Shear Rate-Dependent Rheological Properties of Mine Tailings: Determination of Dynamic and Static Yield Stresses

In this paper, shear rate-dependent rheological properties of mine tailings taken from abandoned mine deposits prone to mass movements are examined using a commercial ball-measuring rheological system. The yield stresses (i.e., dynamic and static yield stresses) and viscosity of sand-rich materials are examined by the shear rate-controlled flow curve and time-dependent stress growth methods. Before yielding, the shear stress reaches a peak value (i.e., yield stress) observed for all flow curves. In the steady-state condition, the materials have a minimum shear stress (i.e., dynamic yield stress). The static yield stress can be determined under a constant applied shear rate with different initial values ranging from 10−4 to 10−1 s−1. As a result, the Bingham yield stress and viscosity can be used as a first approximation for estimating the debris flow mobility of post-failure materials. However, the Bingham yield stress is competitive with the static yield stress measured from stress growth methods. Upon comparison of the dynamic and static yield stresses, the static yield stress is approximately 35–45 times greater than the dynamic yield stress, and may be strongly related to microstructural changes (i.e., thixotropy). In this context, special attention must be paid to the determination of yield stresses in debris flow mitigation programs.

Experimental study on the critical initiation of reservoir fluid mud motions

Reservoir fluid mud is the stagnant or near-stagnant suspension distributed at the bottom of a reservoir after variable-density flows. It consists of fine particles that flocculate easily, and its thickness can be maintained unchanged over a long time period. Because the formation and the movement of reservoir fluid mud are affected by various factors, and given that measured data relevant to the dynamic conditions of the reservoir fluid mud are lacking, current results can hardly explain scientific questions that have to be resolved. These include the development of reservoir fluid mud and its response mechanisms to subsequent floods. In this study, a pressurized sealed water flume is employed to simulate deep-water conditions and to facilitate the conduct of experiments on the initiation of reservoir fluid mud motions. The results demonstrate that the critical shear stress required for the initiation of motion of fluid mud increases exponentially as a function of the volumetric weight of fluid mud and water depth. The critical shear stress is much smaller than the Bingham yield stress and the two are associated according to a power-function relationship. The findings provide technical support for the utilization of the reservoir fluid mud and the optimization of reservoir operations.

Rheology of aqueous boehmite-coated silicon nitride suspensions and gels

The rheological properties of boehmite-coated silicon nitride aqueous suspensions and gels are reported. In unidirectional rheological tests, it was found that the boehmite coating reduces the viscosity of the suspensions over a wide range of shear rates and volume fractions of particles. The suspension shear stress as a function of shear rate can be described by the Bingham model, and the Bingham yield stresses of boehmite-coated silicon nitride suspensions are lower than those of the uncoated suspensions. The reduction in the viscosity and the Bingham yield stress is attributed to a shallower secondary minimum in the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential between coated particles than that for uncoated silicon nitride particles. Moreover, at low values of pH, the coated silicon nitride suspensions gelled over time, and the viscoelastic behavior of the gels was studied by dynamic oscillatory tests. It was found that the shear modulus (G′) and loss modulus (G″) remain constant up to a certain strain amplitude, γ°, beyond which G′ and G″ begin to vary. The value of G′ in the linear region increases exponentially, whereas γ° decreases exponentially with the volume fraction of coated silicon nitride particles. The exponential behavior of the shear modulus G′ of the gels is similar to the exponential pressure-density relationship found in the previous pressure filtration study, indicating that particulate rearrangement occurs as volume fraction of particles is increased.

Influence of polysaccharides on kaolinite structure and properties in la kaolinite–water system

The influence of soil organics on the development of soil structure and behaviour of soils is the motivation for this study. Xanthan and dextran, two soil polysaccharides, were examined in relation to their influence on the development of the structure of kaolinite–polysaccharide systems and also on resultant physicochemical properties. The kaolinite–polysaccharide soils were formed in suspensions under anaerobic and low redox potential conditions to simulate environmental conditions that exist in subsurface soils. Information on the rheology of the kaolinite–polysaccharide system, zeta potential, aggregate size, settling properties, and specific surface area were obtained. Both polysaccharides were shown to develop aggregate groups (floes) which participated to a greater or lesser extent in the overall rheological properties of the kaolinite–polysaccharide complexes. The ability of the polysaccharides to develop flocs is seen as a factor to be considered in studies of soil permeability and transport processes in soil. Together with information from infrared spectroscopy and scanning electron microscopy, and in combination with the results obtained from the physicochemical tests, models of interaction between kaolinite and both polysaccharides are proposed. Key words: soil organics, polysaccharides, kaolinite, rheology, floes, xanthan, dextran, soil structure, zeta potential, kaolinite–polysaccharide system, differential viscosity, Bingham yield stress.