Initiation of movement of quartz particles

A theoretical description of the initiation of movement of sediments consisting of uniform-size, quartz particles is developed. These sediments behave in a noncohesive manner for coarse-grained particles but show cohesive behavior for fine-grained particles, i.e., as the particle size decreases, the critical shear stress increases and also becomes strongly dependent on the bulk density. The analysis includes gravitational, lift, drag, and cohesive forces as well as changes in bulk density and is uniformly valid for the range of particle sizes investigated, from fine-grained, cohesive particles to coarse-grained, noncohesive particles. Excellent agreement between theory and experiments is obtained. The analysis is also extended to quartz particles with small amounts of an added clay, bentonite, which makes the mixture more cohesive. This increase in cohesivity is greatest for intermediate size particles. An additional binding force due to the bentonite must then be included in the analysis.