Measurement of Changes in Beach Sand Soil Stiffness due to Fluctuating Tides

Understanding the penetration resistance and geotechnical properties of beach sands is important for erosion modeling and performance-based design; however, data for beach sand penetration resistance with depth is sparse, and particularly temporal and spatial investigations are lacking. This study presents the results of a series of instrumented dynamic cone penetrometer (DCP) tests that were performed in the intertidal zone of the Atlantic-side sandy beach in Duck, North Carolina, over a tidal cycle. Spatial (cross-shore) and temporal (tidal) changes in soil stiffness in the upper 2 m of the beach surface were investigated. In total, 13 DCP soundings that measured the sediment resistance at two cross-shore locations were approximately 3 m apart. The soundings were performed every 30 min, which alternated between the two locations and started with the ebb tide, followed by the low tide and flood tide. The results showed changes in the soil penetration resistance that were related to the tidal cycle. Location A, which was closer to the dune, had overall lower penetration resistance values compared with Location B, which was closer to the water. Near low tide, the differences between the penetration resistance diminished, and differences between the penetration resistance values increased during the ebb and flood tides. Therefore, the observed changes in penetration resistance could be related to groundwater dynamics that follow the tidal water elevations.