Scalp-and-replacement of oversize particles on internal stability erosion assessment

Internal stability (IS) erosion tests were conducted on broadly graded materials to better understand the effect of the scalped-and-replaced method of removing oversize particles on IS erosion testing. Four gradations (three internally unstable and one internally stable), and their scalped-and-scalped gradations were evaluated under upward flow in three large rigid-wall permeameters (300, 500, and 1000 mm in diameter). The limited test results suggest that scalp-and-replacement of oversize particles coarser than 4 × d₃₀ has no significant effect on IS assessment of the original gradation. The permeameter-to-particle diameter ratio (D/d_max) in the tests varied from 5 to 13. The limited test results suggest D/d_max ≥ 6·5 is acceptable for IS erosion tests in a rigid-wall permeameter if proper measures are taken to minimise preferential erosion along the permeameter wall.