A rugged marine impact penetrometer for sea floor assessment

The utilization of marine sediments, as well as the awareness of hazards originating from unstable sea floor conditions, has increased since the second half of the last century. Accordingly, testing methods and especially in situ tests have been developed to assess the geotechnical properties of marine sediments, with the static velocity Cone Penetration Testing (CPT) method being the most prominent testing technique. The values determined by CPT are the cone resistance, being the resistance of the sediment to the forced penetration of a measurement cone, and sleeve friction, being the friction between the sediment and the measurement. Pore water pressure during penetration may also be measured, in which case the testing is termed CPTu. However, the lower handling requirements of dynamic velocity impact penetrometers have led to investigations of their applicability for the determination of properties of marine sediments since the 1970s. Thereby, CPT instrumentation or alternate sensors were used within the impact penetrometers. The aim of the research in this thesis is to investigate the applicability of an impact penetrometer – the Lance Insertion Retardation meter (LIRmeter) – for assessing geotechnical properties of marine sediments. The design of the LIRmeter needed to cover certain requirements regarding operating conditions, such as ruggedness, insensitivity to hydrostatic pressure, ability to cover a wide range of sediment types without retooling, and a fast measurement progress. To cover these requirements, a measurement approach has been chosen that uses solely acceleration sensors to determine geotechnical properties on the basis of the measured acceleration data. Qualitative field trials in the German Bight (North Sea) successfully evaluated the instrument. These field trials were followed by a verification study in the Southern North Sea, where dynamic CPT measurements were taken simultaneously and additional data of a conventional, static velocity, CPT were available. During the verification study, it was shown that dynamically acquired cone resistance data was interchangeable with processed data from the acceleration sensors. Moreover, a correlation of dynamically acquired data with data from a nearby static velocity CPT station was established. This allowed for sediment classification on the basis of dynamic measurements by using common interpretation models. The measurement approach was further investigated by conducting laboratory tests on dense sands with low amounts of cohesive fines. For this laboratory study, a model penetrometer was developed that, likewise, accommodates conventional sensors for measuring cone resistance and sleeve friction in addition to the acceleration during penetration. It was again shown that the acceleration data mimics the data of the cone resistance sensor to a high degree. Geotechnical testing on the mixed soil was undertaken to establish a correlation to the dynamically acquired data using the state parameter relationship. The laboratory study showed a good correlation between the reference data from geotechnical tests and the dynamically