Rheological analysis of mud from Port of Hamburg, Germany

Abstract

Purpose An innovative way to define navigable fluid mud layers is to make use of their rheological properties, in particular their yield stress. In order to help the development of in situ measurement techniques, it is essential that the key rheological parameters are estimated beforehand. Is there only one yield stress? In which shear rate/shear stress range is yield expected to occur? How is yield stress dependent on depths and locations in the harbor? In order to answer these questions, we investigated the changes in the rheological properties of mud from along the river stream in the Port of Hamburg, Germany, using a recently developed laboratory protocol. Materials and methods In this study, a detailed rheological analysis was carried out on the mud samples collected from different locations and depths of the Port of Hamburg. A variety of rheological tests was performed including: stress sweep tests, flow curves, thixotropic tests, oscillatory amplitude, and frequency sweep tests. Results and discussion The yield stresses of sediments from different locations were significantly dissimilar from each other due to differences in densities and organic matter content. Two yield stresses (termed static and fluidic) were observed for every sample and linearly correlated to each other. The thixotropic studies showed that all mud samples, except from one location, displayed a combination of thixotropic and anti-thixotropic behaviors. The results of frequency sweep tests showed the solid-like character of the sediments within the linear viscoelastic limit. The yield stresses, thixotropy, and moduli of the mud samples increased by going deeper into the sediment bed due to the increase in density of the sediments. Conclusions This study confirmed the applicability of the recently developed protocol as a fast and reliable tool to measure the yield stresses of sediments from different locations and depths in the Port of Hamburg. The fluid mud layer, in all the locations it was observed, exhibited relatively small yield stress values and weak thixotropic behavior. This confirms that despite the fact that rheology of fluid mud is complex, this layer can be navigable.

Is density enough to predict the rheology of natural sediments?

“…However, to compare the yield stress values of different samples, the approach reported by (Zhu et al 2001) was used to obtain the yield stress values by extrapolation. The stress values associated with the first decline are referred to as "static" yield stress, τ s y , while the second decline stress values are termed as "fluidic" yield stress, τ f y (Shakeel et al 2019a). The static yield stress value is most probably associated with the relaxation of the sample structure, including the breakage of large flocs into smaller flocs, as already suggested by Nosrati et al 2011.…”

Using in situ density and strength measurements for sediment maintenance in ports and waterways

“…Both the density and the yield stress of the samples were measured. The yield stress was assessed using a recently developed rheological protocol (Shakeel et al 2019(Shakeel et al , 2020. A HAAKE MARS I rheometer (Thermo Scientific, Germany) was used to perform the rheological experiments with concentric cylinder (Couette) geometry.…”

“…A previous analysis revealed the existence of significantly different rheological properties for mud samples collected from the selected locations. These differences are due to different densities and organic matter content (Shakeel et al 2020).…”

“…Claeys et al (2015) proposed another protocol with vane geometry that includes preconditioning of mud. Recently, Shakeel et al (2020) conducted an extensive laboratory rheological study on mud samples of different densities. Different measuring geometries, rheological methods and protocols were compared.…”

“…Different measuring geometries, rheological methods and protocols were compared. The authors found that a stress sweep test with Couette geometry is a practical and time-efficient method for determining the yield stress values of mud samples with weak yield stress values (Shakeel et al 2020).…”