CELE Geotechnical Engineering — Soil ClassificationRevision Notes
Condensed revision notes for Soil Classification, built for the final weeks before the CELE 2026. These are the distilled key points you need when there is no time left for full study notes — just the concepts, formulas, and traps Professional Regulation Commission (PRC) — Board of Civil Engineering tests.
Exam context
The Civil Engineer Licensure Examination is conducted by Professional Regulation Commission (PRC) — Board of Civil Engineering and is scheduled for May and November 2026. The Geotechnical Engineering subtest is marked as "Core" in the official pattern, and Soil Classification appears in position 2nd of 11 in the CELE Geotechnical Engineering review rotation. Passing mark: 70% weighted average, no sub-test below 50%. Recent CELE 2026 papers have drawn roughly a meaningful share of questions from this subject.
Soil Classification - Revision Notes
Soil classification is one of the most consistently tested topics in the PRC Civil Engineer Licensure Examination under Geotechnical Engineering. It provides a standardized language for describing soil behavior, enabling engineers to anticipate how a soil will perform as a foundation material, fill, or subgrade. Two systems dominate the board exam: the Unified Soil Classification System (USCS) and the AASHTO system (used for highway subgrades). Mastery of grain-size parameters (Cᵤ and Cc), Atterberg limits (LL, PL, PI), the plasticity chart A-line, and the AASHTO Group Index formula is non-negotiable for exam success. This chapter synthesizes all classification criteria into a structured, formula-driven review.
Sections
Formulas
Example
Given D₁₀ = 0.15 mm, D₆₀ = 1.5 mm → Cᵤ = 1.5 / 0.15 = 10. Since 10 ≥ 6, the Cᵤ criterion for well-graded sand is satisfied.
Formula
Cᵤ = D₆₀ / D₁₀
Variables
D₆₀ = diameter at 60% passing (mm); D₁₀ = diameter at 10% passing (mm)
Application
Measures the range of particle sizes in a soil. Used as the first criterion for well-graded classification in USCS.
Example
D₁₀ = 0.15, D₃₀ = 0.45, D₆₀ = 1.5 mm → Cc = (0.45)² / (0.15 × 1.5) = 0.2025 / 0.225 = 0.90. Since Cc = 0.90 < 1, the Cc criterion FAILS → soil is poorly graded (SP) despite high Cᵤ.
Formula
Cc = (D₃₀)² / (D₁₀ × D₆₀)
Variables
D₃₀ = diameter at 30% passing (mm); D₁₀ = diameter at 10% passing (mm); D₆₀ = diameter at 60% passing (mm)
Application
Describes the shape of the gradation curve. Must satisfy 1 ≤ Cc ≤ 3 for well-graded classification. Values outside this range indicate gap-graded or poorly graded conditions.
Exam Tips
- On the board exam, gradation data is usually given as a table or described verbally. Extract D₁₀, D₃₀, D₆₀ first, then compute Cᵤ and Cc sequentially.
- If only Cᵤ is given and Cc is unknown, you CANNOT classify as well-graded — report as data insufficient or assume SP/GP.
- Memorize: Gravel threshold Cᵤ = 4; Sand threshold Cᵤ = 6. The Cc range 1–3 is the SAME for both.
- The No. 4 sieve (4.75 mm) separates gravel from sand within coarse-grained soils.
Key Points
- The particle-size distribution (gradation) curve plots percent passing (y-axis) vs. particle diameter in mm (x-axis, log scale).
- Three characteristic diameters are read directly from the curve: D₁₀ (10% passing), D₃₀ (30% passing), D₆₀ (60% passing).
- D₁₀ is called the effective size — it governs permeability and drainage behavior.
- Coefficient of Uniformity (Cᵤ) measures the spread of particle sizes; a large Cᵤ means a wide range of sizes.
- Coefficient of Curvature (Cc), also called the coefficient of gradation, describes the shape of the gradation curve.
- Well-graded classification requires BOTH Cᵤ AND Cc criteria to be satisfied simultaneously — meeting only one is insufficient.
- For GRAVEL: well-graded if Cᵤ ≥ 4 AND 1 ≤ Cc ≤ 3.
- For SAND: well-graded if Cᵤ ≥ 6 AND 1 ≤ Cc ≤ 3.
- Gap-graded (skip-graded) soils have Cc outside 1–3 even if Cᵤ is large.
- Coarse-grained soils retain more than 50% on the No. 200 sieve (0.075 mm); fine-grained soils have 50% or more passing the No. 200 sieve.
Definitions
Term
Effective Size (D₁₀)
Definition
The particle diameter corresponding to 10% passing on the grain-size distribution curve.
Importance
Governs the permeability of a granular soil; used in Hazen's formula k ≈ C(D₁₀)². Always read from the gradation curve, never calculated.
Term
Well-Graded Soil
Definition
A soil with a good representation of all particle sizes from coarse to fine, satisfying BOTH Cᵤ ≥ 4 (gravel) or ≥ 6 (sand) AND 1 ≤ Cc ≤ 3.
Importance
Well-graded soils compact better, have higher density, and are more suitable as fill and subgrade materials. Denoted W in USCS (GW or SW).
Term
Poorly Graded Soil
Definition
A soil dominated by particles of similar size (uniformly graded) or missing an intermediate size range (gap-graded). Fails at least one of Cᵤ or Cc criteria.
Importance
Denoted P in USCS (GP or SP). Has higher void ratio, lower compaction density, and lower bearing capacity than well-graded equivalent.
Term
No. 200 Sieve (0.075 mm)
Definition
The boundary sieve separating coarse-grained from fine-grained soils in USCS. Soils with > 50% retained are coarse; soils with ≥ 50% passing are fine-grained.
Importance
The most critical sieve in soil classification — determines which half of the USCS chart applies.
Section Title
Grain-Size Distribution and Shape Parameters
Common Mistakes
- Applying the Cᵤ ≥ 4 criterion (gravel) to sand — sand requires Cᵤ ≥ 6 for well-graded.
- Classifying a soil as well-graded based on Cᵤ alone without checking Cc — BOTH criteria must be satisfied.
- Confusing 'percent retained' with 'percent passing' when reading the gradation curve — D₆₀ is read at 60% PASSING.
- Forgetting that gap-graded soils (Cc outside 1–3) are classified as poorly graded (P), not a separate category in USCS.
- Using Cc = D₃₀ / (D₁₀ × D₆₀) — the D₃₀ must be SQUARED in the numerator.
Formulas
Example
LL = 45%, PL = 22% → PI = 45 − 22 = 23%. Plot point (45, 23) on the plasticity chart.
Formula
PI = LL − PL
Variables
LL = Liquid Limit (%); PL = Plastic Limit (%)
Application
Quantifies the plastic range of a fine-grained soil. Used with LL to locate the soil on the plasticity chart. PI = 0 means the soil is non-plastic (NP).
Example
At LL = 45: PI_A-line = 0.73(45 − 20) = 0.73 × 25 = 18.25. If actual PI = 23 > 18.25 → soil plots ABOVE A-line → clay. Since LL = 45 < 50 → CL (low-plasticity clay).
Formula
A-line: PI = 0.73(LL − 20)
Variables
PI = Plasticity Index (%); LL = Liquid Limit (%)
Application
Boundary line on the plasticity chart. Points ABOVE → clay minerals (C-symbol). Points BELOW → silt or organic minerals (M or O symbol). Points ON the A-line → use C designation.
Example
At LL = 40: PI_U-line = 0.9(40 − 8) = 0.9 × 32 = 28.8. If a soil plots PI > 28.8 at LL = 40, re-check laboratory data.
Formula
U-line: PI = 0.9(LL − 8)
Variables
PI = Plasticity Index (%); LL = Liquid Limit (%)
Application
Upper boundary of the plasticity chart. No naturally occurring soil plots above the U-line. Used to verify data quality and identify laboratory errors.
Exam Tips
- In board problems, after computing PI, always compute the A-line PI value at the given LL to determine above or below classification.
- The hatched zone between the A-line and U-line at low LL (borderline CL-ML) is classified CL-ML in USCS — expect one board question on this.
- Memorize four USCS fine-grained symbols: ML, CL, MH, CH — and their plasticity chart quadrant positions.
- CH soils (fat clays) are the most problematic: high compressibility, high shrink-swell, low bearing capacity — expect qualitative questions about their engineering behavior.
Key Points
- Atterberg limits define the moisture content boundaries between consistency states (solid, semisolid, plastic, liquid) of fine-grained soils.
- Liquid Limit (LL): moisture content at which soil transitions from plastic to liquid state; measured by Casagrande cup or fall cone.
- Plastic Limit (PL): moisture content at which soil transitions from semisolid to plastic state; measured by the thread-rolling test (3 mm thread crumbles).
- Shrinkage Limit (SL): moisture content at which further drying causes no more volume change.
- Plasticity Index (PI) = LL − PL: the range of moisture content over which soil behaves plastically. Higher PI = more clay-like behavior.
- The Plasticity Chart (A-line chart) is the graphical tool for classifying fine-grained soils in USCS.
- The A-line equation: PI = 0.73(LL − 20) separates CLAYS (plot above or on the A-line) from SILTS and ORGANIC soils (plot below the A-line).
- The vertical line at LL = 50 separates LOW plasticity (L) from HIGH plasticity (H) soils.
- The U-line (PI = 0.9(LL − 8)) is the upper boundary — no natural soil plots above the U-line. Used for outlier checking.
- Organic soils (OL, OH) plot below the A-line; they are distinguished from inorganic silts by the oven-dry LL test (LL drops > 30% when oven-dried for organic soils).
- Liquid limit of 50 is the boundary: LL < 50 → Low (L); LL ≥ 50 → High (H).
Definitions
Term
Liquid Limit (LL)
Definition
The water content (%) at which a fine-grained soil transitions from the plastic state to the liquid state. Measured using the Casagrande cup (25 blows closes a 13 mm groove) or the fall cone apparatus.
Importance
Primary axis of the plasticity chart. Determines L vs. H designation (LL < 50 = Low, LL ≥ 50 = High). Also used in AASHTO Group Index calculation.
Term
Plastic Limit (PL)
Definition
The water content (%) at which a soil thread of 3.2 mm diameter just begins to crumble when rolled on a glass plate.
Importance
Used with LL to compute PI. A soil with PL ≥ LL (PI ≤ 0) is classified as non-plastic (NP).
Term
Plasticity Index (PI)
Definition
The numerical difference between LL and PL; represents the range of water content over which the soil remains in the plastic state.
Importance
The vertical axis of the plasticity chart. Higher PI indicates more expansive, compressible, and problematic clay behavior. Critical for AASHTO group index and USCS fine-grained classification.
Term
A-line
Definition
The empirical boundary on the plasticity chart defined by PI = 0.73(LL − 20), separating clay-type soils (above) from silt- and organic-type soils (below).
Importance
The single most important line on the plasticity chart for USCS fine-grained classification. Must be memorized exactly — coefficient 0.73 and offset 20 are frequently tested.
Section Title
Atterberg Limits and the Plasticity Chart
Common Mistakes
- Using PI = LL + PL instead of PI = LL − PL — always subtract PL from LL.
- Writing the A-line as PI = 0.73(LL) — the correct form subtracts 20 from LL first.
- Confusing 'above the A-line = clay' with 'above LL = 50 line = high plasticity' — these are two independent boundaries on the plasticity chart.
- Classifying organic soils (OL, OH) as ML or MH — organic soils plot below the A-line but are identified by the oven-dry LL test, not position alone.
- Reporting PI for a non-plastic soil as negative — PI is always reported as NP (non-plastic) when LL ≤ PL.
Exam Tips
- Draw the USCS flow chart from memory as your first step in any classification problem — it prevents branching errors.
- The board exam frequently gives partial data (e.g., only Cᵤ) — if Cc is not given, you cannot assign W; default to P.
- Remember: for fine-grained soils, the LL and PL data are used directly. No sieve data (Cᵤ, Cc) is used for fine-grained classification.
- Borderline soils (5–12% fines) with dual symbols appear in advanced board questions — know the rule.
Key Points
- USCS (ASTM D2487) uses a two-letter symbol: first letter = soil type, second letter = qualifier.
- First step: determine if soil is coarse-grained or fine-grained using the No. 200 sieve (0.075 mm).
- Coarse-grained (> 50% retained on No. 200): Gravel (G) if > 50% of coarse fraction retained on No. 4 (4.75 mm); Sand (S) if > 50% of coarse fraction passes No. 4.
- Second letter for clean coarse soils: W (well-graded) or P (poorly graded), determined by Cᵤ and Cc criteria.
- Second letter for coarse soils with fines: M (silty fines, plot below A-line) or C (clayey fines, plot above A-line); determined by plasticity chart of the fines fraction.
- Fine-grained (≥ 50% passing No. 200): M (silt), C (clay), O (organic), with L/H determined by LL = 50 boundary.
- Peat and highly organic soils are classified as Pt — identifiable by dark color, organic odor, and fibrous texture.
- USCS borderline symbols use dual symbols (e.g., SP-SM) for soils near classification boundaries.
- The USCS system is the preferred system for geotechnical design; AASHTO is used for highway subgrade evaluation.
- For coarse soils with 5–12% fines, dual symbols are used (e.g., SW-SM, GW-GC); < 5% fines → W or P; > 12% fines → M or C.
Definitions
Term
GW — Well-Graded Gravel
Definition
Gravel with less than 5% fines; Cᵤ ≥ 4 AND 1 ≤ Cc ≤ 3.
Importance
Excellent foundation and fill material; high bearing capacity, good drainage, good compactability.
Term
GP — Poorly Graded Gravel
Definition
Gravel with less than 5% fines; fails Cᵤ ≥ 4 OR Cc criterion (or both).
Importance
Lower density and strength than GW; still acceptable for drainage applications.
Term
GM — Silty Gravel
Definition
Gravel with more than 12% fines that plot below the A-line (non-plastic or silt-like fines).
Importance
Reduced drainage due to fines; lower bearing capacity than GW/GP; common in Philippine residual soils.
Term
GC — Clayey Gravel
Definition
Gravel with more than 12% fines that plot above the A-line (plastic, clay-like fines).
Importance
Poor drainage; susceptible to frost heave (less relevant in Philippine context) and swelling. Must be compaction-controlled.
Term
SW — Well-Graded Sand
Definition
Sand with less than 5% fines; Cᵤ ≥ 6 AND 1 ≤ Cc ≤ 3.
Importance
Good foundation material when dense; widely used as structural fill and drainage layer.
Term
SP — Poorly Graded Sand
Definition
Sand with less than 5% fines; fails Cᵤ ≥ 6 OR Cc criterion.
Importance
Susceptible to liquefaction under earthquake loading — highly relevant for Philippine seismic design (NSCP 2015 Section 208).
Term
SM — Silty Sand
Definition
Sand with more than 12% fines that plot below the A-line.
Importance
Reduced shear strength when saturated; common in Philippine coastal and riverine deposits.
Term
SC — Clayey Sand
Definition
Sand with more than 12% fines that plot above the A-line.
Importance
Low permeability, moderate plasticity; behavior dominated by clay fines.
Term
ML — Low-Plasticity Silt
Definition
Fine-grained soil plotting below the A-line with LL < 50.
Importance
Quick undrained shear strength loss under vibration; sensitive to disturbance; common in Philippine alluvial plains.
Term
CL — Low-Plasticity Clay
Definition
Fine-grained soil plotting above the A-line with LL < 50.
Importance
Moderate plasticity; most common clay in Philippine lowland areas. Satisfactory compacted fill when moisture-controlled.
Term
MH — High-Plasticity Silt
Definition
Fine-grained soil plotting below the A-line with LL ≥ 50.
Importance
High compressibility, dilatant behavior; problematic as foundation soil.
Term
CH — High-Plasticity Clay (Fat Clay)
Definition
Fine-grained soil plotting above the A-line with LL ≥ 50.
Importance
Most problematic clay — high shrink-swell potential, high compressibility, low shear strength. Requires treatment (lime stabilization, replacement) for road subgrade in the Philippines.
Section Title
Unified Soil Classification System (USCS)
Common Mistakes
- Forgetting to check the No. 4 sieve (4.75 mm) to distinguish gravel from sand within coarse-grained soils.
- Applying gravel Cᵤ criterion (≥ 4) to classify a sand — sand requires Cᵤ ≥ 6.
- Using the total percent passing No. 200 directly instead of computing the coarse fraction for the No. 4 sieve split.
- Classifying a fine-grained soil as M (silt) simply because it has low PI — must check both LL and position relative to A-line on the plasticity chart.
- Forgetting dual symbols for soils with 5–12% fines (e.g., SW-SC not just SW or SC).
Formulas
Example
F = 60%, LL = 45%, PI = 23 → GI = (60−35)[0.2+0.005(45−40)] + 0.01(60−15)(23−10) = 25[0.2+0.025] + 0.01(45)(13) = 25(0.225) + 5.85 = 5.625 + 5.85 = 11.475 ≈ 11
Formula
GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10)
Variables
F = percent passing No. 200 sieve (expressed as a whole number, e.g., 60 not 0.60); LL = Liquid Limit (%); PI = Plasticity Index (%)
Application
Quantifies the quality of a fine-grained or granular soil with fines as a highway subgrade. Apply to A-2-6, A-2-7, A-4, A-5, A-6, A-7-5, and A-7-6 groups. GI ≥ 0 always.
Exam Tips
- In the GI formula, compute both terms separately then add — reduces arithmetic errors.
- Always check: if GI < 0, report GI = 0.
- For A-1, A-3, A-2-4, A-2-5: GI = 0 by definition. Do not apply the formula.
- To distinguish A-7-5 from A-7-6: compute LL − 30 and compare to PI. If PI > LL − 30 → A-7-6 (worse, swelling).
- Board questions on AASHTO often give all three pieces of data (F, LL, PI) — compute GI directly without needing to identify the group first.
Key Points
- AASHTO M145 classifies soils from A-1 (best subgrade) to A-8 (peat, worst) based on suitability for highway pavement subgrade.
- The system uses percent passing sieves No. 10 (2.0 mm), No. 40 (0.425 mm), and No. 200 (0.075 mm), plus LL and PI.
- A-1 through A-3: granular materials (≤ 35% pass No. 200); A-4 through A-7: silty-clay materials (> 35% pass No. 200).
- A-1-a: well-graded gravel/sand mixtures; A-1-b: gravel/sand/silt mixtures; A-3: fine sand (non-plastic).
- A-2 soils (A-2-4, A-2-5, A-2-6, A-2-7) are granular with significant fines — classified by LL and PI of the fines.
- A-4 through A-7 are fine-grained soils; A-7 is subdivided into A-7-5 and A-7-6 based on PI relative to LL.
- A-7-5: PI ≤ LL − 30 (elastic silts); A-7-6: PI > LL − 30 (swelling clays) — A-7-6 is the worst non-organic subgrade.
- A-8: peat or highly organic soil — not rated by Group Index.
- The Group Index (GI) is a numerical refinement within each AASHTO group. GI = 0 is best; higher values indicate poorer subgrade.
- For A-1, A-2-4, A-2-5, and A-3, GI = 0 by definition.
- Report GI as a whole number (round to nearest integer); GI is never reported as negative (minimum = 0).
- In AASHTO classification, the soil group with the lower group number (e.g., A-4 vs. A-6) and lower GI is the better subgrade.
Definitions
Term
Group Index (GI)
Definition
A numerical index used within the AASHTO system to further evaluate soil quality as a highway subgrade. Ranges from 0 (excellent) to 20 (very poor). Computed from percent passing No. 200, LL, and PI.
Importance
Reported in parentheses after the AASHTO group symbol, e.g., A-6(11). A GI of 0 does not necessarily mean the soil is a good subgrade — check the group symbol first.
Term
A-7-6
Definition
AASHTO subgrade classification for highly plastic swelling clays where PI > LL − 30. More expansive and damaging to pavements than A-7-5.
Importance
Requires the most expensive subgrade treatment in Philippine highway construction. Associated with black cotton soils and volcanic clays found in some provinces.
Term
F (percent passing No. 200)
Definition
The percentage by dry mass of soil particles finer than 0.075 mm (No. 200 sieve). Used in the GI formula as a whole number (e.g., 60, not 0.60).
Importance
Critical input to the GI formula. Using F as a decimal fraction instead of a percentage is the most common calculation error in board exam problems.
Section Title
AASHTO Classification System and Group Index
Common Mistakes
- Inputting F as a decimal (0.60) instead of a percentage (60) in the GI formula — this gives a GI nearly 100× too small.
- Reporting a negative GI — always set GI = 0 if the formula gives a negative result.
- Not rounding GI to the nearest integer — the AASHTO standard requires whole number reporting.
- Ignoring the caps built into the GI formula: the partial GI from each term cannot exceed 40 (i.e., (F−35) capped at 40, (F−15) capped at 40, (LL−40) capped at 20, (PI−10) capped at 20). These caps rarely affect typical board problems but appear in extreme data sets.
- Confusing A-7-5 and A-7-6 — A-7-6 (PI > LL − 30) is the worse of the two, not A-7-5.
Exam Tips
- Read the question carefully — does it ask for USCS symbol (e.g., CL), AASHTO group (e.g., A-6), or Group Index (e.g., 11)?
- Note the key AASHTO boundary: 35% passing No. 200 (granular ≤ 35%; silty-clay > 35%). In USCS, it is 50%.
- A soil can be 'acceptable' in USCS but 'poor' in AASHTO — the two systems serve different purposes.
Key Points
- USCS (ASTM D2487) is preferred for geotechnical design (foundations, embankments, retaining walls, dams).
- AASHTO M145 is specifically designed for rating highway pavement subgrade quality.
- USCS gives more precise information about soil behavior (plasticity, gradation); AASHTO gives a direct subgrade quality rating.
- Both systems use grain-size analysis and Atterberg limits, but they weight the data differently.
- A CL (USCS) soil roughly corresponds to A-6 or A-7-6 (AASHTO) — both indicate plastic, problematic clay.
- A SW (USCS) soil corresponds to A-1-b or better in AASHTO — both indicate good granular material.
- USCS does not have a built-in quality ranking; AASHTO explicitly ranks A-1 as best and A-8 as worst.
- In Philippine practice, both systems appear in DPWH specifications: USCS for borrow fill classification, AASHTO for subgrade layer requirements.
- The board exam tests both systems with equal frequency — be equally fluent in both.
Section Title
Comparison: USCS vs. AASHTO
Common Mistakes
- Applying AASHTO group index logic to a USCS classification problem — the two systems are independent.
- Assuming USCS and AASHTO classifications always correspond consistently — they often disagree on borderline soils.
- Using the No. 200 sieve criterion differently — in USCS it is 50% boundary; in AASHTO it is 35% boundary for granular vs. silty-clay groups.
Connections
- Grain-size parameters (Cᵤ, Cc) directly determine USCS coarse-grained symbols (GW, GP, SW, SP) and feed into AASHTO granular groups (A-1, A-3).
- Atterberg limits (LL, PL, PI) determine both the USCS fine-grained symbol (ML, CL, MH, CH) via the plasticity chart AND the AASHTO group and Group Index for fine-grained soils.
- Soil classification connects to compaction design: well-graded soils (GW, SW) achieve higher dry unit weight at lower OMC than poorly graded or plastic soils (CH, MH).
- Classification connects to permeability: GW > GP > GM/GC > SM/SC > ML/CL > MH/CH in terms of hydraulic conductivity — critical for drainage layer design.
- SP (poorly graded sand) classification directly relates to liquefaction susceptibility under seismic loading — relevant to NSCP 2015 Section 208 site classification.
- AASHTO Group Index connects to pavement design — higher GI requires thicker base course and subbase layers per DPWH highway design standards.
- The plasticity index (PI) from Atterberg limits connects to expansive soil identification and treatment (lime stabilization, geotextile reinforcement) in Philippine road construction.
- Soil classification is the prerequisite for understanding consolidation (CH/MH are most compressible), shear strength (CL has moderate strength, CH has low strength), and slope stability analysis.
- Classification by USCS and AASHTO systems is required under RA 544 (Philippine Civil Engineering Act) as part of professional geotechnical investigation reports submitted to government agencies.
- The No. 200 sieve (0.075 mm) is the critical boundary sieve — it connects grain-size analysis (used in coarse-grained classification) to Atterberg limit testing (required for soils with significant fines).
Exam Strategy
Approach every soil classification problem with a systematic three-step protocol: Step 1 — Determine percent passing No. 200: if > 50% → fine-grained (use plasticity chart); if ≤ 50% → coarse-grained (use No. 4 sieve and Cᵤ/Cc). Step 2 — Apply the appropriate system (USCS or AASHTO) based on what the question asks; never mix criteria. Step 3 — Verify your answer makes engineering sense (e.g., a well-graded gravel should not have a poor AASHTO rating). For AASHTO Group Index calculations, always write out both terms of the GI formula separately, check for caps, and ensure GI ≥ 0. For plasticity chart problems, always compute the A-line PI value at the given LL before deciding above/below. In the PRC board exam, soil classification questions typically account for 3–5 items per exam sitting — they are straightforward formula-application problems if you have memorized: Cᵤ = D₆₀/D₁₀; Cc = D₃₀²/(D₁₀×D₆₀); PI = LL−PL; A-line: PI = 0.73(LL−20); GI = (F−35)[0.2+0.005(LL−40)] + 0.01(F−15)(PI−10). Write these on your scratch paper at the start of the exam.
Quick Review Questions
A gravel sample has D₁₀ = 0.5 mm, D₃₀ = 4 mm, D₆₀ = 12 mm. Compute Cᵤ and Cc. Is the gravel well-graded?
For gravel, the USCS requires Cᵤ ≥ 4 AND 1 ≤ Cc ≤ 3. Both must be checked. Here both pass, confirming GW. Note Cc = D₃₀²/(D₁₀ × D₆₀) = 16/6 = 2.67, which is within [1, 3].
A fine-grained soil has LL = 60% and PL = 25%. Classify using USCS.
Two checks determine fine-grained USCS symbol: (1) Is the point above or below the A-line? — determines C or M; (2) Is LL below or above 50? — determines L or H. Here: above A-line (C) + LL ≥ 50 (H) = CH.
Compute the AASHTO Group Index for a soil with F = 82%, LL = 50%, PI = 28.
Note: GI has a maximum reported value of 20. The caps per term: (F−35) max = 40, (F−15) max = 40, (LL−40) max = 20, (PI−10) max = 20. Here (F−35) = 47 > 40, so cap at 40: GI = 40(0.25) + 0.01(40)(18) = 10 + 7.2 = 17.2 ≈ 17. (Some references cap overall GI at 20; apply the per-term caps consistently.)
A sand has Cᵤ = 8 but Cc = 1.8. Is it well-graded or poorly graded?
Both conditions must be checked for sand: Cᵤ ≥ 6 AND 1 ≤ Cc ≤ 3. Since both pass here, the classification is SW. If either failed, the result would be SP.
A fine-grained soil plots at (LL = 45, PI = 16) on the plasticity chart. What is its USCS classification?
The soil plots in the lower-left quadrant of the plasticity chart — below the A-line (M not C) and to the left of LL = 50 (L not H). Note: if PI were between 4 and 7 and the point fell in the hatched zone (near A-line), dual symbol CL-ML would apply.
What is the minimum Cᵤ for a sand to qualify as well-graded in USCS, and what is the Cc range required?
Gravel requires Cᵤ ≥ 4; sand requires a higher threshold of Cᵤ ≥ 6 because sands have inherently finer particles. The Cc criterion of 1 ≤ Cc ≤ 3 is identical for both gravel and sand.
For AASHTO classification, what is the percent-passing-No. 200 boundary that separates granular materials from silty-clay materials?
This 35% boundary in AASHTO differs from the 50% boundary used in USCS. In USCS, > 50% retained on No. 200 = coarse-grained. In AASHTO, > 35% passing No. 200 triggers the silty-clay groups — the systems are not interchangeable.
A soil has LL = 55% and PI = 28%. Determine if it is A-7-5 or A-7-6 in the AASHTO system.
The A-7-5 vs. A-7-6 boundary: if PI ≤ LL − 30 → A-7-5 (elastic silts); if PI > LL − 30 → A-7-6 (swelling clays). A-7-6 is the more problematic subgrade due to high swell potential.
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