Mahaska County Bridge Replacement Using ABC Techniques

IA-92 over the Muchakinock Creek Bridge is located approximately one mile west of the intersection of IA 92 and IA 163 on the west side of Oskaloosa, Iowa. The Mahaska County Bridge Replacement project replaced the existing 140′ long continuous concrete slab bridge constructed in 1965. The existing bridge needed major deck repairs, and because the roadway was regularly overtopped during flood events, the roadway grade needed to be raised. The Iowa DOT hired HR Green to update the preliminary design, perform final design and plan preparation, and assist with construction phase design-related services. The replacement bridge was ultimately a 230′ long two-span continuous welded girder bridge with a 44′ roadway.

The Iowa Department of Transportation (Iowa DOT) performed a concept study to evaluate replacement alternatives. After considering using an off-site detour or a two-lane runaround at the bridge site, the preferred alternative was to replace the bridge using Accelerated Bridge Construction (ABC) techniques. The Iowa DOT Bridges and Structures Bureau (BSB) has encouraged a policy of considering the evaluation of ABC techniques to determine if the increased construction costs can be offset by reduced construction time and decreased cost to society caused by construction delays.

During the concept phase of Mahaska County Bridge Replacement project, the benefits of conventional versus ABC approaches to construction were considered. The BSB, the Design Bureau, and District 5 followed the evaluation procedure outlined in the Bridge Design Manual to calculate an ABC Rating Score. The ABC Rating Score is a numerical value from 0 to 100, indicating the viability of bridge construction projects considering ABC concepts. The higher the score, the more suited a project is for ABC. Bridges with a score of 50 or more are automatically further evaluated for consideration of ABC techniques. The ABC rating score for this project was 57.

After further evaluation, the site conditions and project delivery supported an ABC approach if the road closure period could be reduced from the anticipated 150 days for conventional construction with an off-site detour to a maximum of 21 days. To limit the critical closure period to no more than 21 days, the Iowa DOT recommended the bridge be constructed using a lateral slide method where the superstructure of the new bridge is built on temporary supports adjacent to the existing bridge. Once the superstructure construction is completed, the road is closed, the existing bridge is removed, and new substructures are slid into place. The bridge approach grading and paving finishes the construction.

In addition to the lateral slide, several other ABC techniques were used. The pier was designed so the drilled shafts could be constructed outside the limits of the existing bridge prior to the road closure. The plan preparation included several precast and cast-in-place alternative details to give the contractor as much flexibility as possible. Special Provisions were written for the prefabricated bridge superstructure move and the use of ultra-high-performance concrete.

The goal of the Mahaska County Bridge Replacement project was to save user costs and reduce maintenance costs by implementing ABC techniques to reduce the road closure period to a maximum of 21 days. The final construction cost was $70,000 less than the engineers’ estimate of $4.85 million. The new bridge opened to traffic after being closed for only 19 days, two days ahead of schedule.