Seismic Soil-foundation Interaction Analyses of the New

A new bridge that will replace the existing Woodrow Wilson Bridge is under construction over the Potomac River in Washington, DC. The river piers are supported on cylindrical steel piles of 72, 54 and 48 inches in diameter. The longest piles (at the bascule piers) are approximately 200 ft long. Outside the river, nearly all piers are supported on 24-inch square prestressed concrete piles. Although the seismicity of the region is modest, seismic issues were thoroughly addressed in the design to ensure acceptable performance during a 500 and a 2,500-year earthquake. The results from 3-D dynamic soil-foundation interaction analyses presented demonstrate the effect of site conditions and pile foundations on the design seismic motions. In the presence of scour, pile foundations in the river were found to move significantly more than the free field, especially for motions with periods close to the fundamental period of the soil-pile system. Conversely, for foundations outside the river, pile cap motions were found to be smaller than free-field motions. Furthermore, strong kinematic pile bending due to passage of seismic waves was observed at deep elevations below the surface, close to interfaces with large stiffness difference between adjacent soil layers. Introduction The Woodrow Wilson Memorial Bridge is the only Potomac River crossing in the southern half of the Washington Metropolitan area. Consisting of fixed spans and a movable (bascule) span, it carries the Capital Beltway (I-495), which is part of I-95, the main north-south interstate route on the East Coast. Approximately 6,000-ft long, the new bridge (Figure 1) has been designed for HS25 loads consisting of six lanes for local traffic, four lanes for express traffic, and two HOV lanes. Provisions are made for future replacement of the HOV lanes with rail. The bascule span is 370 ft long with a navigation channel of 175 ft wide. The water depth at the channel is 35 ft. In 1998, Maryland State Highway Administration awarded Parsons Transportation Group (PTG) the design and construction support services of the new bridge. Mueser Rutledge Consulting Engineers performed the geotechnical engineering services. 1 Northeastern University, Boston, MA, 2 Parsons Transportation Group, New York, NY, 3 Mueser Rutledge Consulting Engineers, New York, NY, 4 URS Corp., St. Louis, MD An arch-like appearance has been achieved by introducing V-shaped piers with curved legs, which support haunched girders. This structural system consists of independent structural units (V-piers), and produces zero horizontal thrust forces under dead and live loads. Proper arrangement of the girder spans balances the dead loads and produces minimal bending moments at the piers. This system eliminates the need for using batter piles and results in significant savings in the foundations, especially at the bascule piers located in the deepest part of the river. Figure 1. Computer generated photograph of the new Woodrow Wilson Bridge. Figure 2 shows the soil profile along the longitudinal axis of the bridge and the locations of the various piers. Bedrock is 500 to 700 feet below sea level. The subsurface soil profile consists of 50 to 80 ft of soft or silty clay underlain with deep deposit of hard sandy clay. The soft clay is vulnerable to significant scour in the 500-year flood, especially at the main navigation channel where, the entire soft layer was estimated to undergo scour. Figure 2. Soil profile and bridge pier locations. Although the seismicity of the region is modest, because of the importance of the bridge, it was decided that seismic issues would be addressed in detail throughout the design. This paper describes geotechnical earthquake engineering aspects of the design and analysis of the bridge. In particular, the paper presents the design rock spectra and the soil-pile interaction analyses used to obtain the foundation motions, the pile loads induced kinematically by seismic waves, and the foundation spring coefficients representing the soil-structure interaction (SSI) in the global analysis of the bridge. Free-Field Ground Motions There is significant spatial variability in the soil profile and in the scour potential along the bridge axis. Thus, the foundations of the bridge are quite different. The piers over the river are on 72, 54 and 48 inch diameter cylindrical steel piles with caps that are as much as 30 feet +20 ft 0