Grain Size Uniformity Coefficient Calculator
Free Grain size uniformity coefficient Calculator for soil & sediment mechanics. Enter variables to compute results with formulas and detailed steps.
Formula
Cu = D60 / D10; Cc = (D30)^2 / (D10 * D60)
This Grain Size Uniformity Coefficient Calculator computes results from your provided inputs using the calculator's underlying model.
Frequently Asked Questions
What is the coefficient of uniformity (Cu)?
The coefficient of uniformity Cu is the ratio of D60 to D10, where D60 is the grain diameter at which 60% of the soil by weight is finer, and D10 is the diameter at which 10% is finer. Cu measures the range of grain sizes present in the soil. A Cu value close to 1 indicates a uniform soil with particles of similar size. For sands, Cu greater than or equal to 6 indicates well-graded soil, while for gravels the threshold is Cu greater than or equal to 4. Well-graded soils generally have better engineering properties because smaller particles fill voids between larger ones.
What is the coefficient of curvature (Cc)?
The coefficient of curvature Cc, also called the coefficient of gradation, is calculated as (D30)^2 / (D10 * D60). It measures the shape of the grain size distribution curve between D10 and D60. For a soil to be classified as well-graded, Cc must fall between 1 and 3 in addition to meeting the Cu criterion. A Cc outside this range indicates a gap-graded or skip-graded soil that is missing certain intermediate grain sizes, even if Cu is high. Both Cu and Cc must satisfy their criteria simultaneously for well-graded classification.
How is grain size analysis performed?
Grain size analysis uses two complementary methods. For particles larger than 0.075 mm (retained on No. 200 sieve), mechanical sieve analysis is performed by passing soil through a stack of sieves with progressively smaller openings and weighing the retained material on each sieve. For particles finer than 0.075 mm, hydrometer analysis uses Stokes law to determine particle sizes from their settling velocity in a water suspension. The combined results produce the grain size distribution curve, from which D10, D30, D50, and D60 values are read.
Why does grain size distribution matter in geotechnical engineering?
Grain size distribution affects virtually every engineering property of coarse-grained soils. Well-graded soils achieve higher compacted densities, have greater shear strength, lower permeability, and less settlement under load compared to uniformly graded soils. The D10 size (effective size) is used to estimate hydraulic conductivity through correlations like the Hazen formula. Grain size distribution is the primary basis for USCS soil classification and directly influences filter design criteria for earth dams, drainage systems, and erosion control.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
Is Grain Size Uniformity Coefficient Calculator free to use?
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