The Vietnam Veterans Memorial Bridge, which crosses the James River directly downstream of the Port of Richmond, is the gem among the 15 bridges along the new 8.8-mile Route 895 Connector, known as the Pocahontas Parkway.

The new toll road was planned to alleviate congestion and traffic delays by linking I-95 in Chesterfield County to I-295 in Henrico County.

The project cost $324 million, of which approximately $111 million was for the Vietnam Veterans Memorial Bridge. Completion of the bridge was the key to meeting the ambitious 45-month schedule for the entire parkway project. Delivered under budget, and completed on schedule, the project was implemented 15 years sooner than it would have been under the Virginia Department of Transportation's normal construction budget processes.

The 1,475-foot-long, high-level fixed bridge, which extends from the west abutment in Richmond, over the Route 150 interchange and the I-95 mainline, to the east abutment in Henrico County, features a 672-foot main span with 145 feet of vertical clearance for marine traffic using the deepwater port. The bridge also includes nearly 3,500 feet of high-level approach spans and three new, high-level ramp structures that connect to I-95.

A series of complex criteria, from accommodating ocean-going ships and complying with seismic requirements to maintaining river and vehicular traffic during construction, were among the challenges that called for innovative approaches to design and construction. Among these was the use of three distinct concrete structure types - cast-in-place on the main and approach spans; precast cantilever on the approaches; and precast span by span on straight portions of the ramps (curved steel girders were used on the tighter radii). To leave the river bottom undisturbed and to avoid cofferdams as well as ship collisions, the bridge was designed without the use of piers in the river.

Due to the magnitude of the project, alternative designs for steel and concrete were evaluated for the main span and approaches. During the conceptual design phase, Parsons Brinckerhoff, the designer for the river crossing, considered several options for both alternatives. The steel design alternatives evaluated for the main span included steel truss, steel tied arch, steel plate and box girders, and steel cable-stayed bridges. The concrete main-span alternatives evaluated included cast-in-place balanced cantilever segmental concrete box girder and concrete cable-stayed bridges, while the alternatives studied for the approach spans included precast segmental box girders, prestressed concrete AASHTO beams, and steel plate and box girder spans.

After further study of selected alternatives following conceptual design, a hybrid segmental concrete option was chosen as the most economical solution for the final design. For both the cast-in-place and precast segmental spans, 6,500-psi compressive strength concrete was used.

Cast-in-place span

The bridge is believed to be the second-longest cast-in-place span in the United States. To achieve the great length required for the main span, the design team selected cast-in-place concrete-haunched twin cell boxes cast in balanced cantilever. The girder depth ranges from 41 feet at the piers to 16 feet at midspan.

The smaller span lengths of the approach spans permitted the use of precast constant-depth boxes. The deck width varies between three and four lanes, with full shoulders. This width was accommodated by placing two boxes side by side and casting a wet joint between the wings. Over 1,300 segments were required, weighing between 32 metric tons and 50 metric tons each.

Both the east and west approaches were erected as balanced cantilevers. The west side primarily used an overhead truss to place the segments, which allowed construction to proceed over the heavily used six lanes of I-95 without interrupting traffic. The east approach was erected using cranes.

The single-lane ramp structures use both precast concrete segments and structural steel for the sharp radius portions of the superstructures. These precast segments were erected using a span-by-span method and an underslung truss.

Complex footings

A contaminated groundwater plume under portions of the western end of the bridge required the construction of most of the footings above the existing grade, supported on piles driven to rock. Single-column cylindrical piers resting on steel H-pile foundations support the approach span and ramp superstructures. The construction of the footings on top of the ground and the use of the H-piles driven through the contaminated plume avoided the need to excavate contaminated material or groundwater and minimized disturbance of the existing soils. The single-column piers, up to 130-feet tall, provided additional flexibility to cope with seismic loads.

The main span foundations consist of blade-type pier shafts supported by up to 20 6- or 8-foot drilled shafts. Fortunately, these shafts were located in an area outside the contaminated plume. The twin-blade design was used at the two main river piers to more effectively resist out-of-balance construction loads. The river piers incorporate multiple cylindrical rebar cages to provide for adequate confinement during a seismic event. The drilled shafts penetrate to bedrock and are designed to support the bridge in a 500-year scour event.

Mass concrete pours

The size of the foundations for this monumental structure required the development of state-of-the-art specifications for the use of mass concrete. All substructure elements for this project were considered to be mass concrete. Pier footings up to 16-feet thick required single pours of up to 3,900 cubic yards of concrete that each took 24 hours to complete.

The design team prepared specifications to allow the contractor to monitor the curing rate and temperature history of these elements in order to ensure that specified temperature rise and gradients were not exceeded in the pours, and to determine a form stripping time. Studies were undertaken to determine acceptable temperature gradients in various elements, including the effects of various reinforcing ratios and form insulation under a wide variety of ambient temperature conditions.

To avoid any potential weakening of the concrete due to overheating during the curing process, the concrete supplier recommended the use of blast-furnace slag in place of some of the cement to lower the heat of hydration. The heat transfer analysis, use of blast-furnace slag, and instrumentation monitoring avoided the use of cooling pipes in the pours.

The bridge's columns and piles are designed to resist seismic loads, including uplift. California Department of Transportation detailing practices were adopted for the larger columns to more effectively resist the seismic loads with simpler details. The project is located in a Category B seismic zone, with a lateral acceleration coefficient of 0.13. Multi-modal (response spectrum) analysis was used for the seismic investigations in order to properly distribute the seismic forces to the different height foundations.

Column detailing incorporated the first use in Virginia of seismic-rated bar couplers for the seismic hoops. Cap and column shapes and rebar details were developed with input from the contractor in order to produce a design that was efficient and economical for the contractor to build, and allowed for multiple reuses of formwork.

Innovative funding

In addition to the technical elements, the monumental structure was a pioneering effort in other ways. It was the first project to be completed under the Commonwealth of Virginia's 1995 Public Private Transportation Act, which allows for the Virginia Department of Transportation to accept proposals from private entities to build needed transportation facilities when public funding is not forthcoming. Just $26 million of the $324 million for the Route 895 project derives from public funds; the rest was raised through the sale of private bonds and will be repaid through toll revenues. The private partner, developer FD/MK LLC, a joint-venture firm created by Fluor Daniel and Morrison Knudsen, was responsible for the sale of the bonds.

Following the conceptual design phase, VDOT turned the project over to the developers, which used a design-build method of delivery to complete the entire project in the shortest possible time. Final design was completed on a fast-track basis, and the design team also prepared an eight-stage construction sequence scheme that allowed the contractor to speed up construction activities while traffic moved unimpeded through the site throughout the building process. The designers also assisted the developer during construction by providing segmental experts on call, as well as the more usual shop drawing reviews and responses to contractor inquiries.

Fred Parkinson is a project manger for Parsons Brinckerhoff in Norfolk, Virginia.