Until the late 1930's, the accepted practice for the construction of drainage installation included the use of a large, approximately No. 2 size, aggregate as porous backfill. However, a lage number of drains ceased to function as result of clogging or "silting"*, and many highway departments practically eliminated sub-surface and intercepting drainage. The following report summarizes some of the more important studies that have dealt with "filter" (backfill material) to be used in drainage

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... truction. The scope of it is limited, dealing primarily with the questions of proper gradation. At present Kentucky Specifications (1) require that the gradations of the aggregate for backfill over pipe underdrains be as "specified for Class 'A' concrete", (page 462), that is No. 6 size coarse aggregate (No. 8 to 1-1/2"). For stone underdrains, it is required (page 408) that a No. 2 coarse aggregate be used (1-1/2" to 3").