Abstract:
North Carolina Department of Transportation (NCDOT) awarded their U-5026/R-5720 Design-Build project to Barnhill Contracting Company, with HDR as their lead designer. This project consists of the widening of Eastern Avenue to a four-lane, divided facility with a raised median from SR 1003 (Red Oak Road) to SR 1544 (North Halifax Road). The project will also convert the existing Sunset Avenue bridge over Interstate 95 into a new grade-separated interchange with ramps and loops that will provide direct access to the interstate.
The project is located at the Atlantic Seaboard Fall Line where the Piedmont and Atlantic Coastal Plain meet. The geologic conditions are markedly different in the Piedmont and Coastal Plain regions. Soil testing borings indicate metavolcanic rocks (Piedmont Eastern Slate Belt) are overlain in part by Terrace deposits (Coastal Plain) in this transition zone. Terrace deposits generally consist of layers of very loose to medium dense sand and soft to medium stiff silt and clay. Residual soils underlie the Terrace deposits and generally consist of layers of very soft to hard silt and clay and loose to medium dense silty sand. The deeper borings transition to weathered metavolcanic rocks.
Approximately 20 to 25 feet of fill were anticipated to be placed along the new roadway embankment near the bridge end bents. MSE walls up to 20 feet high were proposed to be installed at both end bents. The settlement analyses indicate that over a foot of settlement would occur during a period of two years after construction. An approximate 190 kip of drag load is anticipated to occur on a single pile.
In order to reduce the settlement and drag loads on the piles, the following construction sequence was proposed: Install piles at End Bents before MSE wall construction; Install pile sleeves at End Bents and fill with loose sand; Construct MSE wall and embankment to the bottom of cap elevation; Continue embankment construction using a temporary 1.5:1 backslope from the bottom of cap and extend to the proposed grade; Monitor the settlement until 90% complete; Restrike all piles at End Bents; construct the pile cap and place backfill behind the pile cap.
Following the proposed construction sequence, the anticipated long term settlement is on the order of 1 inch, with reduced drag load of 50 kips on piles.