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

Purpose: Fluid mud layers can be substantial in waterways and they can jeopardise navigation when the ship’s keel comes in its vicinity. The nautical bottom has therefore to be properly characterised. Mud density and yield stress are used as criteria to characterise the nautical bottom. For a decade, measuring these parameters in situ has been a challenging task. The goal of this paper is to compare the parameters obtained from recently developed in situ measuring instruments. Materials and methods: Two vertical profilers (Graviprobe and Rheotune) were used in this study. The Graviprobe measures the undrained shear strength, whereas the Rheotune measures the Bingham yield stress and density of mud layers. Four different locations at the Port of Hamburg were used to test the Rheotune. Bingham yield stresses obtained from Rheotune measurements were compared with the laboratory rheological measurements. The mud layer thicknesses profiled by Graviprobe and Rheotune at the Port of Hamburg and at the Port of Rotterdam were compared. The measurements were also compared with a model that predicts the consolidation of weak mud. Results and discussion: The Rheotune data match well with the laboratory experiments at two of the four tested locations at the Port of Hamburg. For the other two locations, the Rheotune data underestimates the Bingham yield stress, as compared with the laboratory data. The reason for the discrepancy is discussed. The comparison between the Graviprobe’s and Rheotune’s profiles showed that the thickness of the fluid mud layer, found from each device, for all locations, is similar. The in situ density measurements were confirmed by the laboratory experiments. It was found that the standard model for consolidation of mud could reproduce the in situ strength and density measurements as a function of time relatively well. The mismatch between model and experimental data is most probably due to a change in constitutive parameters over time. Conclusions: Our results show that the correlation between density and yield stress measurements, as a function of different measuring equipment, strongly depends on the type of mud. Therefore, a proper calibration should be performed by establishing a reference database for each equipment. In order to decrease the amount of in situ survey, it is also necessary to develop models that can accurately predict the consolidation over time.