Index Properties and Soil Classification (USCS and AASHTO)

NCEES FE Civil spec: Geotechnical Engineering, A. Index properties and soil classifications. FE Reference Handbook: Civil Engineering chapter, Geotechnical section (Phase Relationships, AASHTO and Unified Soil Classification tables).

What the FE Civil Spec Covers

Geotechnical section A asks you to describe a soil from its index properties: grain-size distribution, Atterberg limits and moisture state, then assign a group symbol in the Unified Soil Classification System (USCS) or a group in the AASHTO system. Expect one or two quick problems that hinge on reading a gradation or computing a plasticity index correctly.

Key Concepts

Grain size

A sieve analysis gives percent passing versus particle size. In USCS, material retained on the No. 200 sieve (0.075 mm) is coarse; material passing it is fines. Within the coarse fraction, particles retained on the No. 4 sieve (4.75 mm) are gravel and particles passing it are sand. $D_{10}$, $D_{30}$ and $D_{60}$ are the sizes at which 10, 30 and 60 percent of the sample passes.

Atterberg limits

The liquid limit (LL) and plastic limit (PL) are water contents that bound the plastic range of a fine-grained soil. Their difference is the plasticity index (PI). The liquidity index tells you where the natural water content sits in that range.

Core Formulas

USCS decision steps

1. Coarse or fine? If 50 percent or more passes the No. 200 sieve, the soil is fine-grained (M, C, O). Otherwise it is coarse-grained (G or S).
2. Gravel or sand? Compare the coarse fraction retained on the No. 4 sieve. If more than half of the coarse fraction is retained, it is gravel (G); otherwise sand (S).
3. Fines content. Under 5 percent fines: clean, use gradation (W or P). Over 12 percent: use the fines (M or C). Between 5 and 12 percent: dual symbol such as SP-SM.
4. Well graded? Gravel is GW when $C_u\ge 4$ and $1\le C_c\le 3$. Sand is SW when $C_u\ge 6$ and $1\le C_c\le 3$. Failing either test means poorly graded (GP or SP).
5. Fine-grained soils. Plot LL and PI on the plasticity chart. Above the A-line is clay (C), below is silt (M). LL of 50 or more is high plasticity (H); under 50 is low plasticity (L). So CL, CH, ML or MH. For LL under 50, CL also needs PI above 7, ML is PI below 4 or below the A-line, and PI of 4 to 7 above the A-line is the dual symbol CL-ML.

The AASHTO system groups soils from A-1 (granular, excellent subgrade) to A-7 (clayey, poor subgrade) using percent passing the No. 10, No. 40 and No. 200 sieves plus LL and PI. Work left to right across the table and take the first group that fits.

Where It Is in the FE Reference Handbook

Search the Civil Engineering chapter, Geotechnical section. The Cu, Cc and PI definitions sit with the phase relationships, and the USCS and AASHTO classification tables follow later in the same section.

Worked Example

Part 1. A soil has 92 percent passing the No. 4 sieve and 3 percent passing the No. 200 sieve, with $D_{10}=0.15$ mm, $D_{30}=0.45$ mm and $D_{60}=1.05$ mm. Classify it.

Only 3 percent passes the No. 200 sieve, so it is coarse-grained. Gravel (retained on No. 4) is 8 percent of the sample and sand is 92 minus 3 = 89 percent, so sand dominates: S. Fines are under 5 percent, so classify by gradation.

Both criteria pass, so the soil is SW, well-graded sand.

Part 2. An inorganic soil with 70 percent passing the No. 200 sieve has LL = 42 and PL = 20.

PI = 22 plots above the A-line, so it is a clay, and LL = 42 is under 50, so low plasticity: CL, lean clay.

Common Mistakes

• Using 6 as the Cu limit for gravel. Gravel needs Cu of at least 4; sand needs at least 6.
• Checking Cu but forgetting Cc. Both must pass for a W symbol.
• Deciding gravel versus sand from the whole sample instead of the coarse fraction.
• Swapping the A-line axes or forgetting the minus 20. The line passes through PI = 0 at LL = 20.
• Calling a soil with exactly 50 percent fines coarse-grained. At 50 percent or more passing the No. 200 sieve it is fine-grained.

Practice Problems

Try these from the free PolyCalc FE Civil problem bank. Each one has a full worked solution.

• A soil has 65% passing the No. 200 sieve, LL = 45 and PL = 25. The soil is inorganic. Its USCS classification is most nearly: See the worked solution or practice the Soil Mechanics deck
• A clean sand has D10 = 0.20 mm, D30 = 0.50 mm and D60 = 1.20 mm. Its USCS classification is most nearly: See the worked solution or practice the Soil Mechanics deck
• A fine-grained soil has LL = 55 and PL = 25. Its plasticity index is: See the worked solution or practice the Soil Mechanics deck
• An inorganic fine-grained soil has LL = 60 and PI = 15. Its USCS classification is: See the worked solution or practice the Soil Mechanics deck
• A clean gravel has D10 = 5.0 mm, D30 = 9.5 mm and D60 = 15 mm. Its USCS classification is most nearly: See the worked solution or practice the Soil Mechanics deck

Related Calculators and Examples

• Phase relations lesson
• Laboratory and field tests lesson
• Soil mechanics practice deck
• FE Reference Handbook section guide

Frequently Asked Questions

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