What is a Sieve Analysis?
A sieve analysis involves stacking a series of wire mesh screens with progressively smaller openings. A dried, weighed soil sample is placed at the top, and a mechanical shaker vibrates the stack. Particles fall through the screens until they are retained by a mesh smaller than their diameter.
By weighing the soil retained on each sieve, technicians create a gradation curve showing the percentage of soil finer than each corresponding mesh size. This curve is the basis for the Unified Soil Classification System (USCS).
Understanding D10, D30, and D60
To mathematically classify the shape of the gradation curve, engineers extract three specific diameters from a semi-logarithmic plot of the data.
- D10 (Effective Size): The particle diameter where exactly 10% of the soil mass is finer.
- D30: The diameter where 30% of the soil is finer.
- D60: The diameter where 60% of the soil is finer.
How to Calculate Cu and Cc
Once D10, D30, and D60 are interpolated from the gradation curve, you use them to calculate the Coefficient of Uniformity (Cu) and the Coefficient of Curvature (Cc).
The Formulas: Cu = D60 / D10. Cc = (D30)² / (D10 x D60).
Worked Example: After plotting a sand sample, you determine D60 = 2.0 mm, D30 = 0.8 mm, and D10 = 0.2 mm. Cu calculation: 2.0 / 0.2 = 10. Cc calculation: (0.8)² / (0.2 x 2.0) = 0.64 / 0.4 = 1.6. The Cu is 10, and the Cc is 1.6.
Well-Graded vs. Poorly-Graded Soils
The calculated Cu and Cc values are checked against USCS thresholds to determine if a soil is 'well-graded' or 'poorly-graded'. Well-graded soils have a wide distribution of particle sizes, allowing small grains to fill the voids between large grains, resulting in excellent compaction.
For gravels to be considered well-graded (GW), Cu must be > 4 and Cc must be between 1 and 3. For sands to be well-graded (SW), Cu must be > 6 and Cc must be between 1 and 3. In our worked example, the sand has Cu=10 and Cc=1.6, satisfying both conditions, making it a well-graded sand (SW).
Frequently asked questions
What is the difference between gap-graded and poorly-graded?
Poorly-graded (or uniformly-graded) soil consists almost entirely of particles of the same size (like beach sand). Gap-graded soil has both large and small particles, but is missing an intermediate size range, showing a 'step' in its gradation curve.
Why is D10 called the 'Effective Size'?
D10 is critical because the hydraulic conductivity (how fast water flows through the soil) is largely governed by the size of the smallest 10% of the particles, which dictate the size of the void channels.
What do you do with soil finer than the No. 200 sieve?
Particles passing the No. 200 sieve (0.075 mm) are silts and clays. Mechanical shaking is ineffective for these. They must be separated via a wet-wash process, and their precise gradation is determined using a hydrometer analysis based on sedimentation velocity.
Can you calculate Cu and Cc if 15% of the soil passes the No. 200 sieve?
No, if more than 10% of the soil is finer than the No. 200 sieve, the curve never crosses the 10% passing line on the graph, meaning D10 cannot be determined. In these cases, Cu and Cc cannot be calculated.