Geotechnical Investigation of Sediment Remobilization Processes using Dynamic Penetrometers

Abstract

The understanding of naturally and anthropogenically induced subaqueous sediment remobilization processes is of high importance for the increasing industrial and touristy usage of coastal zones, estuaries, rivers and lakes. Geotechnical properties such as sediment strength play an important role in sediment dynamical processes, however, they are poorly represented in theoretical approaches and field studies, because geotechnical in-situ measurements are highly complicated in such challenging areas (e.g., strong hydrodynamics, close to offshore constructions). From previous studies it can be assumed that dynamic penetrometers might be capable of such measurements. The aim of this thesis was to find out (i) whether dynamic penetrometers are suitable for measurements in areas of sediment remobilization, and (ii) if so, what kind of complementary data can be delivered for the research of sediment remobilization. Different types of dynamic penetrometers were introduced. However, none of the existing penetrometers matched all of the requirements assumed for the detection of sediment remobilization. Following that, a new device was designed: the Nimrod, equipped with accelerometers (measuring deceleration and inclination), pore pressure and temperature sensor. Different penetration signatures could be found for quartz sand compared to carbonate sand. Furthermore, an approach was presented to estimate quasi-static bearing capacity from the dynamic deceleration depth profiles of the penetrometer to consider the change of penetration velocity and penetration surface area. In data sets from the Jade tidal inlet channel (North Sea) and shore breaks in Hawaii hints of sediment remobilization were detected in the penetrometer signatures. More detailed surveys in areas of sediment remobilization followed: (i) subaqeuous dunes in the Danish Wadden Sea, (ii) sorted bedforms close to Tairua, NZ, (iii) a shifting sandbar at Raglan s harbor mouth, NZ, (iv) scouring at offshore wind energy converter foundations (North Sea), (v) mud accumulation and mud layers in different ports, (vi) disposal sites and (vii) geothermal lakes in New Zealand. First, the results confirmed that layers of mixed up sediment or fresh sediment redeposition are reflected in the sediment strength depth profiles derived from the dynamic penetrometer and that a quantification of these layers is possible with an accuracy of ~ 1 cm. In doing so, e.g., variations of sediment remobilization over a tidal cycle were observed. Additionally, changes of sediment strength patterns over time were monitored: over a tidal cycle along subaqueous dunes, and especially, at offshore wind energy converters (WEC) over a few months after WEC erection. Furthermore, areas of sediment erosion and sediment accumulation were localized and quantified providing a suitable base for the development of a conceptual model of the formation and/or maintenance of the respective sediment dynamic feature. This succeeded for sandy areas (e.g., sorted bedforms, shifting sandbar) as well as for muddy areas (e.g., ports, lakes). In the latter the results may also play an important role for decisions about the further industrial use of areas or further interventions such as dredging. In summary, the new penetrometer Nimrod proved its suitability for the investigation of sediment remobilization processes and delivered complementing data about ongoing sediment remobilization with time (e.g., tides, timeline after WEC erection) and space (indication of areas of sediment erosion or accumulation). During the different surveys the dynamic penetrometer results were supported by acoustic methods, sediment sampling and/or numerical modelling of hydrodynamics. A stronger geotechnical perspective was introduced into the investigation of the sediment remobilization processes. However, some questions could not be answered by the field experiments due to a lack of continuity of boundary conditions such as currents, stability of the vessel or sediment homogeneity. Examples are the detailed investigation of penetration rate effects, or the finding of a correlation between in-situ density and measured sediment strength. To address such issues, in this thesis also first attempts of physical modelling in a wave channel and of numerical modelling using the geotechnical code FLAC3D are presented and discussed as an outlook to future works.