On the Outflow Boundary Opening Size for Internal Erosion Testing of Widely Graded Sand and Gravel

Abstract

Abstract Permeameter testing of a widely graded soil to evaluate its potential for internal instability requires the opening size of the outflow boundary to allow unstable finer fraction particles to pass while retaining the stable coarser fraction. Results from a parametric study, on three widely graded soils and five outflow boundaries, yielded paired-data for opening size Om and d85,e of eroded soil mass passing through the outflow boundary. Gradation shape analysis established d85′ of the finer fraction of each soil used in testing. An opening size ratio 5 ≤ Om/d85′ ≤ 10 correlates with a grain size ratio 1 ≤ d85,e/d85′ ≤ 2 and thus an eroded soil mass believed representative of the finer fraction of the widely graded soil. The novel Om/d85′ criterion for sizing the outflow boundary in permeameter testing is found consistent with an alternate criterion using concepts of base-filter incompatibility.