Basic Properties of Sand and Gravel Filters

Abstract

Laboratory experiments have shown that uniform filters will catch particles with diameter of about $0.11D_{15}$ when the particles are carried in suspension in seeping water. Smaller particles will be carried through the filter pores and larger particles will not enter. The main current filter criterion, $D_{15}/d_{85}⩽5\text{,}$ is shown to be conservative, but not excessively. Its use should be continued. Filter criteria using the ratios $D_{50}/d_{50}$ and $D_{15}/d_{15}$ are not supported by experiments or theory. Filters of angular particles of crushed rock are as satisfactory as those of rounded alluvial particles. It is not necessary for the filter particle size distribution to have a shape generally similar to that of the base particle size distribution.