Fluid mud can be described as a mixture of water, organic matter and mainly cohesive mineral sediment that is usually found in estuaries and in rivers with low-intensity currents. It is in a transient state and will densify over time unless mixing energy is added to mud layers by means of mechanical and/or natural forces. Typically, fluid mud exhibits bulk densities between 1080 and 1200 kg/m3. It has a weak strength that develops over time forming a structured bed of considerably higher rigidity.
The fluid mud layers can be substantial in harbours and waterways. Hence, the PIANC maritime regulations have been developed to guarantee safe navigation in muddy environments. The PIANC Working Group 30 defined the nautical bottom as “the level where physical characteristics of the bottom reach a critical limit beyond which contact with a ship's keel causes either damage or unacceptable effects on controllability and manoeuvrability.” Accordingly, the nautical depth was defined as “the instantaneous and local vertical distance between the nautical bottom and the undisturbed free water surface.” The application of these definitions requires insight in the physical characteristics of the mud that can characterize the effect of mud layers on the behaviour of a ship. Based on experimental research in the Port of Rotterdam in 1974, the density of 1200 kg/m3 has been chosen in the harbour as the physical characteristic that defines the nautical bottom. In other harbours this density value is ranging approximately from 1150 to 1300 kg/m3 or the definition of nautical bottom is determined by echo-sounding or rheological criteria. One of the goals of this paper is to compare the criteria that are used for determining the nautical bottom in harbours.
The fluid mud layers have to be maintained to ensure the entrance of ships to the harbours. Often the dredged sediment is relocated to the open sea making current maintenance method expensive. A remedial maintenance concept of keeping the sediment in place was applied at the Port of Rotterdam. The water injection dredging method was employed for liquefying the top layers of the sediment and mobilizing the fluid mud from the bottom of the 8th Petroleumhaven to a deepening of the total area of 500x100 m2. In this paper we show that fluid mud layers up to 1.5m can be created using the water injection dredging method.
The detection and monitoring of fluid mud layers is of primary importance to safeguard navigation at ports and waterways. In our monitoring campaign the conventional low-frequency (38 kHz) and high frequency (200 kHz) acoustic sounding was compared to the output of Rheocable, Graviprobe and DensX. The SILAS system was used to enhance the low-frequency sounding. The Rheocable survey is based on the physics of a towing body. Towed within a certain velocity window the position of the Rheocable is then related to the interface between fluid and consolidated mud. The penetrometers DensX and Graviprobe are used to provide vertical profiles of the density and undrained shear strength, respectively. The Graviprobe instrument measures the cone penetration resistance and pressures while falling free in a water-mud column. The cone penetration resistance is then correlated to the undrained shear strength of the fluid mud layer. DensX is an X-ray based profiler that measures the densities of a water-mud column between 1000 kg/m3 and 1500 kg/m3. The SILAS system correlates the measured acoustic impedances of a multi-beam echo-sounder to in-situ density measurements. Typically, this density measurements are done with penetrometer-type tools or mud samplers.
Our study provides a new insight into rheological mapping of fluid mud layers. The Rheocable and Graviprobe measurements showed a good agreement regarding navigable depth. The non-linear relationship between density and shear strength of mud was investigated by means of laboratory consolidation and rheological experiments. It was found that the undrained shear strength of mud develops slower with time than the density. This finding is confirmed by theoretical and scaled laboratory study. In particular, the output of the Graviprobe is linked to the critical yield stress (100 Pa) that is currently used as a critical parameter for determining the nautical bottom at the Harbour of Emden and indicated in PIANC (2014) as a reliable rheological criterion.
The water injection dredging method can be used for liquefying and mobilizing weak fluid mud layers. In-situ measuring tools are available for characterizing the behaviour of fluid mud. Based on experimental and theoretical investigation, we can conclude that new cost effective port maintenance strategy is feasible in the ports and waterways with muddy environments.