Fairfax County Parkway/Fair Lakes Parkway Interchange
Fairfax County Parkway/Fair Lakes Parkway Interchange
In the early 1990s, developers had their eyes on several tracts of land in west-central Fairfax County, Virginia, planning to build shops, offices, restaurants, homes, schools and the other physical features of modern suburban life. Simultaneously, the Virginia Department of Transportation (VDOT) had been planning a north-south parkway through the western part of the county. Utilizing the "proffer system", in which developers are required to pay for (or directly construct to VDOT standards) certain necessary infrastructure components in exchange for approval of their development plans, several segments of the eventually 35-mile Fairfax County Parkway were constructed. As the area grew, the Fair Lakes Parkway was added to the road network, crossing the Fairfax County Parkway at a signalized intersection that quickly became a major bottleneck requiring additional improvements. To handle the sixty thousand vehicles per day already using this intersection as design began in 2009, and to calm growing rush-hour traffic jams, VDOT chose to grade-separate the intersecting roadways and widen a portion of the Fairfax County Parkway.
There are many excellent types of projects in which to use Reinforced Earth mechanically stabilized earth technology, but designers of a grade-separated diamond interchange should always start by thinking "mechanically stabilized earth". Better yet, they should automatically think "Reinforced Earth". Reinforced Earth walls and abutments are the preferred choice due to the typically congested nature of such a project location and the significant advantages these walls offer: speed of construction, most work completed from inside the structure, long-term engineered durability and predictable structural performance, ease of applying aesthetic treatment to enhance the walls and the community, reduced cost and, of course, environmental conservation through the use of thin precast facings and no in-ground foundations. Designers of the Fairfax County Parkway/Fair Lakes Parkway interchange, Whitman Requardt & Associates of Richmond, VA, made "the MSE choice" and their project literally rises to the occasion on Reinforced Earth walls and abutments.
Geotechnically speaking, the project site was typical for Northern Virginia and did not require any special design considerations for the Reinforced Earth wall foundations. Bearing capacity was not of concern and the applied bearing pressure was easily estimated by the long-accepted rule of thumb for Reinforced Earth structures: the bearing pressure is approximately 135% of the dead weight of the reinforced soil backfill plus surcharge. The detailed design calculations for Abutment A, performed by The Reinforced Earth Company by taking into account all loads on the structure (excluding the bridge, which was supported on piles), determined the applied bearing pressure to be 4.5 ksf. The rule of thumb obtained the same result at the same location: for 105 pcf backfill, the rule of thumb bearing pressure was also 4.5 ksf (1.35 x 30 ft. x 105 pcf, + 250 psf surcharge). The only area of any geotechnical concern was at the box culvert that passed through Wall C. The foundation beneath the box culvert was undercut to reduce the chance of this rigid structure settling. An extra level of assurance was provided by the hidden slip joint design where the Reinforced Earth panels framed into the box behind a covering lip – allowing the rigid box and the flexible wall to move independently without distress (Figure 1).
As mentioned above, the bridge was supported on piles driven prior to erection of the Reinforced Earth abutments and wing walls. Pre-driving the piles allows the reinforced backfill to be better compacted around them and minimizes the risk of wall facing movement or misalignment that could result from pile driving vibrations. The piles were capped by a semi-integral abutment. While the bridge superstructure and approach slab were allowed to move laterally to accommodate thermal loads, the abutments were restrained to produce minimal lateral movement. This was accomplished by connecting 3 rows of standard 50 mm x 4 mm galvanized steel ribbed reinforcing strips to the backs of the pile caps (Figure 2,Figure 3) – a technique pioneered by The Reinforced Earth Company and used successfully on bridges around the country.
Traffic barriers atop MSE walls are typically designed as a cast-in-place or precast barrier shape (e.g., "Jersey" shape or similar) supported on a cast-in-place moment-resisting slab beneath the roadway or shoulder. VDOT plan requirements note that the design of the moment slab is the responsibility of the MSE wall system supplier. Therefore, RECo designed the cast-in-place moment slab for the roadway approach walls (wing walls leading to the bridges), including special design cases required at approach slab and sleeper slab conflicts. These special designs are a value-added design capability routinely offered by RECo, something not always available from other MSE suppliers.
Construction of the Fairfax County Parkway/Fair Lakes Parkway interchange brought together a team with a long history of working together successfully across the Mid-Atlantic Region. The general contractor, Shirley Contracting Company (Lorton, VA) has been building Reinforced Earth walls for more than 35 years, while precaster K. J. Williams Concrete Co., Inc. (Cresaptown, MD) has a 40-year casting relationship with RECo. The backfill used was No. 57 stone, an MSE backfill long valued by Virginia's contractors for its consistency, compactibility, ease of handling, ease of placing in poor weather and, most important, the excellent alignment and stability of the resulting Reinforced Earth walls. Since the first VDOT contractor experimented with this backfill in 1978, No. 57 stone has been the "gold standard" that costs no more because, in the end, it provides so many cost-saving benefits during construction.
The architectural treatment for this project was ashlar stone, providing a natural quarried-stone look and having jointing that matches well with RECo's 5 ft. x 5 ft. square precast panels (and one K. J. Williams pioneered in the late 1980s, on nearby I-270 in Montgomery County, MD). The same architectural treatment was also used on soundwalls and other structures along the Fairfax County Parkway/Fair Lakes Parkway interchange. After application of the required field-applied concrete stain to all ashlar stone surfaces, the result was a beautiful consistency throughout the project corridor.