Filter Design Criteria for Graded Cohesionless Bases

An attempt has been made to develop filter design criteria that can be adopted for nonuniform particle sizes of different gradation for both cohesionless bases and filter material. It is a deviation from the existing empirical approaches of filter design, which mostly rely upon the ratios of certain particle sizes of the filter and base materials. This concept explains the behavior of filters more clearly and the practical validity of this concept has been amply demonstrated by a number of tests on various base-filter combinations. The results of the laboratory investigations were found comparable with the theoretical predictions. Based on the logical configuration of the filter mass and experimental observations, a procedure for the design of filter has been developed. For this purpose an empirical equation has been established for estimating the design controlling size d* on the basis of the base gradation. This d* is a guiding size for design of the filter mass and depends on the packing pattern of nonuniform particle sizes. Results of this investigation have been found to be consistent with the results of some earlier investigations.