Driving on blades

May 16, 2003

An innovative approach by the Virginia Department of Transportation (VDOT) and FD/MK LLC enabled the completion of the Rt. 895 Connector, now known as the Pocahontas Parkway, in Richmond, Va., some 15 years earlier than using the traditional process for acquiring project funding. This was the first project to be initiated and completed under the Public Private Transportation Act, passed by the Virginia Legislature in 1995.

An innovative approach by the Virginia Department of Transportation (VDOT) and FD/MK LLC enabled the completion of the Rt. 895 Connector, now known as the Pocahontas Parkway, in Richmond, Va., some 15 years earlier than using the traditional process for acquiring project funding. This was the first project to be initiated and completed under the Public Private Transportation Act, passed by the Virginia Legislature in 1995. FD/MK LLC, a firm created by Fluor Daniel and Morrison Knudsen (now Washington Group), which is the private sector owner/partner with VDOT, completed the $324 million east-west tollway project connecting I-295 and I-95. A major part of the project was opened to traffic in May 2002 and the remainder in September 2002.

It's not going anywhere

Initially, the four-lane Pocahontas Parkway is expected to carry 24,000 vehicles daily with an eventual ramp-up to 50,000. The drivers will pass through a toll plaza equipped with Smart Tag technology.

The 8.8-mile-long highway has several new bridges, the most significant of which is the 4,765-ft-long James River Bridge. The bridge over the James River was built by the cantilever system using cast-in-place concrete segments. The 675-ft clear span over the river represents the third longest cast-in-place cantilever construction in the U.S. The main span also provides a vertical clearance of 145 ft over a 300-ft-wide channel for shipping. The superstructure for the river crossing consists of twin cell boxes with depths of 41 ft at the piers and 16.5 ft at midspan.

The main span foundations consist of blade-type pier shafts supported by 8-ft-diam. drilled shafts. The piers adjacent to the river are of a twin-blade design to more effectively resist out-of-balance construction loads. The blade-type columns incorporate multiple cylindrical rebar cages to provide for adequate confinement during a seismic event. The drilled shafts penetrate over 15 ft into bedrock, and are designed to support the bridge in a 500-year scour event.

The approach spans of the James River Bridge consist of precast concrete box segments of constant depth, joined together by post-tensioning. Single-column cylindrical piers resting on steel H-pile foundations support the approach span superstructure. The footings are generally above the finished grade to minimize the removal of contaminated soils from the site. The columns and piles are designed to resist seismic loads, including uplift.

The project is located in a Category B seismic zone, with a lateral acceleration coefficient of 0.13. Several features were incorporated into the design of the James River Bridge to mitigate the impact of seismic forces, including the following:

* Except for the end spans, the continuous spans of the river crossing are fixed to the piers, which allow a more uniform distribution of longitudinal forces;

* The continuous spans of the approach structures are pinned to the piers, which allow a much better distribution of seismic forces to the varying height foundations;

*   The race-track-shaped piers for the approach structures, as developed in the preliminary design, were too stiff requiring unreasonably heavy foundations to resist the seismic forces. Alternative designs with hollow cylindrical columns and solid cylindrical columns were investigated to make the piers more flexible. This effort resulted in a substantial reduction in seismic forces leading to more economical foundations and piers. Single-column cylindrical piers were selected from a constructability point of view;

*   The design of the steel pile foundations for the approach structures were subject to large uplift forces. Tension load tests were performed in the field to determine more accurately the frictional resistance and tension capacity of the piles. These tests resulted in optimizing the design with significantly fewer piles and smaller  pier footings;

* A three-dimensional seismic model, which considered the various features of the structural system, including the configurations, sizes and connections of the superstructure, piers and foundations and the behavior of the soil/structure interaction, was used in the analysis of seismic forces. The response spectrum analysis provided an accurate determination of the effect of seismic forces, which helped in selecting the appropriate sizes for structural members; and

* The use of large-diameter welded hoop-reinforcing steel for the river piers, as required in the specifications, posed significant problems for fabrication and transportation. An alternative design using seismic-rated bar couplers, which had been tested in California, was recommended and accepted by VDOT. Seismic-rated bar couplers were used for the first time in Virginia.

Parsons Brinckerhoff of New York City, as a subconsultant to Site Blauvelt Engineers of Richmond, Va., under contract with FD/MK LLC, was responsible for the design of the James River Bridge. The joint venture of Recchi America Inc. (now Condotte America), Miami, and McClean Contracting Co., Glenburnie, Md., under contract with FD/MK LLC, was responsible for the construction of the James River Bridge.

The project has won several awards including:

*  2003 Engineering Excellence Award from the Consulting Engineers Council of Virginia;

* 2003 National Finalist from the American Consulting Engineers Council;

* 2002 Outstanding Engineering Achievement Award from the Engineers Club of Hampton Roads; and

*  2002 Excellence in Concrete Award - Innovation, from the Virginia Chapter of the American Concrete Institute.