’s floodplain permit requirements for a no-rise condition. Additionally, 2D modeling guided the design of riprap scour countermeasures, providing localized protection at the piers and abutments.
Rural communities face unique challenges in maintaining critical infrastructure, often managing vital assets like roads and bridges with limited staff and funding. For Logan County, securing resources for such an essential bridge project required guidance through the intricate processes of funding and compliance.
Working closely with CDOT, each phase of the project was carefully managed to meet regulatory standards, ensuring Logan County maintained funding eligibility throughout. This collaborative approach allowed the county to focus on the impact of the bridge for their community, while the technical and procedural details that often deter rural clients from pursuing essential infrastructure upgrades were expertly handled.
Despite the project’s remote location and complex challenges, the strategic approach to funding, design, and construction ensured that a safe, resilient, and cost-effective solution was delivered. The new bridge not only meets the immediate needs of Logan County’s infrastructure but is also poised to serve the community reliably for generations. By seamlessly integrating advanced hydraulic analysis with thoughtful design and foundation solutions, the project has set a new standard for bridge replacements in the region, demonstrating the importance of both technical expertise and careful environmental consideration in every step of the process.
Project
Logan County Road 93 Bridge Replacement
Location
Logan County Road 93 over the Northern Tributary of South Platte River
Client
Logan County
Features
Services