IC10_II

Initial questions and objectives of the project
Sediment strength is a function of effective stress, which, after the principle of Terzaghi, is dependent on total stress (lithostatic stress and other cohesive forces) and pore (water) pressure. The more the pore pressure increases at constant total stress rates, the more the effective stress decreases. Thus pore pressure influences sediment stability and even subtle increases in pore pressure (p. ex. during high tides) in saturated sediments can lead to an overpressure which again may facilitate sediment remobilisation in the topmost part of the sub-seafloor.
For offshore sites, free-fall cone penetration tests (FF-CPT) are a time- and cost-efficient method to investigate important sediment physical properties determining sediment strength. The CPT lance is lowered through the water column either in free-fall mode or at a defined winch speed and eventually penetrates the seafloor by its own momentum. During the penetration sensors measure the in-situ tip resistance, sleeve friction, pore pressure, acceleration/deceleration and tilt on which basis the classification of sediment types and determination of their undrained shear strength, sensitivity, consolidation state or other properties can be carried out. Due to the non-linear penetration velocity and the resulting strain rate effects, sediment strength values derived from a dynamic CPT data set have to be corrected so that they can be compared to values from static CP testing or geotechnical laboratory tests.
A particularly complex phenomenon is the episodic and tidally driven variation of navigable depth as a result of fluid mud settlement. Fluid mud is a highly concentrated suspension of clay minerals and organic matter with concentrations between several 10 to 100 g/L. It has a non-Newtonian flow behavior, a one to four times larger viscosity than water and its yield stress correlates positively with increasing SSC at a given shear rate (Seifert & Kopf, 2012).
For an in situ, non-acoustic identification of suspended matter of 90 g/L or more and the transition from fluid to consolidating mud at about 150 g/L a CPT lance with a modified disk shaped tip can be used. By changes in both disk resistance and thus sediment strength and pore pressure measurements during penetration, these different layers of fluid mud can be accurately identified (Seifert & Kopf, 2012).
For my PhD project I will first work on an already existing data set of dynamic deployments of different CPT devices (FF-CPT and NIMROD developed at the MARUM, GraviProbe developed by DotOcean) into controlled mud layers at a test facility of DotOcean at the port of Zeebruegge, Belgium. In addition to the in situ measurements, geotechnical lab tests will be run. The results of the benchmarking of free-fall penetrometer in a well-defined environment will function as a proof of concept and could be published as my first paper.As a second step I will collect data by CP testings in natural settings. Possible sites are the tidally affected Ems river estuary in Northern Germany and the Firth of Thames area on the North Island of New Zealand. By including information about p. ex. the mineralogy of the clay minerals, salinity, flow regimes, etc. and results from other tests such as water-sampling, CTD, ADCP and rheometer tests, the differences and/or similarities between the two (or more) sites at different geographical locations can be distinguished and interpreted. My work could be connected to the work of Dr. Bryna Flaim, one of the INTERCOAST post-docs, who is dealing with the formation of internal waves at the fluid mud – water boundary at said locations and/or the past and current work of Dr. Annedore Seifert. The results could be published in one to two papers.
As an addition to the research on fluid mud, research work on morphodynamic processes in near shore areas in both Germany and New Zealand could be carried out. Of special interest is the distribution of sediment in defined coastal areas such as ebb tidal deltas or offshore sandbar systems after extreme (storm) events. This work could be connected to the work of Shawn Harrison (NZIC 4) and/or Brice Blossier (IC 1). The results could be published in one or more papers.
Currently I focus on literature research to deeply understand the scientific background of my project and to further define scientific questions. Moreover I will get familiar with the CPT devices I will or might use for my research work by performing first test measurements at test sites near campus, supervised by experienced colleagues (older PhD students, technician). Furthermore I have to get familiar with processing the dynamic CPT raw data by a MATLAB routine for further interpretation.