Abstract
Natural mud sediments display complex rheological behaviour like thixotropy, viscoelasticity and yield stress. These rheological characteristics can significantly vary over depth, from one mud layer to another, as each layer can have a different density and composition. Fast and reliable measurements of yield stresses of mud samples are important for maintenance operations in ports and waterways. These protocols, performed in the laboratory, should give a rheological fingerprint which is representative of the in-situ behaviour of the mud. In this article, we show that our recently developed stress ramp-up rheological protocol is a time-efficient and well-grounded protocol to determine the yield stresses of natural mud samples by comparing with other existing well-grounded protocols. In this study, we also refine the stress ramp-up protocol such as to reduce the experimental time for different mud layers based on their densities. The protocol was tested on a large number of mud samples obtained from different locations/depths of the Port of Hamburg, Germany. An empirical model is proposed to fit the two-step yielding behaviour that the mud samples exhibit. The model captures the two-step yielding phenomenon in mud samples quite well, within the density range of 1050–1200 kg. m−3. This two-step yielding is a feature of mud samples as found in various harbours and estuaries worldwide in rheometry.
Highlights
Mud sediments consist of water, clay minerals, sand, silt, and organic matter
The two representations of the flow curves for mud sample are shown in Fig. 4a and b, using different protocols with Couette ge ometry
The differences at shear stress/shear rate values above the static yield point are quite significant. This sug gests that the mud sample ‘L6-3’, in the partially disturbed state, still has some structure depending on its shear history
Summary
Mud sediments consist of water, clay minerals, sand, silt, and organic matter. Usually, (fluid) mud exhibits a complex rheological response including viscoelasticity, yield stress and thixotropy. It is already known that the rheological and cohesive properties of mud sediments are dependent on their density and the amount of organic matter (Malarkey et al, 2015; Parsons et al, 2016; Paterson et al, 1990; Paterson and Hagerthey, 2001; Schindler et al, 2015; Shakeel et al, 2019; Tolhurst et al, 2002; Wurpts and Torn, 2005) Knowledge of these rheological properties of mud sediments is very important to predict density cur rents and fluid mud flows, which in turn impact turbulence, navigation in channels, maintenance of ports (dredging activities) and general quality control of water in coastal areas. The nautical bottom was until recently defined as a critical fluid density (McAnally et al, 2007) There is another criterion (i.e., yield point) which is quite important for the ports where the organic matter content is significantly varying as a function of location within the port. In order to calibrate and improve the in-situ equipment, extensive laboratory tests are still needed
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