Soil Classification - Cheat Sheet
Your last-minute reference for grain-size parameters, Atterberg limits, USCS and AASHTO classification systems. Everything tested on the PRC Civil Engineer Licensure Exam, condensed.
Grain-Size Distribution & Gradation Parameters
Formulas
C_u = D_60 / D_10
Meaning
C_u = uniformity coefficient; D_60 = diameter at which 60% passes; D_10 = effective diameter (10% passes)
Watch out
C_u alone does NOT guarantee well-graded — ALSO need 1 ≤ C_c ≤ 3. High C_u with poor C_c = poorly graded.
When to use
Always calculate first to assess grain-size spread. Well-graded soil has C_u ≥ 4 (gravel) or ≥ 6 (sand)
C_c = (D_30)² / (D_10 × D_60)
Meaning
C_c = coefficient of curvature; D_30 = diameter at 30% pass; numerator is D_30 squared
Watch out
D_30 SQUARED in numerator — don't forget to square it. If C_c < 1 or > 3, soil is poorly graded even if C_u is high.
When to use
Second gradation check; must satisfy 1 ≤ C_c ≤ 3 for well-graded classification
D_10 (effective size or effective diameter)
Meaning
Diameter in mm at which 10% of soil particles are finer (90% retained). From gradation curve.
Watch out
Must be read from cumulative % finer curve, not % retained. If D_10 is very small, C_u becomes huge — verify sieve data.
When to use
Read directly from sieve analysis plot. Use as divisor in C_u and C_c formulas.
Common values
Value
75 mm
Symbol
D > 75
Quantity
Coarse gravel threshold
Value
4.75 mm (sieve #4)
Symbol
Sand boundary
Quantity
Fine gravel limit
Value
0.075 mm (sieve #200)
Symbol
Fines boundary
Quantity
Fine sand limit
Important facts
- D_60 / D_10 (uniformity) and (D_30)² / (D_10·D_60) (curvature) are the two gradation parameters always tested.
- Well-graded REQUIRES BOTH: C_u threshold AND 1 ≤ C_c ≤ 3. Either fails → poorly graded.
- Gravel well-graded threshold: C_u ≥ 4; Sand: C_u ≥ 6 (important distinction for board exams).
- Uniformity coefficient increases if largest particles are much bigger than smallest → indicates gap-graded or well-graded depending on C_c.
- Curvature < 1 or > 3 means soil is gap-graded (missing intermediate sizes).
- D_10, D_30, D_60 are READ from cumulative % finer curve, NOT calculated.
- Gradation affects permeability, compaction, settlement, and bearing capacity.
Key definitions
Grain-Size Distribution
Example
50% of sand particles pass the #200 sieve (0.075 mm); 10% pass 0.15 mm → D_10 = 0.15 mm
Cumulative % of soil particles finer than a given diameter; plotted on semi-log curve from sieve analysis.
Well-Graded Soil
Example
Sand with C_u = 10, C_c = 1.2 → well-graded (SW or SP depends on fines)
Coarse soil with good representation of all sizes from coarse to fine; satisfies C_u ≥ 4 (gravel) or ≥ 6 (sand) AND 1 ≤ C_c ≤ 3.
Poorly Graded Soil
Example
Sand with C_u = 3 (fails C_u ≥ 6) or C_u = 10, C_c = 0.8 (fails C_c) → poorly graded
Coarse soil lacking intermediate grain sizes; fails either C_u or C_c criterion.
Effective Size
Example
Fine sand D_10 = 0.1 mm vs coarse sand D_10 = 0.5 mm → fine sand more impermeable
D_10 diameter; used to characterize soil's drainage and permeability.
Diagrams to know
- Semi-log gradation curve with grain-size axis (log scale) and cumulative % finer axis (linear) — label D_10, D_30, D_60
- Coarse vs fine grain-size triangle — clay, silt, sand, gravel size ranges in mm
Atterberg Limits & Plasticity Index
Formulas
PI = LL - PL
Meaning
PI = plasticity index; LL = liquid limit; PL = plastic limit (all in %)
Watch out
For organic soils, PI computed from Atterberg limits is meaningless; PI should be very small. If computed PI is large for organic soil, something is wrong.
When to use
Always compute to classify fine-grained soils; high PI → high plasticity (clay); low PI → low plasticity (silt)
A-line: PI = 0.73(LL - 20)
Meaning
Slope = 0.73; intercept at LL = 20; separates clay (above) from silt (below) on plasticity chart
Watch out
Slope is 0.73, NOT 0.75 or 1.0 — common rounding error. At LL = 40, A-line PI = 14.6; at LL = 60, PI = 29.2.
When to use
Plot (LL, PI) point. If above A-line → clay (C). If below → silt (M). Classification depends on LL = 50 for L/H split too.
Liquid Limit (LL)
Meaning
Water content (%) at which soil transitions from plastic to liquid state; measured by Casagrande cup or cone penetration test.
Watch out
LL is a property measured in field/lab, not calculated. Do not confuse with PL or SL.
When to use
LL = 50 is the threshold: LL < 50 → Low plasticity (L); LL ≥ 50 → High plasticity (H)
Plastic Limit (PL)
Meaning
Water content (%) at which soil transitions from plastic to semi-solid; lower bound of plastic range.
Watch out
PL is always less than LL. If PL ≥ LL in data, lab error — reject and retest.
When to use
Used to compute PI = LL - PL. Soils with PL near LL have low PI (silts); large (LL - PL) = high PI (clays).
Shrinkage Limit (SL)
Meaning
Water content (%) at which soil stops shrinking with further drying; rarely tested but part of Atterberg suite.
Watch out
Don't confuse SL with PL or LL. If question says shrinkage, it's asking about SL — less common on PRC exam.
When to use
SL < PL < LL always. Mainly for reference; not used in USCS or AASHTO classification on exam.
Common values
Value
PI = 0
Symbol
Starting point of A-line
Quantity
A-line intercept (at LL = 20)
Value
0.73
Symbol
Exact slope for clay-silt separation
Quantity
A-line slope
Value
LL = 50%
Symbol
Divides L and H categories
Quantity
Low/High plasticity threshold
Important facts
- Atterberg limits apply ONLY to fine-grained soils (silt, clay, organic). Do NOT compute or report for sand/gravel.
- A-line (PI = 0.73(LL−20)) is the classification boundary: clay above, silt below.
- LL = 50 is the second boundary: L (low) if LL < 50; H (high) if LL ≥ 50.
- Plasticity index increases with clay content — higher PI = more clay, more swelling, more settlement.
- PI = 0 or very low (< 5) indicates silt or non-plastic soil; PI > 20 indicates clay.
- Organic soils (O) plot on or below A-line despite high PI due to decomposed matter; high water absorption.
- Shrinkage limit (SL) is the lowest water content where volume stops changing — less commonly tested than LL/PL.
Key definitions
Liquid Limit (LL)
Example
LL = 45% means soil becomes liquid-like at 45% moisture content; below 45% it is plastic or semi-solid
Water content at which soil flows like a liquid; threshold between plastic and liquid states.
Plastic Limit (PL)
Example
PL = 20% means below 20% moisture, soil cannot be rolled into a thread; above 20% it behaves plastically
Water content at which soil crumbles and loses plasticity; threshold between semi-solid and plastic states.
Plasticity Index (PI)
Example
LL = 50, PL = 20 → PI = 30 (high); LL = 35, PL = 30 → PI = 5 (low, silt-like)
Range of water content (LL - PL) over which soil exhibits plastic behavior; indicator of clay content and compressibility.
Low Plasticity Soil (L)
Example
CL (low-plasticity clay), ML (low-plasticity silt)
Fine-grained soil with LL < 50; typically less compressible, better for subgrades.
High Plasticity Soil (H)
Example
CH (high-plasticity clay), MH (high-plasticity silt)
Fine-grained soil with LL ≥ 50; higher compressibility, higher swelling potential.
Diagrams to know
- Plasticity chart with A-line: vertical axis PI, horizontal axis LL; regions for CL, CH, ML, MH, OL, OH
- Atterberg limit consistency states: liquid (w > LL) → plastic (PL < w < LL) → semi-solid (SL < w < PL) → solid (w < SL)
USCS (Unified Soil Classification System)
Common values
Value
50%
Symbol
Boundary between coarse and fine
Quantity
Coarse soil threshold (% passing #200)
Value
0–5%, 5–12%
Symbol
Affects third letter assignment
Quantity
Fines content: clear to marginal
Value
12%
Symbol
M or C dominates over W/P
Quantity
Fines content: significant
Important facts
- First letter: G = gravel (≥50% retained on #4), S = sand (≥50% passes #4), M = silt, C = clay, O = organic
- For coarse soil (G/S): second letter W/P based on gradation; third letter M/C based on fines plasticity OR no third if fines < 5%
- For fine soil (M/C/O): add L/H based on LL = 50 threshold. Examples: CL, CH, ML, MH, OL, OH
- Fines in coarse soil: if 5–12% → both W/P and M/C reported (e.g., SW–SM); if >12% → M/C dominates
- Organic soils (O) have LL typically 75+ and plot at or below A-line due to decomposed material.
- GW = best coarse soil for subgrade; CH = worst fine soil for subgrade (high compressibility, low bearing capacity).
- USCS does NOT use group index; AASHTO does (next section).
Key definitions
USCS Classification
Example
SW = well-graded sand; CH = high-plasticity clay; GM = silty gravel
Two-letter system based on grain size (coarse vs fine) and fines/plasticity; most widely used in geotechnical engineering.
Coarse-Grained Soil
Example
If 60% retained on #4 sieve (4.75 mm) → gravel-based; if 60% passes #4 but 40% retained on #200 → sand-based
≥ 50% of material retained on #200 sieve (> 0.075 mm); designated G (gravel) or S (sand)
Fine-Grained Soil
Example
CH = high-plasticity clay (plots above A-line, LL ≥ 50); ML = low-plasticity silt (plots below A-line, LL < 50)
≥ 50% passing #200 sieve (< 0.075 mm); designated M (silt), C (clay), or O (organic)
Well-Graded Coarse Soil (W)
Example
SW = well-graded sand with fines < 5%; SWM = well-graded sand with silt; SWC = well-graded sand with clay
Second letter for coarse soil; satisfies C_u and C_c criteria (SW for sand, GW for gravel)
Poorly Graded Coarse Soil (P)
Example
SP = poorly graded sand; GP = poorly graded gravel
Second letter for coarse soil; fails C_u or C_c criterion (SP for sand, GP for gravel)
Silty (M) vs Clayey (C) Fines
Example
SM = sand with silt fines (silt fines not plastic); SC = sand with clay fines (clay fines plastic)
Third letter for coarse soil depends on fines plasticity: M (silt) if fines plot below A-line; C (clay) if above A-line
Diagrams to know
- USCS flowchart: grain-size split (#200) → coarse (G/S) vs fine (M/C/O) → secondary splits (W/P for coarse; L/H for fine)
- Fines chart for coarse soil: plot fines % vs plasticity (PI, LL) to determine if M or C in third letter
AASHTO Classification & Group Index
Formulas
GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10)
Meaning
GI = group index; F = % passing #200; LL = liquid limit; PI = plasticity index. All percentages and indices as whole numbers, not decimals.
Watch out
Terms in brackets have caps: (F−35) capped at 40, (F−15) capped at 40, (LL−40) capped at 20, (PI−10) capped at 20. If value exceeds cap, use cap. Also note: if F < 15, first term is 0; if PI < 10, second term is 0.
When to use
Refines AASHTO rating A-1 to A-8. Higher GI → poorer subgrade. Always round to nearest integer, minimum 0.
Common values
Value
≤ 35%
Symbol
Upper limit for coarse classification
Quantity
% passing #200 for coarse-grained AASHTO (A-1 to A-3)
Value
35%
Symbol
Lower limit for fine classification
Quantity
% passing #200 for fine-grained AASHTO (A-4 to A-7)
Value
40%
Symbol
Threshold for silt/clay high vs low LL
Quantity
LL boundary for A-4 vs A-5 (and A-6 vs A-7)
Important facts
- AASHTO groups: A-1-a, A-1-b (coarse), A-2-4, A-2-5, A-2-6, A-2-7 (coarse with fines), A-3 (fine sand), A-4, A-5, A-6, A-7-5, A-7-6 (fine-grained)
- GI capping: (F−35) max 40, (F−15) max 40, (LL−40) max 20, (PI−10) max 20. If computed value exceeds, use cap.
- GI floor: round to nearest integer; if negative, report 0. Never report negative GI.
- For F < 15%, first term vanishes; for PI < 10, second term vanishes.
- A-1 to A-3 are coarse-grained sands/gravels, generally good subgrades. A-4 to A-7 are fine-grained clays/silts, generally poor.
- A-4 soil = low-PI silt (GI typically 0–8); A-5 = high-LL silt; A-6 = low-PI clay; A-7 = high-PI clay (worst for highways).
- GI determines subgrade rating in highway design (CBR, modulus corrections).
Key definitions
AASHTO Classification
Example
A-1-a = best (well-graded gravel/sand); A-7-6 = worst (high-PI clay); A-2 categories are intermediate
Seven-group system (A-1 to A-7 with A-2 split) used for highway subgrade evaluation; includes group index for refined rating.
Group Index (GI)
Example
A-2-4(3) means AASHTO A-2-4 with group index 3; A-7-6(15) = high-PI clay, very poor for subgrade
Numerical index (0–20+) that further refines AASHTO group; higher GI = poorer subgrade. Reported as whole number.
A-1 Soil (Best Subgrade)
Example
GW, GP, SW, SP with F < 15% and PI = 0 → A-1 classification
Well-graded gravel/sand with little or no fines; GI = 0. Excellent drainage and bearing capacity.
A-7 Soil (Worst Subgrade)
Example
CH with F = 80%, LL = 60, PI = 28 → A-7-6(high GI) — very problematic for highway base
High-PI clay (A-7-6 if PI > LL−30) or intermediate PI (A-7-5 if PI ≤ LL−30); GI often 15+. High swelling, poor drainage.
Diagrams to know
- AASHTO classification table: grain-size sieve %, LL, PI criteria vs group A-1 through A-7 assignment
- Group index scale: GI = 0 (best, A-1) to GI ≈ 20+ (worst, A-7-6)
Classification Decision Trees & Quick Identification
Important facts
- USCS START: Is soil coarse (>50% on #4) or fine (≥50% on #200)? This determines first letter.
- Coarse soil path: measure fines %; plot fines on chart (M vs C) using LL, PI; check C_u, C_c for W vs P.
- Fine soil path: plot (LL, PI) on plasticity chart; above A-line = C; below = M; then L/H by LL = 50.
- AASHTO START: same as USCS but use LL, PI, GI formula to refine group rating.
- Field visual: sand = gritty, coarse feel; silt = smooth, non-plastic; clay = sticky, plastic, high strength when dry.
- Do NOT skip the % fines check — fines can change W → SM/SC (W-series with fines).
Key definitions
Grain-Size Hierarchy
Example
Particle of 0.2 mm = sand; 0.05 mm = silt; 0.001 mm = clay
Boulders (>300 mm) → Cobbles (75–300 mm) → Gravel (4.75–75 mm) → Sand (0.075–4.75 mm) → Silt (0.002–0.075 mm) → Clay (<0.002 mm)
Field Identification of Coarse vs Fine
Example
If 60% of soil passes #200 sieve → fine-grained (M/C/O classification path)
Sieve #200 (0.075 mm): retain >50% on #200 = coarse; pass ≥50% through #200 = fine. Lab-verified by sieve analysis.
Diagrams to know
- Flowchart for USCS vs AASHTO classification decision path
- Visual grain-size scale: from clay (< 0.002 mm) to boulders (> 300 mm)
Common USCS & AASHTO Symbols Quick Reference
Important facts
- Two-letter codes are standard in USCS; letter 1 = primary constituent (G/S/M/C/O); letter 2 = gradation (W/P) or plasticity (L/H) or fines type (M/C)
- Third letter (if applicable) for coarse soil = M or C (fines type), e.g., SW–SM (sand, well-graded, with silt fines)
- Common exam mistakes: SW vs SM (gradation vs fines type), CH vs CL (LL threshold = 50), GM vs GC (silt vs clay in gravel)
- Avoid OL/OH unless explicitly mentioned in problem; most exams focus on M, C, G, S classifications.
- Borderline soils (e.g., fines right at 5% or 12%, or LL right at 50) should be classified per majority and noted on exam.
Key definitions
GW / GP
Example
Pit-run gravel with D_60/D_10 = 12, C_c = 1.8, <5% fines → GW
Gravel, well-graded / poorly graded. First choice for foundations and subgrades.
GM / GC
Example
Gravel with 10% clay passing #200, low PI → GC
Gravel with silt / clay fines. Second and third choices; fines reduce drainage.
SW / SP
Example
Beach sand with C_u = 8, C_c = 2.2, <5% fines → SW
Sand, well-graded / poorly graded. Good subgrade if clean (<5% fines).
SM / SC
Example
Sand with 8% silt fines (LL = 32, PI = 6, plots below A-line) → SM
Sand with silt / clay fines. More prone to settlement; SM drains better than SC.
ML / MH
Example
Silt with LL = 35, PI = 4 (below A-line, LL < 50) → ML
Silt, low / high plasticity. Non-plastic to moderately plastic; compressible but not as much as clay.
CL / CH
Example
Clay with LL = 65, PI = 32 (above A-line, LL ≥ 50) → CH
Clay, low / high plasticity. CL is better for foundations; CH has high swelling and settlement risk.
OL / OH
Example
Peat or highly decomposed material with high LL → OH (avoid for structural fill)
Organic soil, low / high plasticity. Dark color, odor, low bearing capacity. Avoid if possible.
Must remember
- BOTH C_u AND C_c must satisfy well-graded criteria (C_u ≥ 4 or 6, AND 1 ≤ C_c ≤ 3). High C_u alone is NOT well-graded if C_c fails.
- A-line equation: PI = 0.73(LL − 20), slope = 0.73 (NOT 0.75 or 1.0). Plots above A-line = clay (C); below = silt (M).
- LL = 50 is the threshold for L (low plasticity) vs H (high plasticity). LL < 50 → L; LL ≥ 50 → H.
- Group index formula: GI = (F−35)[0.2 + 0.005(LL−40)] + 0.01(F−15)(PI−10). Cap each term's parent: (F−35) max 40, (F−15) max 40, (LL−40) max 20, (PI−10) max 20. Round GI to nearest integer, minimum 0.
- USCS first letter: G/S coarse (>50% retained on #200 means coarse), M/C/O fine (≥50% passing #200). AASHTO uses same #200 split but different group logic.
- Fines in coarse soil: < 5% → no third letter (SW, GW); 5–12% → both W/P AND M/C (SW–SM); > 12% → M/C dominates (SM, SC, GM, GC).
- Plasticity index PI = LL − PL. High PI (> 20) indicates clay; low PI (< 5) indicates silt or non-plastic soil.
- D_10 (effective size) is READ from the gradation curve, NOT calculated. Always read % finer = 10%, then read diameter on x-axis.
- GW, GP (gravel), SW, SP (sand) = best coarse soils. CH (high-PI clay) = worst fine soil for subgrades (highest GI, lowest bearing capacity).
- PRC Exam Common Pitfall: Confusing PI = 0.73(LL−20) as the exact value EQUAL to A-line. A-line is the BOUNDARY; soils plot above/below it to determine C vs M.
Last minute tips
- If a problem gives D_10, D_30, D_60 directly, ALWAYS compute both C_u AND C_c before classifying as well-graded or poorly graded. Do NOT assume high C_u = well-graded.
- When reading a plasticity chart question: plot the (LL, PI) point first. If above A-line = C; if below = M. Then apply LL = 50 for L/H. This two-step check eliminates ~30% of board errors.
- For AASHTO GI formula, remember that if F < 15, the first term becomes negative or zero (cap at 0); if PI < 10, the second term becomes zero. Check these conditions to avoid sign errors.
- In fines classification for coarse soil (GM, GC, SM, SC), always determine if fines plot above or below A-line using their LL and PI. Plot the fines' Atterberg limits, NOT the whole soil's.
- If a soil is exactly at a boundary (e.g., LL = 50.0, or 50% passing #200), round DOWN or classify by majority—use engineering judgment and note it on your exam; examiners understand boundary ambiguity.
Comparison tables
Well-Graded (W) vs Poorly Graded (P) Coarse Soil
| Criterion | Well-Graded (W) | Poorly Graded (P) |
|---|---|---|
| Uniformity Coefficient C_u | ≥ 4 (gravel) or ≥ 6 (sand) | < 4 (gravel) or < 6 (sand) |
| Curvature Coefficient C_c | 1 ≤ C_c ≤ 3 | C_c < 1 or C_c > 3 |
| Grain-Size Spread | Good representation of all sizes coarse to fine | Gap-graded or uniform sizes; missing intermediate |
| Compaction Density | Higher maximum density (better packing) | Lower maximum density (poor packing) |
| Permeability | More predictable (depends on fines %) | More erratic; gap-grading reduces permeability |
| Subgrade Suitability | Better; SW/GW preferred for highways | Worse; SP/GP acceptable but not optimal |
Clay (C) vs Silt (M) — Plasticity Chart Separation
| Property | Clay (C) — Above A-Line | Silt (M) — Below A-Line |
|---|---|---|
| A-Line Equation (Plasticity Chart) | PI > 0.73(LL − 20); plots ABOVE A-line | PI ≤ 0.73(LL − 20); plots BELOW A-line |
| Plasticity Index (PI) | Generally high (> 10); expands/contracts with moisture | Generally low (< 10); minimal volume change |
| Appearance (Dry) | Hard, brittle chunks; sticky when wet | Crumbly, friable; smooth but non-plastic |
| Bearing Capacity | Lower; high compressibility, swelling (especially CH) | Higher than clay; less compressible |
| USCS Low-Plasticity | CL (low-plasticity clay, LL < 50) | ML (low-plasticity silt, LL < 50) |
| USCS High-Plasticity | CH (high-plasticity clay, LL ≥ 50) | MH (high-plasticity silt, LL ≥ 50) |
| Subgrade Use | Avoid if possible, especially CH (poor for highways) | Better than clay but still problematic if high LL |
USCS vs AASHTO Classification Overview
| Aspect | USCS (Unified Soil Classification) | AASHTO (American Association State Highway) |
|---|---|---|
| Primary Use | General geotechnical engineering (foundations, dams, cuts) | Highway subgrade evaluation and design |
| Classification Format | Two letters (e.g., SW, CH, ML); up to 3 with fines modifiers | Group A-1 to A-7 with optional group index GI |
| Grain-Size Cutoff for Fine | #200 sieve (0.075 mm): ≥ 50% passes = fine-grained | #200 sieve (0.075 mm): > 35% passes generally → fine group |
| Plasticity Chart | Central to classification (A-line separates C from M) | Used but not plotted; LL and PI applied to group criteria |
| Fines Effect | Affects second/third letter (M/C for clay vs silt type) | Used in GI formula to refine group quality |
| Best Soil | GW (well-graded gravel) | A-1-a (coarse, well-graded, minimal fines, GI = 0) |
| Worst Soil | OH or CH (organic or high-PI clay) | A-7-6 (high-PI clay, high GI ≈ 15–20) |
Low-Plasticity (L) vs High-Plasticity (H) Fine Soil — LL = 50 Threshold
| Property | Low-Plasticity L (LL < 50) | High-Plasticity H (LL ≥ 50) |
|---|---|---|
| Liquid Limit | LL < 50% moisture content | LL ≥ 50% moisture content |
| Compressibility | Lower; more suitable for construction | Higher; more settlement expected |
| Swelling Potential | Moderate; acceptable for some uses | High; problematic for rigid structures (pavements, buildings) |
| Bearing Capacity | Better than H; CL preferred over CH for foundations | Lower; CH and MH require treatment or avoidance |
| USCS Examples | CL (clay), ML (silt), OL (organic) | CH (clay), MH (silt), OH (organic) |
| AASHTO Examples | A-4, A-6 (low PI; GI typically < 8) | A-5, A-7-5, A-7-6 (high PI or high LL; GI often 8–20+) |
| Highway Subgrade | Acceptable if properly compacted; CL/ML better than CH/MH | Requires special design (geotextile, lime treatment, or avoidance) |
Diagrams
USCS Soil Classification Decision Tree
Step-by-step flowchart for classifying soil in USCS. Starts with #200 sieve split (coarse vs fine), then branches into grain-size (G/S) or plasticity (C/M/O) evaluation, and finally refines with gradation (W/P) or plasticity level (L/H) criteria.
Soil Classification Key Parameters & Relationships
Mind map showing the hierarchical relationships between grain-size parameters, Atterberg limits, classification systems, and the plasticity chart boundary conditions. Useful for understanding how each parameter guides classification decisions.
AASHTO Subgrade Quality Distribution by Group Index
Pie chart showing relative distribution of AASHTO soil groups by quality for highway subgrades. A-1 is excellent (near 0% of samples); A-2/A-3 are good; A-4–A-6 are fair; A-7 is poor. Illustrates why high-PI clays (A-7-6) are avoided in highway design.
Soil Consistency States vs Atterberg Limits
State diagram showing the progression of soil consistency from solid to liquid as water content increases. Atterberg limits (SL, PL, LL) mark the transitions. This is critical for understanding why plasticity index PI = LL − PL defines the plastic range.