Geotechnical investigations of sandy seafloors using dynamic penetrometers

Geotechnical in-situ characterization of the strength of the shallowest sub-seafloor sediment is an important factor in offshore engineering (e.g., scouring at wind energy plants), coastal engineering (e.g., sediment erosion close to the shores and beaches), navy applications (e.g., mine burial) and research (e.g., dunes in tide-affected areas). Dynamic penetrometers are well known as time- and cost-saving means to derive sediment physical properties in-situ and to detect layering or changes of strength of the shallow marine deposits. However, until now such instruments were rarely used on hard sandy seafloor because of their small penetration depth. The aim of this study is to unravel how applicable dynamic penetrometers are on sand and what kind of information they can deliver. Deceleration - depth signatures of the devices are used to compute quasi-static bearing capacity and related to governing parameters such as mineralogical composition, grain size distribution and sedimentary layering. We present the results of measurements on sand with two different types of dynamic penetrometers (FF-CPT and Nimrod) developed at MARUM (University of Bremen, Germany). The devices were operated with different penetration velocity, with different deploying technique and in variable sedimentary conditions. The parameters monitored during penetration are deceleration and tip resistance. Data analysis follows two approaches. First, we directly compared deceleration - depth profiles from both instruments to extract typical profiles for the different materials and to quantify areas of sediment remobilization. Second, dynamic bearing capacity is derived from the deceleration (Nimrod) and from the tip resistance (FF-CPT) respectively. Following an empirical approach (Dayal and Allen, 1975; Can. Geotech. J.) dynamic bearing capacity can be converted into quasi-static bearing capacity for a chosen threshold penetration velocity to consider the varying impact force and penetration rates- of the devices. This allows a better comparison of different dynamic penetrometers to each other and to standard CPT records. A comprehensive data set was acquired in four research areas, which differ in type of sand, grain size and appearance of processes of sediment remobilization. The first research area stretches along a dune in a tidal inlet channel in the Jade estuary, North Sea. The sediment consists of medium to coarse quartz sand (after Udden-Wentworth scale). Acoustic subbottom profiling indicates sediment movement induced by the tides. The second area is in the North Sea, 50 km north of the island of Borkum (Germany) in the planned wind energy farm Alpha Ventus. Sediment sampling yielded fine-grained sand and acoustic sub-bottom profiling showed very homogeneous sediment with no hints for significant sediment mobilization. The third (Kailua Bay) and fourth (Waimanalo Bay) regions are located at the windward side of O´ahu, Hawaii, USA. Sediment in both areas is carbonate sand, with the Kailua material generally being finer-grained. In Waimanalo Bay, deployments focussed on the zone of the shorebreak to shed light on sediment mobilization. In our study, we observed short penetration times of 0.1 - 0.25 s and small penetration depths (< 0.5 m) with the consequences of a high risk of tilting and disturbances by ship movement in case of winch-lowered deployments. In contrast, true free-fall, fluid-dynamical shape, tip-oriented center of mass and high sampling rate were identified as advantages during the deployments. However, we found typical impact signatures for the different research areas with either penetrometer and could further show that owing to the high sampling rate, layering can be detected at a cm-scale. Mineralogical composition has a significant effect on the penetrometer´s response. The carbonate sand (Waimanalo Bay, Kailua Bay) caused a significantly higher maximum deceleration (Nimrod: carbonate sand: 120 - 210 g, quartz sand: 25 - 85