Bereket Yohannes

Effects of coarse grain size distribution and fine particle content on pore fluid pressure and shear behavior in experimental debris flows

Debris flows are typically a saturated mixture of poorly sorted particles and interstitial fluid, whose density and flow properties depend strongly on the presence of suspended fine sediment. Recent research suggests that grain size distribution (GSD) influences excess pore pressures (i.e., pressure in excess of predicted hydrostatic pressure), which in turn plays a governing role in debris flow behaviors. We report a series of controlled laboratory experiments in a 4 m diameter vertically rotating drum where the coarse particle size distribution and the content of fine particles were varied independently. We measured basal pore fluid pressures, pore fluid pressure profiles (using novel sensor probes), velocity profiles, and longitudinal profiles of the flow height. Excess pore fluid pressure was significant for mixtures with high fines fraction. Such flows exhibited lower values for their bulk flow resistance (as measured by surface slope of the flow), had damped fluctuations of normalized fluid pressure and normal stress, and had velocity profiles where the shear was concentrated at the base of the flow. These effects were most pronounced in flows with a wide coarse GSD distribution. Sustained excess fluid pressure occurred during flow and after cessation of motion. Various mechanisms may cause dilation and contraction of the flows, and we propose that the sustained excess fluid pressures during flow and once the flow has stopped may arise from hindered particle settling and yield strength of the fluid, resulting in transfer of particle weight to the fluid. Thus, debris flow behavior may be strongly influenced by sustained excess fluid pressures controlled by particle settling rates.

Evolution of the microstructure during the process of consolidation and bonding in soft granular solids.

The evolution of microstructure during powder compaction process was investigated using a discrete particle modeling, which accounts for particle size distribution and material properties, such as plasticity, elasticity, and inter-particle bonding. The material properties were calibrated based on powder compaction experiments and validated based on tensile strength test experiments for lactose monohydrate and microcrystalline cellulose, which are commonly used excipient in pharmaceutical industry. The probability distribution function and the orientation of contact forces were used to study the evolution of the microstructure during the application of compaction pressure, unloading, and ejection of the compact from the die. The probability distribution function reveals that the compression contact forces increase as the compaction force increases (or the relative density increases), while the maximum value of the tensile contact forces remains the same. During unloading of the compaction pressure, the distribution approaches a normal distribution with a mean value of zero. As the contact forces evolve, the anisotropy of the powder bed also changes. Particularly, during loading, the compression contact forces are aligned along the direction of the compaction pressure, whereas the tensile contact forces are oriented perpendicular to direction of the compaction pressure. After ejection, the contact forces become isotropic.

Mechanistic modelling of tests of unbound granular materials

Various tests are used to characterise the strength and resilience of granular materials used in the subbase of a pavement system, but there is a limited understanding of how particle properties relate to the bulk material response under various test conditions. Here, we use discrete element method (DEM) simulations with a mechanistically based contact model to explore influences of the material properties of the particle on the results of two such tests: the dynamic cone penetrometer (DCP) and the resilient modulus tests. We find that the measured resilient modulus increases linearly with the particle elastic modulus, whereas the DCP test results are relatively insensitive to particle elastic modulus. The DCP test results are also relatively insensitive to inter-particle friction coefficient but strongly dependent on the particle shape. We discuss strengths and weaknesses of our modelling approach and include suggestions for future improvements.

Discrete particle modeling and micromechanical characterization of bilayer tablet compaction

A mechanistic particle scale model is proposed for bilayer tablet compaction. Making bilayer tablets involves the application of first layer compaction pressure on the first layer powder and a second layer compaction pressure on entire powder bed. The bonding formed between the first layer and the second layer particles is crucial for the mechanical strength of the bilayer tablet. The bonding and the contact forces between particles of the first layer and second layer are affected by the deformation and rearrangement of particles due to the compaction pressures. Our model takes into consideration the elastic and plastic deformations of the first layer particles due to the first layer compaction pressure, in addition to the mechanical and physical properties of the particles. Using this model, bilayer tablets with layers of the same material and different materials, which are commonly used pharmaceutical powders, are tested. The simulations show that the strength of the layer interface becomes weaker than the strength of the two layers as the first layer compaction pressure is increased. The reduction of strength at the layer interface is related to reduction of the first layer surface roughness. The reduced roughness decreases the available bonding area and hence reduces the mechanical strength at the interface. In addition, the simulations show that at higher first layer compaction pressure the bonding area is significantly less than the total contact area at the layer interface. At the interface itself, there is a non-monotonic relationship between the bonding area and first layer force. The bonding area at the interface first increases and then decreases as the first layer pressure is increased. These results are in agreement with findings of previous experimental studies.

Unified mechanistic approach for modeling tests of unbound pavement materials

AbstractSeveral tests are used for the characterization of unbound materials for pavement applications. The resilient modulus has been one of the most common tests for design specification of unbound materials. The California bearing ratio (CBR) is another laboratory test that is frequently used. The dynamic cone penetrometer (DCP) test is a more common test for in situ quality assessment/quality control of unbound materials. For better connection between design and quality assurance (QA)/quality control (QC), it would be helpful to have a reliable, mechanistic method for correlating test results. This is particularly true for the use of new materials, for which there is not an extensive body of data to empirically draw such connections. This paper presents a framework for a unified approach for modeling these tests. A discrete-element method (DEM) is used to simulate the CBR test, the DCP test, and the resilient modulus test. An initial evaluation demonstrated that the simulations can account for the eff...

Preliminary investigations on the rheology and boundary stresses associated with granular mixtures

Recent advances in rheological models for monodisperse dense granular materials are exciting. However, they do not account for the effect of local particle size distributions on the rheology mixtures of particles. It is well‐known that particulate mixtures tend to unmix, and their rheological properties are dependent on species concentration. Typically, expressions for the rheology of dense granular flows are explicitly dependent on particle size. However, there is no indication of what may be used for a representative size in a mixture of particles of different sizes. We find, in the absence of gravity, plane Couette cells present an effective geometry for investigating the rheology of binary mixtures of different sized particles. Unlike the behavior of more sparse systems, we find that the dense systems do not segregate much, indicating the usefulness of the geometry for studying the dependence of the mixture rheology on particle sized distribution systematically. In our preliminary studies we find that...