Filters for Silts and Clays

An investigation was made of the filters needed in dams for fine‐grained clays and silts. The “critical” downstream filter in a central core dam should be capable of controlling and sealing a concentrated leak through the core, and should also be stable in conventional laboratory filter tests under a relatively high gradient, such as 1,000. Two different types of laboratory tests were developed to simulate the action of a critical filter (slot and slurry tests). Both gave identical and reproducible results. For fine‐grained clays a sand filter with $D_{15}$ of 0.5 mm is conservative. For sandy clays and silts the filter criterion $D_{15}/d_{85}⩽5$ is conservative and reasonable. The Atterberg limits of a clay have no significant influence on the needed critical filter. For nondispersive and dispersive clays having similar particle size distribution the needed critical filters are the same. For “noncritical” filters, such as filters upstream of a clay core, quantitative filter criteria are not necessary.