Feature ArticleIn-Situ Geotechnical Investigation Of Sediment Remobilization Processes
By Nina Stark • Achim Kopf
Different modes of sediment remobilization and their correspondence to geotechnical properties.
Measurements in such areas are difficult due to strong hydrodynamic forcing and an easily disturbed seafloor surface. Geotechnical methods have been used to investigate sediment remobilization processes, with the deployment of the small-scale dynamic penetrometer Nimrod developed by the Center for Marine and Environmental Sciences (MARUM) at the University of Bremen.
Sediment Properties Versus Remobilization Processes
The seafloor can be characterized by a number of geotechnical properties, such as cohesion, friction angle and pore pressure.
To initiate sediment movement, the shear stress applied by currents and waves has to overcome a threshold, which depends on the friction angle and cohesion. Approaches to assess the threshold shear stress predominantly use the grain size to estimate the friction angle. However, the friction angles of surficial seafloor sediments are also highly influenced by particle packing, grain shape and protrusion of particles.
As soon as sediment transport is initiated, particles in the so-called active layer will be entrained and moved. Thus, the existing particle fabric will be broken up, and properties such as friction angle, pore pressure and density will change within the range of the active layer, allowing observers to distinguish the active layer from the stable underlying seabed.
During sediment transport, areas of sediment erosion will be distinguishable from areas of deposition. Freshly eroded seafloors will be characterized by a hard surface due to the fact that looser particles were removed, whereas in areas of sediment deposition, a loosely packed layer will be found on top of the original sediment surface.
Measuring Sediment Remobilization In Situ
One option to get a better idea of the friction angles of surficial seafloor sediments are laboratory measurements and shear tests conducted under water-saturated conditions and low normal stresses. However, it is difficult to simulate the sensitive and quickly varying packing conditions at the seafloor sediment surface.
As such, in-situ methods are required but face a number of challenges. Devices used in situ must be operable in highly turbulent waters, easily deployable from small and navigable platforms, and able to provide vertical profiling at high resolution of loosely packed layers (e.g., fluid mud), as well as spatial resolution as active layers are often in a range of a few centimeters.
In-situ vane shear apparatus (often diver operated) and standard engine-driven cone penetration testing are often not applicable due to vigorous wave and current environments. The latter also faces difficulties in the case of shallow waters, which do not allow navigation of large platforms. Dynamic penetrometers are a promising technology that are easily deployable and can estimate geotechnical properties, such as sediment strength and pore pressure, with high vertical resolution.
Design of the Dynamic Penetrometer Nimrod
The Nimrod follows the measurement strategy of existing small-scale dynamic penetrometers: It free-falls through the water column, reaching velocities up to 11 meters per second, depending on water depth, type of tether and length, and hydrodynamic conditions, and penetrates the seafloor until its kinetic energy is dissipated.
Penetration depths vary, depending on the sediment, from a few centimeters to 3 meters. It records deceleration, pressure and, optionally, temperature versus time. Velocity and penetration depth can be determined by single- and double-integration of deceleration, respectively. Assuming the monitored deceleration of the probe during sediment penetration results from sediment resistance only, the deceleration can represent sediment resistance force and, consequently, sediment strength. To account for the nonlinear backcoupling between decreasing penetration velocity and varying impact velocity, an approach has been presented to estimate an equivalent of quasistatic bearing capacity (QSBC).
Nimrod's steel tip and hull design ensure stable free-falls even in turbulent waters and under unstable deployment conditions (e.g., drifting kayak, jet ski, deployment by divers and submersibles), making it particularly suitable for investigating sediment remobilization processes. Its size (81 centimeters long and 11 centimeters in diameter) and mass (13 to 15 kilograms, depending on chosen tip geometry) allow deployment and recovery by hand, and do not require an engine-driven winch. The choice of three tip geometries—cone, hemisphere or cylinder—allows for increased sensitivity for very soft layers, such as fluid mud, by using the cylindrical tip, or increased penetration depth into harder sediments, such as fine sand, using the conical tip.
Recently, an optional tail was developed in collaboration with the École Polytechnique Fédérale de Lausanne. For deployments from the Russian MIR submersibles in Lake Geneva, the original cross-finned tail was replaced by a handle which could be held, released and grabbed by the MIR's robotic claw. Potentially, this modification would also enable deployments from ROVs or other underwater vehicles in the future.
Data acquisition for the Nimrod is designed for high-frequency, and thus, high-resolution profiling. The customized data logger from Avisaro AG (Hannover, Germany) records 16 channels at 1 kilohertz, leading to a spatial resolution of less than 1 centimeter in the vertical profiles. Five differently ranged microelectronic-mechanical systems acceleration sensors monitor the Nimrod's acceleration or deceleration from 0.1 g, where g is the gravitational acceleration, up to 250 g with high resolution. The acceleration sensors, two of which are three axes, also give information about the tilt of the probe during deployment. Using the pressure transducer data, water depth can be determined, and pore pressure behavior (subhydrostatic or excess pore pressure) can be estimated. To continue this article please click here.
Nina Stark, now a postdoctoral fellow at Dalhousie University, received her Ph.D. from the marine geotechnics working group at MARUM, University of Bremen, where she developed the Nimrod. She accomplished her master's thesis at the University of Münster, in collaboration with the Naval Research Institute for Geophysics and Water Acoustics.
Achim Kopf is a full professor for marine geotechnics at MARUM, University of Bremen. His research and developments focus on sediment physical behavior in the geotechnical laboratory, by in-situ measurements and from borehole observatories.