Design of Filters for Clay Cores of Dams

The clay core of the Balderhead Dam Suffered cracking and internal erosion in 1967. Segregation of the eroded debris was observed and the filter failed to retain the finer particles that reached it. This observation has lead to the development of a method of filter design for clays in flocculating conditions, based on the retention of clay flocs, which are the smallest particles which can arise during erosion. Such a filter will operate irrespective of cracking and the amount of segregation of the eroded debris. Factors controlling floc size are described, and an experimental relationship between permeability of a filter and the size of particle which will pass through it is presented. It is suggested that the retention characterists of a filter can be described more generally by its permeability than by its grading.