Print Email Facebook Twitter Deposition of high density tailings on beaches Title Deposition of high density tailings on beaches Author Van de Ree, T.H.B. Contributor Van Rhee, C. (mentor) Talmon, A.M. (mentor) Van Kesteren, W.G.M. (mentor) Chassagne, C. (mentor) Faculty Mechanical, Maritime and Materials Engineering Department Maritime and Transportation Technology Programme Dredging Engineering Date 2015-09-02 Abstract One of the biggest uncertainties affecting the operations of mine and land reclamation activities is related to tailings / slurry management, for which the processes of beach deposition play a critical role. Deposited tailings produce both sheet and channelised flow, causing varying velocity profiles which lead to differential settlement. This is impacting slurry management operations (e.g. fines capture, capacity of a storage facility) as well as the mine closure and/or reclamation planning (e.g. strength of the deposit, differential consolidation, total settlement and reclamation topography). Even though critical, little is known about the physics of non-Newtonian slurry flow over beaches. In order to improve this knowledge, Deltares initiated a research program. This program starts with large scale experiments, aiming to mimic and understand the flow behavior observed in practice. This study is based on the large scale experiments at Deltares and consists of a literature survey, experiments and numerical modeling. In the experiments a mixture of clay, silt, sand and salty water is discharged over a 2% slope in which the composition is varying during the experiments. In the subsequent experiments water is added to the initial mixture causing a decrease in strength. Along with decreasing strength, the flow behavior changes from robust sheet flow to more dynamic channel flow. The change in flow behavior corresponds to a change in observed surface shear-profile. During (slow) sheet-flow, perpendicular and abrubt shear planes are observed wheras smooth and parabolic shear planes were observed during (fast) channel flow. Accompanying the change of strength, flow behaviour and shear mechanism, a shift from non-segregating to segregating slurry is observed. A hypothesis formulated in this thesis states that the observed surface shear profile is similar to the vertical shear profile, this could explain why the slurry is segregating or not. Rheology measurements were conducted with mixture used in the flume experiments. The measurements reveal rheopectic behavior with cumulative shear, after a certain time (added shear) the rheology reaches an equilibrium state (i.e. constant flowcurve). From these measurements an empirical function was developed relating the flow-curve to the mixture composition. Shear stress calculations based on these rheology measurements in combination with observations show a great agreement with the equilibrium bottom shear stress during the last part of the experiments. However during the first part the dynamic shear stress was exceeding the equilibrium bottom shear stress of the inclined flow, this is most likely caused by the developing rheology which was not yet at the equilibrium flow curve in the beginning of the experiments. After channel formation, cores were taken in both flowing and stagnant (or very slow flowing) parts. These cores were analysed on water content and particle size distribution. In addition vane (strength) measurements were conducted at nearby locations. A comparison between flowing and stagnant parts reveal no significant difference in water content, particle size distribution and strength. Though an equivalent variation (increasing strength and particle size, decreasing water content) was found (from top to bottom) over the height of the mud layer due to settling and consolidation of solids. From bathymetry measurements it was observed that the overall slope of the mud-line was steeper at the location of the flowing part than at the stagnant part. In addition the equilibrium slope was decreasing during the experiments caused by a decreasing strength. In previous research on flowing tailings, an analytical model(1DV) was developed aiming to predict the deposition of sand. The ingenuity of this model is a rheology-based viscosity profile which lead to a settling-velocity distribution over the height of the flowing layer. The 1DV model developed in this study assumes a similar (and constant) settling-velocity profile, however the problem is solved numerically. Hence it is possible to include more aspects like hindered settling, a multi-sized particle size distribution and morphological development. The input for the model is generated by flume-measurements in combination with the developed rheology function. The model predicts the average sedimentation measured during the flume experiments reasonably well. The experimental data was also compared with the original model (without hindered settling and with a mono-sized particle size distribution (d=2d50), it was found that the developed model performs slightly better than the original model. Subject tailingsnon-Newtoniansegregationrheologyexperimental To reference this document use: http://resolver.tudelft.nl/uuid:6e541dec-2d4a-4514-b521-a01ab709065f Embargo date 2017-09-01 Part of collection Student theses Document type master thesis Rights (c) 2015 Van de Ree, T.H.B. 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