GEOTECHNICAL BORROW SITE CHARACTERIZATION FOR PRECONSTRUCTION ENGINEERING AND DESIGN, WEST ONSLOW BEACH AND NEW RIVER INLET, N.C. COASTAL STORM DAMAGE REDUCTION PROJECT, USACE, WILMINGTON DISTRICT

The West Onslow Beach and New River Inlet (Topsail Beach), N.C. Storm Damage Reduction Project is a shore protection project currently being re-authorized for the Town of Topsail Beach. Topsail Beach lies atop a modern transgressive barrier island, Topsail Island, which is subject to hurricane and seasonal storm erosion of the shoreline. Earlier studies conducted by the Wilmington District or sponsored by the Town of Topsail Beach have delineated several borrow sites that may contain enough material for initial construction and subsequent renourishment cycles. The exploration phase of this project was initiated May 2010 in order to evaluate the primary borrow site (Borrow Site A) for the Topsail Beach shoreline segment of Topsail Island. A total of 167 vibracores recovered using Wilmington District’s Debris Boat Snell drilling platform, 2.5 to 4.0 miles offshore of New Topsail Inlet. The vibracores were processed, soils visually classified in accordance with the Unified Soils Classification System, and gradation testing conducted using ASTM-D422 “Standard Test Method for Particle-Size Analysis of Soils”. Much of the material recovered from Borrow Site A is classified as a SP-SM (poorly-graded silty sand). This material appears to be derived from heavily indurated Oligocene sandstone that floors the ocean bottom within the borrow site. Present and historical boring logs were input into a USACE specific gINT geotechnical database to facilitate data modeling. Isopach mapping, along with 2-D and 3-D fence profiling was conducted to describe and quantify the type of material, its distribution, and thickness within the borrow site. The results and findings of this subsurface investigation are presented in order to give insight into the availability of sand resources for the Town of Topsail Beach.