Value Engineering Analysis Solves an Un-Constructible Design

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KCI designed a temporary work trestle bridge that facilitated drilling operations over Coddle Creek.

Ridership on the North Carolina Department of Transportation’s (NCDOT) rail network has nearly doubled in the last decade. To accommodate the growth, the state is investing time and money in modernizing its railroad system to improve safety and better connect major cities. Projects have included straightening curves, adding additional tracks and replacing old crossings. During construction of a 10-mile stretch of new track to carry trains between Charlotte and Raleigh, Crowder Construction Company called on KCI to perform a value engineering study and evaluate alternatives for a bridge crossing that was deemed impossible to construct as designed.

Just 11 feet from the proposed structure, the existing rail bridge over Coddle Creek carries up to 30 trains per day and had to stay in service during construction.

The contractor was scheduled to begin construction of a 160-foot-long, three-span bridge that would cross Coddle Creek right next to an existing rail structure in Cabarrus County. Crowder quickly realized that the proposed foundations called out in the plans and specifications were in direct conflict with the existing bridge. Because the structural beams had already been fabricated in accordance with the original design, Crowder asked KCI to incorporate the girders into all design alternatives, thereby minimizing the cost and time required to order new beams. In addition, NCDOT, specifically requested that any solution follow the originally proposed horizontal alignment, while allowing the existing bridge to remain in service through construction.

Through quick response and thinking outside of the box, we were able to overcome all of the challenges and provide a constructible design without further delaying the original project schedule.

R. Eric Burgess, PEVice President, Regional Practice Leader

R. Eric Burgess, PE

KCI’s engineers determined that by rearranging the spans, the bridge could be lengthened to avoid the conflict with the existing structure’s foundations. An additional 30 feet of beam was spliced onto one span of girders to provide the additional length needed.

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Switching the girders for Spans A and C, combined with splicing on an additional 30 feet of beam length, allowed the high-wall abutments from the original design to be replaced with constructible abutment caps that could be supported by drilled shaft foundations that were not in conflict with the existing structure.

With a plan in place, the team had to consider any impacts the new design might have on permitting as well as the ongoing construction. “Any encroachment on the stream by the lengthened spans would have required permits to be resubmitted,” said structural engineer Eric D. Anderson, PE. “We were able to match the existing hydraulic analysis by installing an additional retaining wall and prevented a prolonged review and approval process that could have delayed the schedule by as much as 12 months.”

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The proposed solution had to match the existing hydraulic analysis for the original design to eliminate the need to re-submit permits.
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The secant pile shoring wall (at right) designed to support both bridges during construction was the first of its kind to be used by the NCDOT Rail Division.

With travel continuing on the existing bridge only 11 feet away, it also became apparent that the excavation and drilling required for the new structure could weaken the support for the existing crossing and place passing trains in jeopardy. “To put it simply, we needed to prevent the trains from falling off the tracks, and we needed the solution to achieve that goal without causing any downtime in the rail traffic,” said project manager R. Eric Burgess, PE. KCI worked with the contractor, the NCDOT and Norfolk Southern Railroad to develop the final design for a secant pile retaining wall between the two structures that would support both bridges during construction.

In addition to the major technical challenges—alignment, materials, permitting and impacts to the adjacent structure—the timeline also complicated the design process. When KCI was originally approached about the Coddle Creek bridge, the project was already behind schedule. Our team worked nights and weekends in order to submit a revised design in only six weeks, all while identifying and overcoming potential delays and ensuring the project’s original deadlines were met. Once the plans were submitted, KCI engineers conducted a live-edit meeting with NCDOT, Crowder and Norfolk Southern to expedite review and construction. In the end, our team exceeded the expectations of both the contractor and state by quickly turning an impossible situation into a constructible success.