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A creative solution that made up for lost time

Renovating a pony truss bridge over the Cowanesque River required a creative approach to the temporary bridge.

Opportunity

When the pony truss bridge carrying State Route 49 over the Cowanesque River required rehabilitation, a temporary structure was needed to maintain vehicular and pedestrian traffic.

A contractor proposed to use a 132-foot-long Acrow panel bridge to maintain traffic while rehabilitating the adjoining mainline structure. However, due to the poor stability of the adjacent riverbank, the contractor could not obtain approval from PennDOT to use a proposed timber mat foundation.

Several months behind schedule, the contractor was faced with finding an alternative approach that could address the unstable riverbank condition and deal with particularly tight site constraints. These included a privately owned barn that encroached on the northeast corner of the site, a buried wingwall at the southeast corner, and two water mains.

The proposed foundation locations conflicted with existing utilities and structures. The southeast corner of the site presented an unusually challenging condition, as the southeast bearing unit of the temporary bridge was positioned above one of the existing water mains and a wingwall of the truss bridge.

Solution

The contractor contacted Mott MacDonald’s office in Pittsburgh to develop a new approach.

Working closely with the contractor, PennDOT District 3-0, and American Geotechnical and Environmental Services (our geotechnical subconsultant), Mott MacDonald developed an alternative foundation. This design allowed the contractor to finish the project within seven days of the originally scheduled completion date.

As the southeast foundation unit required relatively shallow asymmetrical geometry to straddle the wingwall and water main, we designed a steel-framed pile cap foundation to avoid modifying these existing features. At the contractor’s request, steel-framed pile caps were also developed for the other locations to expedite construction.

In addition, we developed an innovative roadway approach structure, using a geotextile-reinforced soil embankment with timber facing. These elements served as the backwalls of the bridge and reduced the lateral earth load on the bridge foundations.

Outcome

Our design was completed within two months of the notice to proceed, and construction was completed on July 10, 2013. The temporary bridge was used for four months, until October 31, 2013, while the pony truss was rehabilitated. It provided a safe alternative for the river crossing while protecting the riverbank and existing infrastructure.

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