1. Results and Discussion
Impact of Packing Density, Particle Angularity and Uniformity Coefficient on the Erodibility of Coarse-grained Particles
Hyunwook Choo1, Ph.D., Qian Zhao2, Ph.D., Terry Sturm1, Ph.D., P.E., and Susan Burns1, Ph.D., P.E.
1School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia
2Department of Civil & Environmental Engineering, University of Louisville, Louisville, Kentucky
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Introduction
Theoretical Analysis
Combined Rolling vs. Paphitis Envelopes
Effects of Uniformity Coefficient and Particle Shape
Transport of coarse-grained sediments : f(hydrodynamic conditions, geotechnical properties of sediments)
Among various geotechnical properties of soils, the effect of mean grain size on the erodibility of coarse grains : well quantified. However, studies on the impact of other geotechnical properties (e.g., void ratio, uniformity coefficient, and particle shape) on the erosion potential of coarse grains : very limited.
Common erodibility model for coarse grains :
Shields Parameter
Simple Sliding and Rolling Mechanism
Envelopes of Paphitis (2001): compiled existing data;
Lower limit –
Mean –
Upper limit -
As the coefficient of uniformity or particle angularity is increased, the coordination number, or number of interparticle contacts, increased, which results in a higher friction angle and soil stiffness.
Results and Discussion
Combined (Drag + Lift forces) Sliding and Rolling Mechanism
Comparison b/w Theory and Experiments / Regression Analysis
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Effect of Void Ratio
where, E = erosion rate; Kc = erodibility factor; n = empirical constant; τb = applied bottom stress; τc = critical shear stress.
Materials and Method
Soil type
D50 (mm) Gs Cu Cc
Roundness
ASTM 20/30
0.72
2.65
1.17
1.006
0.90
GS22 20/30
0.23
F-80 sand
0.184
1.67
0.938
0.75
F-110 sand
0.121
1.56
0.966
0.70
Coarse 1
2.64
1.44
0.864
0.42
Coarse 2
1.25
1.092
0.32
F80 mix
0.192
2.68
0.663
0.78
ASTM graded
0.365
2.22
1.233
0.80
Reasonable agreement between the theoretical prediction (combined rolling model) and the test results
However, characteristics of different materials cannot be fully captured via the theoretical model.
Regression analysis:
Comparison w/ Experimental Results
Decreasing void ratio will result in a higher coordination number and a higher rotational frustration.
(a) Two Sliding Models
(b) Two Rolling Models
Effect of Particle Size
where, τc is in Pa; D50(mean grain size) is in mm; Cu = uniformity coefficient; R = roundness. 
Acknowledgments
Bed-load transport by rolling is the more likely mechanism for the initiation of particle motion in fluid flow.
Partial funding for this investigation was provided by the Georgia Department of Transportation, and the authors are grateful for their support. The authors especially appreciate the thoughts and insights of Mr. Jon D. Griffith, P.G., P.E.
As D50 increased, individual grains had more weight and more resistance to buoyancy and shearing forces