Soil Gradation Coefficients Cu / Cc (USCS, ASTM D2487)
The coarse-fraction gradation coefficients of the USCS, and the curve SHAPE a fineness modulus cannot see.
Example
You enter
- D10, effective size (mm) 0.15
- D30 (mm) 0.55
- D60 (mm) 1.2
- Coarse fraction passing the #4 sieve (%, <= 50 = gravel) 60
- Fines passing the #200 sieve (%) 3
You get
- Uniformity coefficient Cu 8
- Coefficient of curvature Cc 1.68056
- Hazen k (cm/s) 0.0225
Details, formula, and sources
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.
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.
The Cu / Cc definitions, the USCS well-graded thresholds, and the Hazen relation are public and printed in every soil-mechanics text; ASTM D2487 itself is a purchased standard and is cited by name only.
Estimate. AHJ and licensed professional govern.
Field names used by the API: d10_mm, d30_mm, d60_mm, pct_coarse_passing_no4, pct_fines, cu, cc, hazen_k_cm_s
- Both criteria required well graded needs Cu >= 4 (gravel) or >= 6 (sand) AND 1 <= Cc <= 3; a wide range with a gap in the middle fails on Cc and packs no better than a uniform sandASTM D2487
- Fines decide whether gradation controls < 5% fines the gradation symbol governs; 5-12% takes a dual symbol; > 12% the fines govern and the Atterberg limits decide M vs CASTM D2487
- Hazen is flagged, not silent k = D10^2 cm/s is reported with an in-range / out-of-range flag (Cu < 5, D10 0.1-3 mm, clean) and is order-of-magnitude even when validHazen 1892
- Sizes must increase D10 <= D30 <= D60 comes off one grain-size curve; a decreasing set is a data-entry error and returns an errorscope of this tile