Soil Gradation Coefficients Cu / Cc (USCS, ASTM D2487)
The coarse-fraction half of USCS that atterberg-indices leaves out, and the curve SHAPE a fineness modulus cannot see. From three points on the grain-size curve: the uniformity coefficient Cu = D60/D10 (how wide a range of sizes is present) and the coefficient of curvature Cc = D30^2 / (D10 x D60) (whether the curve is smooth or gapped). Well graded needs BOTH -- Cu >= 4 for a gravel or >= 6 for a sand, and Cc between 1 and 3 -- because a wide range with a gap packs no better than a uniform sand. D10 0.15, D30 0.55, D60 1.2 mm gives Cu 8.0 and Cc 1.68: a well-graded sand (SW) that will compact. The fines content decides whether gradation controls at all: under 5% it governs, 5-12% takes a dual symbol, over 12% the fines govern instead. The Hazen k = D10^2 cm/s permeability estimate is reported only with its validity flag. Three curve points, not the full sieve analysis; the lab gradation report and the geotechnical engineer govern.
Formula and source
Cu = D60 / D10; Cc = D30^2 / (D10 x D60); well graded requires Cu >= 4 (gravel) or Cu >= 6 (sand) AND 1 <= Cc <= 3; Hazen k (cm/s) = D10^2 (D10 in mm).
ASTM D2487 (Unified Soil Classification System) gradation criteria and ASTM D6913 (sieve analysis) by name, with the Hazen (1892) permeability estimate; no ASTM table text reproduced.
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