Soil Classification and Identification Tests

TL;DR

You'll learn how to identify soil types using grain size and plasticity, then classify them with the USCS system. You'll know how sieve analysis, hydrometer tests, and Atterberg limits feed into a soil's final classification symbol. You'll be able to read a report and say "this is a CL" or "this is an SW" with confidence.

1. The Mental Model

Every soil has two personalities: how big its particles are, and how it behaves when wet. Grain size tells you if it's gravel, sand, silt, or clay. Plasticity tests tell you whether fine particles act "sticky" or "crumbly" under moisture. Classification is just a decision tree combining these two pieces of information into one short code engineers trust instantly. If you know the grain size curve and the Atterberg limits, you already know the soil's name.

2. The Core Material

2.1 Grain Size Analysis — Sieve and Hydrometer

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Soil particles are sorted by size. Coarse particles (gravel and sand) are measured with sieves; fine particles (silt and clay) are too small for sieves, so we use a hydrometer based on sedimentation speed.

Sieve Analysis (for coarse-grained soil):
You stack sieves from largest opening on top to smallest on bottom (e.g., 75 mm, 4.75 mm, 2.00 mm, 0.425 mm, 0.075 mm), shake the dried sample through them, and weigh what's retained on each.

Percent passing a given sieve:

$$\%\text{Passing} = \frac{W_{total} - W_{cumulative\ retained}}{W_{total}} \times 100$$

From this you plot the grain size distribution (GSD) curve — sieve size (log scale) on the x-axis, percent passing on the y-axis. Two numbers matter a lot:

• $D_{10}$ — diameter at which 10% of particles are finer (the "effective size")
• $D_{30}$, $D_{60}$ — diameters at 30% and 60% passing

From these you compute:

$$C_u = \frac{D_{60}}{D_{10}} \quad \text{(Coefficient of Uniformity)}$$

$$C_c = \frac{(D_{30})^2}{D_{10} \times D_{60}} \quad \text{(Coefficient of Curvature)}$$

• $C_u$ large → well-graded (wide range of particle sizes)
• $C_u$ close to 1 → poorly/uniformly graded (particles all similar size)
• $C_c$ between 1 and 3, combined with high $C_u$ → confirms "well-graded"

Hydrometer Analysis (for fine-grained soil, passing No. 200 / 0.075 mm sieve):
Soil is mixed in water with a dispersing agent, and a hydrometer measures the specific gravity of the suspension over time. Bigger particles sink faster (Stokes' Law), so the reading over time tells you the % finer than a given diameter. This is how you separate silt from clay.

2.2 Atterberg Limits — The Plasticity Story

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These tests tell you how a fine-grained soil behaves across moisture levels. There are three key states separated by two limits:

• Liquid Limit (LL): moisture content at which soil transitions from plastic to liquid state. Measured with the Casagrande cup — you cut a groove in wet soil, count the number of blows (taps) needed to close it by 12.7 mm (½ inch), and find the moisture content that corresponds to 25 blows (using a flow curve, since multiple trials at different moisture contents are plotted).
• Plastic Limit (PL): moisture content at which soil starts to crumble when rolled into a 3 mm thread. Simple rolling test by hand.
• Shrinkage Limit (SL): moisture content below which further drying causes no more volume change. Less commonly run in routine labs but important for expansive clays.

From LL and PL you get the single most important classification number:

$$PI = LL - PL \quad \text{(Plasticity Index)}$$

PI tells you the range of moisture over which the soil stays plastic (workable but not falling apart, not liquid).

• PI = 0 → non-plastic (silt-like, crumbles immediately)
• PI low (under ~7) → low plasticity
• PI high (over ~17) → highly plastic, often problematic clay (shrink-swell issues)

2.3 USCS Classification — Putting It Together

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The Unified Soil Classification System (USCS) uses a two-letter symbol. First letter = primary soil type, second letter = modifier.

Step 1 — Coarse or fine?
Check % passing No. 200 sieve (0.075 mm):
- Less than 50% passing → coarse-grained (gravel or sand dominant)
- 50% or more passing → fine-grained (silt or clay dominant)

Step 2a — If coarse-grained:
Check if gravel (G) or sand (S) dominates (compare % retained on No. 4 sieve vs No. 200). Then check fines content:
- Less than 5% fines → well-graded (W) or poorly-graded (P) based on $C_u$ and $C_c$ → e.g. SW, SP, GW, GP
- More than 12% fines → the fines control behavior → classify fines as silty (M) or clayey (C) → e.g. SM, SC, GM, GC
- 5–12% fines → dual symbol (e.g. SW-SM)

Step 2b — If fine-grained:
Plot LL and PI on the Plasticity Chart against the A-line:

$$PI = 0.73(LL - 20)$$

• Points above the A-line → clay (C)
• Points below the A-line → silt (M)
• LL < 50 → low plasticity, add "L" → CL, ML
• LL ≥ 50 → high plasticity, add "H" → CH, MH

Organic soils (OL, OH) are identified by odor, dark color, and a drop in LL after oven-drying.

flowchart TD
    A["Soil sample"] --> B{"% passing No. 200 sieve?"}
    B -->|"< 50% = coarse"| C{"Gravel or sand dominant?"}
    B -->|"≥ 50% = fine"| D["Run Atterberg limits (LL, PL, PI)"]
    C --> E{"Fines content %"}
    E -->|"< 5%"| F["Check Cu, Cc → GW/GP/SW/SP"]
    E -->|"5-12%"| G["Dual symbol e.g. SW-SM"]
    E -->|"> 12%"| H["Fines control → GM/GC/SM/SC"]
    D --> I{"Plot on Plasticity Chart vs A-line"}
    I -->|"Above A-line"| J["Clay: CL or CH"]
    I -->|"Below A-line"| K["Silt: ML or MH"]

3. Worked Example

A geotechnical lab runs tests on a soil sample for a highway subgrade project.

Sieve data:
- % passing No. 4 sieve = 95%
- % passing No. 200 sieve = 62%

Since 62% passes No. 200, this is a fine-grained soil (more than 50% fines control behavior). We skip straight to Atterberg limits.

Atterberg limit results:
- Liquid Limit (LL) = 38
- Plastic Limit (PL) = 19

Calculate PI:
$$PI = LL - PL = 38 - 19 = 19$$

Check A-line value at LL = 38:
$$PI_{A-line} = 0.73(38 - 20) = 0.73 \times 18 = 13.1$$

Compare: actual PI (19) is greater than A-line PI (13.1), so the point plots above the A-line → it's a clay.

Since LL = 38 is less than 50, it's low plasticity.

Classification: CL (low-plasticity clay).

Engineering meaning: a CL soil is workable, moderately compressible, and commonly used/accepted as subgrade or fill material if compacted properly — but it's moisture-sensitive, so the lab will likely recommend checking Proctor compaction and CBR (California Bearing Ratio) next before final approval for pavement design.

4. Key Takeaways

4.1 Most Important Concepts

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1. Grain size separates coarse from fine — the No. 200 sieve (0.075 mm) is the single biggest decision point in USCS.
2. $C_u$ and $C_c$ describe gradation shape — they tell you if a coarse soil is well-graded (good for compaction/drainage) or poorly-graded.
3. PI = LL − PL is the plasticity fingerprint of fine soils — small range means the soil is touchy to work with, large range means it's forgiving but can be a high-shrink-swell clay.
4. The A-line is the silt/clay dividing line — above it is clay behavior, below it is silt behavior, regardless of what the soil "looks like" visually.
5. USCS symbols are two letters for a reason — first letter is the dominant material, second letter is the behavioral modifier (gradation or plasticity).
6. Dual symbols exist for borderline soils — 5–12% fines content doesn't fit cleanly into "clean" or "dirty" categories.
7. Classification predicts engineering behavior, not just naming — a CH soil screams "expect shrink-swell problems," an SW soil screams "good drainage, good compaction."

4.2 Common Misconceptions

• "Clay just means fine particles." Not true — clay is defined by plasticity behavior (PI, A-line), not only particle size. A fine silt can have near-zero plasticity.
• "Higher LL always means worse soil." Not necessarily — it's the combination with PI and the A-line position that determines behavior, not LL alone.
• "Well-graded is always better." Well-graded is usually better for compaction and bearing capacity, but poorly-graded uniform sand can be ideal for drainage layers or filters.
• "USCS and AASHTO classification are the same thing." They're different systems with different letter codes and purposes — USCS (used here) is for general geotechnical engineering, AASHTO is specifically for pavement subgrade suitability.

4.3 Compare & Contrast

5. Now Try It

Exam-style question: A soil sample has 70% passing the No. 200 sieve. Lab tests give LL = 55 and PL = 22. Calculate the Plasticity Index, determine the A-line value at this LL, decide whether the point falls above or below the A-line, and state the final USCS classification symbol. Show every calculation step.

A top-band answer must include:
1. $PI = LL - PL = 55 - 22 = 33$
2. A-line equation applied: $PI_{A-line} = 0.73(55-20) = 25.55$
3. Comparison: actual PI (33) > A-line PI (25.55) → clay
4. LL = 55 ≥ 50 → high plasticity
5. Final symbol: CH, with one sentence noting this soil likely has high shrink-swell potential and needs special treatment as subgrade.

Success looks like: you arrive at CH with every number justified, not guessed.