The new Woodrow Wilson Bridge (WWB) will replace the existing bridge over the
Potomac River to connect Alexandria, Virginia to Prince Georges County,
Maryland. The new WWB will extend approximately 1.1 miles across the river,
with a 367-ft long bascule span in the main river channel where the water depth
is about 36 ft. The subsurface soil profile consists of up to 50 ft of a soft organic
silty clay layer that is very vulnerable to scour, underlain by a deep deposit of
hard sandy clay. Foundation construction in the Potomac River required the use
of cofferdams to allow structural concrete work to be performed in the dry.
This paper will the present the types of cofferdams used to construct the
foundations for the WWB replacement project. The bridge is primarily comprised
of fixed spans in relatively shallow water and a bascule span over a navigation
channel in relatively deep water. Foundation construction included the
installation of 48” to 72” open end steel pipe piles and cast-in-place concrete pile
caps. To achieve the desired bridge appearance, pile caps were required to
extend below the water surface and below the existing river bottom in shallow
water locations. Conventional steel sheet pile cofferdams with tremie concrete
seal were used to construct fixed span, shallow water piers. Selection of deep
water bascule span foundation cofferdams was governed by several factors
including contraction scour, environmental considerations and cost. Potomac
River cofferdams, i.e., relatively shallow depth cofferdams that are supported by
the foundation piling were selected. This cofferdam system provided a shallow
profile which minimized contraction scour and the amount of filling in the river.
Design issues include load selection and cofferdam stability in very soft alluvial
soils. Construction issues include erection sequence, tremie concrete mix design
and placement procedures.
Author: Roderic A. Ellman, Jr., Museser Rutledge Consulting Engineers, Jim X. Zhang & Bill Spence, Tidewater Skanska, Inc.
Publisher: DFI