Controlling constriction sizes of granular filters

Controlling constriction size D c * is a size characteristic of the void network in a granular filter and is equal to the diameter of the largest particle that can possibly be transported through the filter by seepage. It provides a quantitative measure of the size of the channels within a granular filter and it is an aid to understanding filtration by drawing attention to processes occurring within the void network. Results of analytical and experimental studies indicate that D c * is largely independent of the shape of the filter's grain size distribution curve and thickness and can be related to the size of particles in the fine fraction.To transport particles equal in size to D c * through a granular medium requires mild vibration and seepage velocities corresponding to values of hydrodynamic number R′ ≥ 10. Smaller velocities will only transport particles smaller than D c *. Filtration capability is thus dependent on both D c * of the filter and hydrodynamic conditions within the filter.It is shown that knowledge of D c * can be helpful when designing filters to protect cohesionless and cohesive soils. Key words: filter, constriction size, hydrodynamics, tests, design.