Vertical pore pressure variations and geotechnical sediment properties at a sandy beach

Abstract The role of geotechnical properties and soil behavior for beach dynamics has been recognized before, but geotechnical field measurements in energetic beach environments are still rare. This study focused on two days of field measurements along a cross-shore transect reaching from the foot of the dunes to the upper subtidal zone at the western sandy beach of the island of Sylt, Germany, just south of the city of Westerland. Sediment properties and geotechnical parameters were obtained from sediment sampling and limited in-situ testing. Pore pressure measurements were conducted along a vertical array in the upper 55 cm of the beach surface in the lower intertidal zone. Pore pressure recordings were then analyzed using a one-dimensional-vertical (1DV) model based on Biot (1956) and Mei and Foda (1981). Laboratory testing results demonstrated slight trends of increasing grain size and friction angles from the subaerial to the lower intertidal zone. In-situ sediment strength testing using a portable free fall penetrometer supported the trends in friction angles for the subaerial and intertidal zone. Additionally, a significant increase in sediment resistance was observed in the swash zone and upper subtidal zone. Pore pressure recordings showed a consistent trend associated with the tidal water elevations. However, data collected during low tide suggested a decoupling of surface water effects and groundwater, possibly associated with gas content and negative pore pressures in the vadose zone. Pore pressure recordings also suggested a more pronounced wave damping in the upper sediment layers and a minor phase lag. The 1DV pore pressure model succeeded to simulate the observed pressures at all sediment depths well, and suggested no liquefaction events during the measurement period, but a reduction of effective weight that may affect sediment dynamics.