«Dissertation zur Erlangung des akademischen Doktorgrades in den Naturwissenschaften Dr.rer.nat. im Fachbereich Geowissenschaften der Universität ...»
In situ detection and characterisation
of fluid mud and soft cohesive
sediments by dynamic piezocone
Dissertation zur Erlangung des akademischen
Doktorgrades in den Naturwissenschaften
im Fachbereich Geowissenschaften
der Universität Bremen
Bremen, Oktober 2010
Gutachter der Dissertation:
Prof. Dr. Achim Kopf
Prof. Dr. Tobias Mörz
Prof. Dr. Heinrich Villinger Prof. Dr. Burghard Flemming
Weitere Mitglieder des Prüfungsausschusses:
Dr. Christian Winter Antje Lenhart
Tag des Promotionskolloquiums:
12.01.2011 Table of Contents I
TABLE OF CONTENTSNarrative of the Project II
IV Kurzfassung VI 1 Introduction 1
1.1 Motivation 1
1.2 Thesis outline 2 2 Formation of mud deposits 4
2.1 General processes and mud states 4
2.2 Fluid mud 7
2.3 Mud of different consolidation states 10 3 Research areas 13
3.1 The river Ems 13
3.2 Baltic Sea 15 4 Cone Penetrometer Testing (CPT) 19
4.1 Introduction to cone penetrometer testing 19
4.2 Research progress to detect soft and fluid mud by dynamic CPTU 21 References 26 Manuscript 1 32 Rheological characteristic of natural fluid mud and the influence of different parameter Manuscript 2a (Original Manuscript)
NARRATIVE OF THE PROJECTIn the beginning of my PhD thesis in summer 2005, intensive talks with the Federal Waterways Engineering and Research Institute (BAW) of Germany in Hamburg and with the Waterways and Shipping Administration of Emden (WSA Emden) led to the conclusion that there is a great demand on research of soft cohesive sediments in waterways, in particular fluid mud. Especially in the river Ems, high suspended sediment concentrations resulted in high dredging volumes to keep the waterways navigable. Authorities were looking for a reliable and efficient system to detect and characterise fluid mud in the waterways and harbours to reduce the rising dredging costs. Therefore, we considered a threefold approach: 1) The idea was developed that a ship might be able to move fluid mud out of the waterway and/or to prevent consolidation due to turbulences and oxygen enrichment when the ship is passing through. If this hypothesis turns out to be correct, then a ship could be used to keep the waterways navigable instead of dredging the sediment. 2) Second, it was planned to study the interaction of ship movements with fluid mud by modelling different ship speeds and ship shapes in a flume in the laboratories of BAW. 3) In a third step it was planned to test the model results in the river Ems by using a ship equipped with instruments to detect fluid mud and its distribution during ship passage. However, problems occurred like filling the flume with fluid mud, to keep fluid mud in natural conditions in the flume, and to scale the sediment to the model ship size. Furthermore, it was difficult to get a ship equipped with sensors which would pass fluid mud layers since there was the possibility to get stuck in the riverbed. My first concept of the PhD research was not possible to realise anymore. Therefore I had to modify the whole research approach.
In 2006 my research was then focused on the development of a special in situ survey technique to detect both soft cohesive sediments and fluid mud. Additionally, laboratory tests of natural fluid mud and sediment cores were planned. At the same time, a dynamic cone penetrometer (CPTU) measuring in situ pore pressure and strength of sediments had been designed at MARUM. The lance was used in several studies to characterise consolidated marine sediments. However, it was not yet deployed to characterise soft cohesive sediments or even fluid mud suspensions, but showed potential to be a practicable and efficient way for testing these types of sediment.
In January and March 2006 I had the opportunity to participate into two research cruises in the Baltic Sea and tested soft gassy cohesive sediments with the new CPTU device. In the following months I analysed data from more than 50 deployments and categorised the pore pressure development. For additional ground thruthing, I determined the sediment properties of several sediment cores taken there. The CPTU proved to be a good and fast instrument to characterise soft cohesive sediments (Manuscript 3 of this thesis). In 2007 I had the opportunity to join two research cruises in the North Sea and carried out another appx. 50 CPTU tests into soft cohesive sediments. For a second approach of my research it was planned to compare the characterisation of mud layers in both Baltic Sea and North Sea. After several months of data analysis it transpired that these tests couldn’t be used for further studies due to the influence of the stormy weather conditions during both cruises (heave obscuring the deceleration signals) and also Narrative of the Project III because of some technical problems of the lance (some transducers partly non-functional). Again, my research topic had to be modified because there was not another possibility to participate cruises into the North Sea to repeat the measurements.
During 2006 and 2007 I also followed the plan to test fluid mud layers in situ. However, the testing of fluid mud suspensions was even more difficult to realise. Initially I wanted to develop an electrical measuring chain which could be deployed for several tidal cycles in the water of the harbour of Emden to detect fluid mud by electrical conductivity changes. Nevertheless, numerous electrical measurements in the laboratory of fluid mud and clay suspensions with different salinities didn’t attest the applicability of this method, but gave very small variations between the samples (and even the end members).
Afterwards, I followed the second approach to deploy the dynamic CPTU. In cooperation with the WSA Emden and later as well with the Emden port authority it was possible to test the deployment of the CPTU from the ship. Unfortunately, in 2006 and 2007 the CPTU tests in the river Ems and in the harbour of Emden were not able to detect fluid mud. On one hand, there was sometimes no sufficient fluid mud concentration in the investigated areas and secondly, the chosen method of deployment of the CPTU and the way of data analysis was unsuccessful at that time. After having almost lost confidence that there is a chance to develop the new in situ testing method by CPTU during my research time and thinking of modifying the research topic, I figured out that new algorithm for fluid mud detection and CPTU data analysis and processing is required. Hence, in 2007/2008 I developed such an algorithm to analyse the obtained CPTU data for fluid mud detection. Before the algorithm for consolidated sediments had only been applied and turned out to be not applicable for fluid mud. And indeed, recalculation of CPTU data for fluid mud suspension and carrying out new tests in the beginning of 2008 proved that the lance was actually able to detect fluid mud as long as the standard cone is replaced by a custom-built plate with special pore pressure port. Laboratory tests of fluid mud properties completed my research until summer
2008. Anyhow, due to difficulties to get the possibility to join ship cruises for proper CPTU deployment, bad weather conditions on sea, technical problems, necessary PhD research topic modifications, and last but not least to develop a new in situ method took more time than expected. Three years of my contract with the University of Bremen terminated in summer 2008. In the second half of 2008 I took a family break and in spring 2009 I got a job in the industry. Hence, my PhD thesis will be published sometimes later after the actual research time.
Cohesive sediments or muds are common in marine and estuarine environments. During formation of mud deposits several mud states may appear, e.g. dilute suspension, fluid mud, and semi- to fully consolidated mud beds. Appearance and composition of cohesive sediments are highly variable. Shortcomings are apparent in current knowledge of the general relationships to predict the physical behaviour and transport mechanisms of the cohesive sediments in natural environments. Especially the influence of partially free gas resulting from degradation of organic material on the in situ sediment strength is not yet fully understood. Accumulation of gas bubbles may affect the stability of such sedimentary deposits largely. In situ techniques for measurements of physical properties are available, but often not suitable for soft cohesive sediments because of the use of heavy rigs which may disturb the sediment while testing. Also research is still needed to improve fluid mud detection with high accuracy. This thesis presents a novel approach to in situ characterise and classify soft consolidated mud as a function of fluid saturation and to detect fluid mud in its natural environment.
Additionally this thesis outlines laboratory results about the relationship of the rheological behaviour of natural fluid mud to different parameters to provide substantial validation for numerical modelling studies. The findings will also help to understand the interaction between the newly applied in situ detection technique and the behaviour of fluid mud suspensions which has to be investigated further.
A dynamic piezocone penetrometer with pore pressure sensors (CPTU) was deployed in the marine environment of the western Baltic Sea and in the tidally influenced Ems estuary (Germany) to measure in situ strength and pore pressure of both consolidated mud and fluid mud suspension. CPTU measurements in the western Baltic Sea were complemented with sediment sampling by nine gravity cores served sedimentological characterisation and analysis of geotechnical properties as well as with high resolution echo-sounder profiles at selected locations.
The measurements in the marine consolidated mud of the western Baltic Sea show very variable changes in both pore pressure and sediment strength during CPTU deployments. Mainly initially sub-hydrostatic pore pressure values during penetration and a delayed response towards positive pressures thereafter occur. This signal is typically found in soft muds with high water content and undrained shear strength of 1.6.-6.4 kPa. Those pore pressure curves are further affected by enhanced consolidation and strength of individual horizons as well as by the presence of free gas. In contrast, a second type of pore pressure curve shows a well-defined peak in both pore pressure and cone resistance. The initial pore pressure increase during sediment penetration is followed by an exponential decay owing to dissipation. This signal is associated with normally consolidated mud, with indurated clay layers showing significantly higher undrained shear strength (up to 19 kPa).
Abstract V The detection of fluid mud suspension by a dynamic CPTU highlights a novel approach to support surveying and management of areas with high suspended sediment concentrations (SSC). To enhance the resistance during penetration the originally conical device is modified with a disk configuration (33 x the area of the cone) for rapid fluid mud detection. The results show that suspended sediment concentrations 90 g/l can be identified by both disk resistance and pore pressure measurements, and furthermore the transition from fluid mud to consolidating mud once concentration exceed 150 g/l.