Evaluating Scour for the Piers on the New Woodrow Wilson Bridge

The Woodrow Wilson Memorial Bridge carries the combined I-95 and I-495 traffic over the Potomac River. The existing bridge, built in 1961 is being replaced with twin bridges with the first one opened to traffic in 2006. This paper will discuss the development and results of the hydraulic models and scour evaluations that were used in for the design of the new piers. This was a coordinated effort of various agencies and consultant firms involved with the bridge design. An Interdisciplinary Scour Team was formed during the early stages of the design studies to determine and guide the development of the technical studies. The original Woodrow Wilson Bridge was constructed by the Federal Highway Administration (then Bureau of Public Roads), and was opened to traffic in 1961. As the transportation system for the Washington D. C. area developed, the bridge was incorporated into the I-95/495 Interstate System serving Washington and the east coast. It became the only Federal bridge on the entire Interstate Highway System. The bridge did not have the capacity to accommodate the Interstate traffic, and it soon became a traffic bottleneck, regularly backing up traffic during commuting hours. During the preliminary planning for a replacement bridge, there were a number of environmental issues to address. These included the impacts to development on the Virginia side of the river and a public concern that a high bridge would dominate the landscape and skyline. Resolution of these issues resulted in a decision to construct a lower bridge with a bascule span. This, in turn, established the essential elements of the TS&L for the bridge. A competition was held among bridge designers to select an appropriate design for the crossing and the winning design was awarded to the Parsons Transportation Group of Baltimore, Maryland. Although the bridge appears to be a series of arches, it is actually composed of continuous haunched steel box girder spans supported on "V" piers. The first 6-lane bridge was opened to traffic in 2006, and completion of the second span is scheduled for 2008. The existing bridge needed to remain in service during the design and construction of the new bridge, and various studies conducted between 1991 and 1996 indicated a potential for significant scour, especially for piers near the Maryland shore. Therefore, a monitoring system that could be accessed by telephone was installed and maintained during this time to measure scour depths, channel velocities and water surface elevations. An Action Plan was set up to provide guidance as to critical scour depths affecting bridge stability and actions to take in the event that the bridge was closed. The design process for the replacement bridge began in the late 1990's and the Maryland SHA assumed the responsibility for coordinating the scour evaluation of the river piers. An interdisciplinary team was formed to work with the consultant. Similarly, a team was formed for geotechnical studies/structural design of the pier foundations. During the scour evaluation of the piers, a pile load testing program was also conducted which helped in the selection of the foundation type. During this testing and evaluation process, there was close coordination between the geotechnical engineers and the hydraulic engineers.