Originally published in ASHE Scanner Magazine
By James Bishoff, PE, and Jonathan Wilfong, PE, J2, ASHE Potomac Section
Loudoun County’s Northstar Boulevard was recognized by the Virginia Department of Transportation (VDOT) as part of a north/south Corridor of Statewide Significance. It had the potential to reduce congestion on parallel north/ south routes between I-95 and Dulles International Airport. It could also connect travelers to places in Loudoun County and beyond. The new four-lane segment of Northstar Boulevard, to be completed by the end of this year, will provide an alternative north/south connection to US 50 from Tall Cedars Parkway.
Loudoun County Department of Transportation and Capital Infrastructure tasked J2 Engineers (J2) with preliminary and final designs for roadway improvements. These consisted of 4,100 linear feet of new alignment, 1,900 linear feet of roadway widening, turn lanes at three intersections and a 40-foot-wide depressed grass median. The design also included 10-foot-wide shared-use paths on both sides of the road. Traffic signals with pedestrian controls will be installed at the US 50 and Tall Cedars Parkway intersections as part of the project.
The project funding (Tier 2 – revenue sharing) required the VDOT Locally Administered Projects process. The $45.8 million project budget included 70 percent regional and 30 percent local funding from the Northern Virginia Transportation Authority and proffers. The project’s scope of work consisted of roadway design, coordination with adjacent developers, minimizing environmental impacts, design of stormwater management facilities and water and sewer design. It also entailed utility coordination, bridge design, traffic and signal design, maintenance of traffic plan, permitting, plat preparation, public involvement and construction administration. The J2 Team used laser scanning to gather survey data for more than 6,500 linear feet of the project's associated roadway.
One feature of the project is the 150-foot-long by 126-foot-wide single-span bridge over the South Fork of Broad Run, designed in coordination with J2’s subconsultant E.L. Robinson. The project required a floodplain model calibration, floodplain alteration and an approved Conditional Letter of Map Revision (CLOMR) from the U.S. Army Corps of Engineers. The design included bridge approaches, guardrail, shared-use path, evaluation of deck drains, scour protection, utility relocations and public input for bridge and abutment aesthetics. Working with Loudoun County and VDOT, the bridge foundation was redesigned to reflect the variable bedrock encountered at depths different from the estimated bedrock depth, which was based on the original subsurface investigation. Even with this site challenge, the project has remained on schedule.
The proposed roadway alignment of Northstar Boulevard required crossing the South Fork of Broad Run Major Floodplain. One of the challenges was that the roadway alignment crossed the floodplain at a 45-degree skew, requiring a 480-foot multispan bridge to cross the existing floodplain. J2 discovered that the 100-year flow distribution at the crossing was located incorrectly within the model. This resulted in an increased 100-year flow by about 600 cubic feet per second at the crossing and a larger structure.
J2 worked with the Loudoun County Floodplain Team to update the existing model. This would correct flow location and provide an analysis approvable by Loudoun County, VDOT and the Federal Emergency Management Agency with a CLOMR. J2 modeled multiple bridge options to provide a design that limited the flood elevation rise and minimized the bridge length. The final design reduced the multispan bridge to a 150-foot single-span bridge, resulting in project savings and permit impacts.
Another challenge with this project was designing an intersection at US 50 that would be both safe and cost-effective. The adjacent properties had previously reserved right-of-way for this new roadway that dictated where the alignment of Northstar Boulevard would intersect with US 50. J2 studied the corridor and found that US 50 is bifurcated between the eastbound and westbound lanes, in some locations as much as 14 feet vertical difference. This geometric constraint required multiple iterations to understand where the new roadway would connect to US 50, and evaluated the future continuation of Northstar Boulevard north. This effort reduced improvement costs on US 50 and promoted a safe transition through the crossover. The intersection accommodated dual turning movements at each approach, intersection crossover design, drainage and signalization. J2 worked with VDOT to ensure the interim improvements would not sacrifice public safety until the ultimate improvements are completed.
An additional challenge with this intersection was the inadequate intersection spacing on US 50 with an existing intersection to the west of the proposed intersection. A VDOT Access Management Waiver was supported by the state’s Location and Design Division and VDOT–Traffic Engineering for the reduction of the crossover spacing. The VDOT maintenance facility was also located on the northeastern quadrant of the intersection, and the design included maintaining access to the facility during and after construction.
Residents and motorists will benefit from additional routes, alleviated traffic congestion and safer roadways once this project is finished. Construction progress is on track, and the project is expected to be completed on time.
Loudoun County is also developing a future segment of Northstar Boulevard from Tall Cedars Parkway to Braddock Road, which will join this segment with the existing roadway in the Dulles area. J2 is lead designer on the project that includes planning, design, right-of-way and construction of the remaining two lanes of Northstar Boulevard between Tall Cedars Parkway and Braddock Road.