CELE Geotechnical Engineering — Soil ClassificationCheat Sheet
A printable cheat sheet for Soil Classification, built for CELE reviewers who want one go-to reference in the final stretch. Covers formulas, key definitions, common question types, and the Professional Regulation Commission (PRC) — Board of Civil Engineering-specific twists you will see on CELE day.
Exam context
On the CELE 2026, the Geotechnical Engineering subtest carries a "Core" weight in Professional Regulation Commission (PRC) — Board of Civil Engineering's pattern. Soil Classification lands at position 2nd out of 11 in the standard review order. Target score is 70% weighted average, no sub-test below 50%, and roughly a meaningful share of items come from Geotechnical Engineering on a typical CELE paper.
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.
Sections
Formulas
Formula
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)
Formula
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
Formula
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
Section Title
Grain-Size Distribution & Gradation Parameters
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
Term
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
Definition
Cumulative % of soil particles finer than a given diameter; plotted on semi-log curve from sieve analysis.
Term
Well-Graded Soil
Example
Sand with C_u = 10, C_c = 1.2 → well-graded (SW or SP depends on fines)
Definition
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.
Term
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
Definition
Coarse soil lacking intermediate grain sizes; fails either C_u or C_c criterion.
Term
Effective Size
Example
Fine sand D_10 = 0.1 mm vs coarse sand D_10 = 0.5 mm → fine sand more impermeable
Definition
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
Formulas
Formula
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)
Formula
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.
Formula
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)
Formula
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).
Formula
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
Section Title
Atterberg Limits & Plasticity Index
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
Term
Liquid Limit (LL)
Example
LL = 45% means soil becomes liquid-like at 45% moisture content; below 45% it is plastic or semi-solid
Definition
Water content at which soil flows like a liquid; threshold between plastic and liquid states.
Term
Plastic Limit (PL)
Example
PL = 20% means below 20% moisture, soil cannot be rolled into a thread; above 20% it behaves plastically
Definition
Water content at which soil crumbles and loses plasticity; threshold between semi-solid and plastic states.
Term
Plasticity Index (PI)
Example
LL = 50, PL = 20 → PI = 30 (high); LL = 35, PL = 30 → PI = 5 (low, silt-like)
Definition
Range of water content (LL - PL) over which soil exhibits plastic behavior; indicator of clay content and compressibility.
Term
Low Plasticity Soil (L)
Example
CL (low-plasticity clay), ML (low-plasticity silt)
Definition
Fine-grained soil with LL < 50; typically less compressible, better for subgrades.
Term
High Plasticity Soil (H)
Example
CH (high-plasticity clay), MH (high-plasticity silt)
Definition
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)
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
Section Title
USCS (Unified Soil Classification System)
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
Term
USCS Classification
Example
SW = well-graded sand; CH = high-plasticity clay; GM = silty gravel
Definition
Two-letter system based on grain size (coarse vs fine) and fines/plasticity; most widely used in geotechnical engineering.
Term
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
Definition
≥ 50% of material retained on #200 sieve (> 0.075 mm); designated G (gravel) or S (sand)
Term
Fine-Grained Soil
Example
CH = high-plasticity clay (plots above A-line, LL ≥ 50); ML = low-plasticity silt (plots below A-line, LL < 50)
Definition
≥ 50% passing #200 sieve (< 0.075 mm); designated M (silt), C (clay), or O (organic)
Term
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
Definition
Second letter for coarse soil; satisfies C_u and C_c criteria (SW for sand, GW for gravel)
Term
Poorly Graded Coarse Soil (P)
Example
SP = poorly graded sand; GP = poorly graded gravel
Definition
Second letter for coarse soil; fails C_u or C_c criterion (SP for sand, GP for gravel)
Term
Silty (M) vs Clayey (C) Fines
Example
SM = sand with silt fines (silt fines not plastic); SC = sand with clay fines (clay fines plastic)
Definition
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
Formulas
Formula
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)
Section Title
AASHTO Classification & Group Index
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
Term
AASHTO Classification
Example
A-1-a = best (well-graded gravel/sand); A-7-6 = worst (high-PI clay); A-2 categories are intermediate
Definition
Seven-group system (A-1 to A-7 with A-2 split) used for highway subgrade evaluation; includes group index for refined rating.
Term
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
Definition
Numerical index (0–20+) that further refines AASHTO group; higher GI = poorer subgrade. Reported as whole number.
Term
A-1 Soil (Best Subgrade)
Example
GW, GP, SW, SP with F < 15% and PI = 0 → A-1 classification
Definition
Well-graded gravel/sand with little or no fines; GI = 0. Excellent drainage and bearing capacity.
Term
A-7 Soil (Worst Subgrade)
Example
CH with F = 80%, LL = 60, PI = 28 → A-7-6(high GI) — very problematic for highway base
Definition
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)
Section Title
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
Term
Grain-Size Hierarchy
Example
Particle of 0.2 mm = sand; 0.05 mm = silt; 0.001 mm = clay
Definition
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)
Term
Field Identification of Coarse vs Fine
Example
If 60% of soil passes #200 sieve → fine-grained (M/C/O classification path)
Definition
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)
Section Title
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
Term
GW / GP
Example
Pit-run gravel with D_60/D_10 = 12, C_c = 1.8, <5% fines → GW
Definition
Gravel, well-graded / poorly graded. First choice for foundations and subgrades.
Term
GM / GC
Example
Gravel with 10% clay passing #200, low PI → GC
Definition
Gravel with silt / clay fines. Second and third choices; fines reduce drainage.
Term
SW / SP
Example
Beach sand with C_u = 8, C_c = 2.2, <5% fines → SW
Definition
Sand, well-graded / poorly graded. Good subgrade if clean (<5% fines).
Term
SM / SC
Example
Sand with 8% silt fines (LL = 32, PI = 6, plots below A-line) → SM
Definition
Sand with silt / clay fines. More prone to settlement; SM drains better than SC.
Term
ML / MH
Example
Silt with LL = 35, PI = 4 (below A-line, LL < 50) → ML
Definition
Silt, low / high plasticity. Non-plastic to moderately plastic; compressible but not as much as clay.
Term
CL / CH
Example
Clay with LL = 65, PI = 32 (above A-line, LL ≥ 50) → CH
Definition
Clay, low / high plasticity. CL is better for foundations; CH has high swelling and settlement risk.
Term
OL / OH
Example
Peat or highly decomposed material with high LL → OH (avoid for structural fill)
Definition
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
Rows
Values
- ≥ 4 (gravel) or ≥ 6 (sand)
- < 4 (gravel) or < 6 (sand)
Property
Uniformity Coefficient C_u
Values
- 1 ≤ C_c ≤ 3
- C_c < 1 or C_c > 3
Property
Curvature Coefficient C_c
Values
- Good representation of all sizes coarse to fine
- Gap-graded or uniform sizes; missing intermediate
Property
Grain-Size Spread
Values
- Higher maximum density (better packing)
- Lower maximum density (poor packing)
Property
Compaction Density
Values
- More predictable (depends on fines %)
- More erratic; gap-grading reduces permeability
Property
Permeability
Values
- Better; SW/GW preferred for highways
- Worse; SP/GP acceptable but not optimal
Property
Subgrade Suitability
Columns
- Criterion
- Well-Graded (W)
- Poorly Graded (P)
Table Title
Well-Graded (W) vs Poorly Graded (P) Coarse Soil
Rows
Values
- PI > 0.73(LL − 20); plots ABOVE A-line
- PI ≤ 0.73(LL − 20); plots BELOW A-line
Property
A-Line Equation (Plasticity Chart)
Values
- Generally high (> 10); expands/contracts with moisture
- Generally low (< 10); minimal volume change
Property
Plasticity Index (PI)
Values
- Hard, brittle chunks; sticky when wet
- Crumbly, friable; smooth but non-plastic
Property
Appearance (Dry)
Values
- Lower; high compressibility, swelling (especially CH)
- Higher than clay; less compressible
Property
Bearing Capacity
Values
- CL (low-plasticity clay, LL < 50)
- ML (low-plasticity silt, LL < 50)
Property
USCS Low-Plasticity
Values
- CH (high-plasticity clay, LL ≥ 50)
- MH (high-plasticity silt, LL ≥ 50)
Property
USCS High-Plasticity
Values
- Avoid if possible, especially CH (poor for highways)
- Better than clay but still problematic if high LL
Property
Subgrade Use
Columns
- Property
- Clay (C) — Above A-Line
- Silt (M) — Below A-Line
Table Title
Clay (C) vs Silt (M) — Plasticity Chart Separation
Rows
Values
- General geotechnical engineering (foundations, dams, cuts)
- Highway subgrade evaluation and design
Property
Primary Use
Values
- Two letters (e.g., SW, CH, ML); up to 3 with fines modifiers
- Group A-1 to A-7 with optional group index GI
Property
Classification Format
Values
- #200 sieve (0.075 mm): ≥ 50% passes = fine-grained
- #200 sieve (0.075 mm): > 35% passes generally → fine group
Property
Grain-Size Cutoff for Fine
Values
- Central to classification (A-line separates C from M)
- Used but not plotted; LL and PI applied to group criteria
Property
Plasticity Chart
Values
- Affects second/third letter (M/C for clay vs silt type)
- Used in GI formula to refine group quality
Property
Fines Effect
Values
- GW (well-graded gravel)
- A-1-a (coarse, well-graded, minimal fines, GI = 0)
Property
Best Soil
Values
- OH or CH (organic or high-PI clay)
- A-7-6 (high-PI clay, high GI ≈ 15–20)
Property
Worst Soil
Columns
- Aspect
- USCS (Unified Soil Classification)
- AASHTO (American Association State Highway)
Table Title
USCS vs AASHTO Classification Overview
Rows
Values
- LL < 50% moisture content
- LL ≥ 50% moisture content
Property
Liquid Limit
Values
- Lower; more suitable for construction
- Higher; more settlement expected
Property
Compressibility
Values
- Moderate; acceptable for some uses
- High; problematic for rigid structures (pavements, buildings)
Property
Swelling Potential
Values
- Better than H; CL preferred over CH for foundations
- Lower; CH and MH require treatment or avoidance
Property
Bearing Capacity
Values
- CL (clay), ML (silt), OL (organic)
- CH (clay), MH (silt), OH (organic)
Property
USCS Examples
Values
- 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+)
Property
AASHTO Examples
Values
- Acceptable if properly compacted; CL/ML better than CH/MH
- Requires special design (geotextile, lime treatment, or avoidance)
Property
Highway Subgrade
Columns
- Property
- Low-Plasticity L (LL < 50)
- High-Plasticity H (LL ≥ 50)
Table Title
Low-Plasticity (L) vs High-Plasticity (H) Fine Soil — LL = 50 Threshold
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