A better way for the Beltway
Geosynthetics | August 2011
By Alan Dinges
Each day close to 50,000 motorists travel through the Telegraph Road interchange on the integral Interstate-95/495 Capital Beltway between Virginia and Maryland, south of Washington, D.C.
In early 2008, extensive upgrades to the interchange began as part of a multi-year project tied to the final phase of the Woodrow Wilson Bridge project. Roughly $236 million was dedicated for multiple improvements at the busy interchange. These capacity-managing improvements involved the construction of a grade-separated interchange, new ramps, and the construction of a bike and pedestrian trail.
A joint venture company was contracted for the work, which focused primarily on 24 lane-miles of roadway with 321,000 yards of paving, 11 bridges, drainage improvements, retaining and noise-abatement walls, and environmental mitigation.
Geogrids and geotextiles were effective construction components in this project’s diverse range of soil reinforcement and separation requirements.
For several portions of this project, the Virginia Department of Transportation (VDOT) specifically requested high-strength geosynthetic reinforcement with an ultimate tensile strength of 38,000lbs/ft.
The selection was a woven geogrid with a high molecular weight and high-tenacity polyester multifilament yarns and an ultimate strength of 41,000lbs/ft. The grid’s polymeric coating also provided mechanical and chemical durability and resistance to biological degradation.
About 14,500yd2 of geogrid was used for basal reinforcement over aggregate piers and under mechanically stabilized earth (MSE) walls and slopes. An additional 10,000yd2 of geogrid was installed across various reinforcement zones of this project.
The 12.8ft.-wide rolls provided coverage, minimizing material waste and maximizing project economics. The project-specified oversized geogrid panels, which varied in length from 350–450ft, were overlapped 2ft and tied together to prevent movement during backfilling. Additionally, all overlaps of geosynthetic panels were “shingled” in the direction of the fill placement. This step ensured that the pushing of fill material did not alter the overlap alignment.
A sand layer was installed below all of the reinforcement and over the aggregate piers. Where needed, wick drains were installed to accelerate soil consolidation.
In addition to the reinforcement materials, the Telegraph Road interchange utilized nonwoven geotextile separators, per VDOT specifications.
At designated locations, geotextile layers were required for separation between natural soils and select fill. Some geotextile material was also used in combination with welded wire mesh for wrap-around wall construction.
The selected nonwoven geotextile is inert to biological degradation and is resistant to chemicals, alkalis, and acids; the 100% polypropylene staple fibers are needlepunched to provide dimensional stability and the required filtration, separation, and protection properties.
The Woodrow Wilson Bridge project continues to move forward, with work on the Telegraph Road interchange concluding as more ramps are opened and the project’s goals are fulfilled. The last of the project’s many components is scheduled for completion in 2013.