CELE Geotechnical Engineering — Soil ClassificationStudy Notes

Grain-size distribution describes the percentage (by mass) of soil particles passing through standard sieves of various sizes. The gradation curve is plotted on a semilogarithmic graph with sieve opening (diameter) on the logarithmic x-axis and percentage passing (finer than) on the arithmetic y-axis. From this curve, three characteristic diameters are identified: • D₁₀ (effective size): diameter at which 10% of soil particles are finer • D₃₀: diameter at which 30% of soil particles are finer • D₆₀: diameter at which 60% of soil particles are finer These values are used to compute two important shape parameters: **Uniformity Coefficient:** Cu = D₆₀ / D₁₀ The uniformity coefficient indicates the range of particle sizes. A high Cu indicates a wide size range (nonuniform), while a low Cu indicates most particles are similar in size (uniform). **Coefficient of Curvature (Gradation Coefficient):** Cc = D₃₀² / (D₁₀ × D₆₀) The coefficient of curvature describes the shape of the gradation curve between D₁₀ and D₆₀. It accounts for whether intermediate-size particles are present in appropriate proportions. **Well-Graded vs. Poorly Graded:** A soil is classified as **well-graded** if it satisfies BOTH conditions simultaneously: • For gravels: Cu ≥ 4 AND 1 ≤ Cc ≤ 3 • For sands: Cu ≥ 6 AND 1 ≤ Cc ≤ 3 If either condition fails, the soil is **poorly graded**. Well-graded soils have good particle size continuity, which typically results in better compaction, higher shear strength, and lower permeability. Poorly graded soils may be uniformly graded (most particles similar size) or gap-graded (missing intermediate sizes). **Physical Significance:** Well-graded soils compact more efficiently because smaller particles can fill voids between larger particles. This makes them ideal for embankments, road bases, and earth dams. Poorly graded soils, particularly uniformly graded sands, are more prone to liquefaction and settlement under seismic loading.

The Atterberg limits are moisture content values that define transitions between the solid, plastic, and liquid states of fine-grained soils (silts and clays). These limits are determined by standardized laboratory tests specified in ASTM D4318 and are critical for understanding soil behavior during construction and over time. **Shrinkage Limit (SL):** The moisture content at which a soil, when dried, reaches its minimum volume. Below this moisture content, further drying does not cause additional volume change. The shrinkage limit is important for predicting cracking in clay embankments and foundations. **Plastic Limit (PL):** The moisture content at which a soil transitions from semi-solid behavior to plastic behavior. At this point, the soil can be rolled into a thread of 3 mm diameter on a glass plate without crumbling. Below the plastic limit, the soil behaves as a brittle material; above it, the soil can deform without cracking. **Liquid Limit (LL):** The moisture content at which a soil transitions from plastic to liquid behavior. At this limit, a pat of soil placed in a standard Casagrande cup requires 25 blows of a falling cam to close a standard groove. At the liquid limit, the soil begins to flow under its own weight. **Plasticity Index (PI):** PI = LL − PL The plasticity index represents the range of moisture content over which the soil exhibits plastic behavior. A high PI indicates a wide plastic range and generally signifies a highly plastic clay. A low PI indicates that the soil transitions quickly from plastic to liquid state. Soils with PI < 4 are classified as non-plastic. **Physical Interpretation:** The plasticity index correlates with clay mineralogy and has strong implications for engineering behavior: • PI < 4: Non-plastic or slightly plastic soils (silts, sandy silts) • 4 ≤ PI < 15: Low plasticity soils (lean clays, silty clays) • 15 ≤ PI < 30: Medium plasticity soils (typical clays) • PI ≥ 30: High plasticity soils (fat clays, montmorillonite-rich soils) Soils with high PI are more compressible, have lower shear strength when wet, and are more prone to shrinkage cracking during drying. The liquid limit LL = 50% is a threshold in both USCS and AASHTO systems, separating low-plasticity (L) from high-plasticity (H) soils.

The plasticity chart is a plotting tool used to classify fine-grained soils based on their liquid limit and plasticity index. The chart's two key features are the **A-line** and the **LL = 50% vertical line**, which together create regions for each soil classification. **The A-Line Equation:** PI = 0.73(LL − 20) This empirical relationship, established through extensive laboratory testing, separates **clays** (soils plotting above the A-line) from **silts** (soils plotting below the A-line). The A-line has a slope of 0.73 and intersects the LL axis at LL = 20. **Physical Meaning:** Clays have higher plasticity relative to their liquid limit than do silts. For a given LL value, clay soils consistently exhibit PI values that plot above the A-line, while silt soils plot below. This distinction reflects differences in mineral composition and surface chemistry. Clays (primarily phyllosilicate minerals) have larger specific surface areas and greater water-holding capacity than silts (primarily quartz and feldspar). **Classification Regions:** The plasticity chart is divided into regions by the A-line and the LL = 50 vertical line: 1. **CL (low-plasticity clay):** Above A-line, LL < 50 2. **CH (high-plasticity clay):** Above A-line, LL ≥ 50 3. **ML (low-plasticity silt):** Below A-line, LL < 50 4. **MH (high-plasticity silt):** Below A-line, LL ≥ 50 Additional regions include: • **OL and OH:** Organic silts and clays (LL after oven-drying is less than original LL) • **CL-ML:** Borderline clays and silts (plotting near the A-line) **Using the Plasticity Chart in Practice:** To classify a soil using the plasticity chart: 1. Determine LL and PI from laboratory tests 2. Locate the point (LL, PI) on the chart 3. Determine which side of the A-line the point falls 4. Note whether LL < 50 or LL ≥ 50 5. Assign the appropriate two-letter code based on region **Practical Note:** In the Philippines, high-PI clays (CH) from tropical weathering are common. These soils often exhibit unusual behavior, including potential for quick-clay formation and significant volume change. The Maynila Clay and Manila Bay soft clay deposits are well-known examples in geotechnical practice.

The Unified Soil Classification System is the primary classification scheme used by geotechnical engineers in design and construction. It was developed jointly by the U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers and has been adopted internationally, including in the Philippines. The USCS classifies soils into two major groups based on the percentage of particles finer than 0.075 mm (the #200 sieve). **Major Division — Coarse-Grained vs. Fine-Grained:** **COARSE-GRAINED SOILS (more than 50% retained on #200 sieve):** Further subdivided by the gravel/sand boundary (4.75 mm sieve): • **Gravel (G):** More than 50% of coarse fraction is larger than 4.75 mm (retained on #4 sieve) • **Sand (S):** 50% or more of coarse fraction is smaller than 4.75 mm (passing #4 sieve) Coarse-grained soils are subdivided into clean (lacking fines) or dirty (containing significant fines) based on the percent passing #200: • **Clean:** Less than 5% passing #200 - GW: well-graded gravel - GP: poorly graded gravel - SW: well-graded sand - SP: poorly graded sand • **Dirty (with fines):** 5% to 12% passing #200 (borderline) or more than 12% passing #200 - GM: gravel with silt - GC: gravel with clay - SM: sand with silt - SC: sand with clay For soils with 5–12% fines, both designations are sometimes used (e.g., SW-SM: well-graded sand with some silt). **FINE-GRAINED SOILS (50% or more passing #200 sieve):** Classified by type and plasticity using the plasticity chart: • **M (Silt):** Below the A-line on the plasticity chart - ML: low-plasticity silt (LL < 50) - MH: high-plasticity silt (LL ≥ 50) • **C (Clay):** Above the A-line on the plasticity chart - CL: low-plasticity clay (LL < 50) - CH: high-plasticity clay (LL ≥ 50) • **O (Organic):** Contains organic material; LL after drying < 4/5 of original LL - OL: low-plasticity organic clay or silt - OH: high-plasticity organic clay or silt • **Pt (Peat):** Primarily organic material; easily identified by dark color, fibrous structure, and distinctive odor **Classification Decision Tree:** The USCS classification requires the following information: 1. Percentage passing #200 sieve (determines coarse vs. fine) 2. Percentage passing #4 sieve (determines gravel vs. sand for coarse soils) 3. D₁₀, D₃₀, D₆₀ (determines Cu and Cc for coarse soils) 4. LL and PI (determines position on plasticity chart for fine soils) 5. Visual inspection (determines if organic or peat) **Physical Significance of Each Class:** **GW (Well-Graded Gravel):** Dense, excellent drainage, high shear strength, minimal settlement. Ideal for filter layers, embankment cores, road base courses. **GP (Poorly Graded Gravel):** Potentially uniform or gap-graded; still has good drainage but may not compact as well as GW. Used in fill and drainage applications. **GM (Gravel with Silt):** Silt fills voids between gravel particles; reduced drainage, moderate compressibility. Used in embankments with controlled permeability. **GC (Gravel with Clay):** Clay binds gravel particles; low drainage, higher cohesion, more stable on slopes. Used in embankment cores and dam construction. **SW (Well-Graded Sand):** Good drainage and compaction properties; often mixed with gravel (SW-GW) for road bases and filters. **SP (Poorly Graded Sand):** Uniform sizing; still drains well but potential for liquefaction in earthquake zones. Requires analysis for seismic hazard. **SM (Sand with Silt):** Silt reduces drainage; variable compressibility. May be problematic if silt content is high (>20%). **SC (Sand with Clay):** Clay provides cohesion; low permeability; suitable for embankment cores and dikes. **ML (Low-Plasticity Silt):** Moderate compressibility, low shear strength when saturated, potential for piping erosion in embankments. Requires stabilization for bearing layers. **MH (High-Plasticity Silt):** Similar behavior to ML but more compressible and less permeable. Often problematic for foundations. **CL (Low-Plasticity Clay):** Moderate to good bearing capacity, moderate compressibility, suitable for general foundation and embankment use. **CH (High-Plasticity Clay):** High compressibility, low permeability, very low strength when saturated, significant volume change. Requires special design consideration in the Philippines due to tropical weathering. **OL & OH (Organic Soils):** Highly compressible, low strength, very slow drainage. Generally unsuitable for foundations; must be removed or treated before construction. **Pt (Peat):** Extremely compressible, very high water content, essentially unsuitable for any structural support. Must be removed from building sites.

The AASHTO (American Association of State Highway and Transportation Officials) classification system is specifically designed to evaluate soils for use in highway pavement construction and embankments. While USCS is the primary system for general geotechnical engineering, AASHTO is the standard for road and highway projects, including projects in the Philippines designed to Philippine road standards. The system rates soils from A-1 (best subgrade material) to A-8 (peat and organic soils, unsuitable). **AASHTO Classification Groups:** **A-1 (Excellent to Good):** - A-1-a: Well-graded gravel and sand with little or no fines - A-1-b: Well-graded sand with little or no fines **A-2 (Good to Fair):** Sandy soils with moderate fines - A-2-4: Sandy materials with silt - A-2-5: Sandy materials with high-plasticity silt - A-2-6: Sandy materials with clay - A-2-7: Sandy materials with high-plasticity clay **A-3 (Fair):** Poorly graded sand with little fines **A-4 (Fair to Poor):** Silt soils **A-5 (Poor):** High-plasticity silt **A-6 (Poor to Very Poor):** Clay soils **A-7 (Very Poor):** High-plasticity clay - A-7-5: PI ≤ LL − 30 - A-7-6: PI > LL − 30 **A-8 (Unsuitable):** Peat and highly organic soils **The Group Index (GI):** Within each AASHTO classification, the **Group Index** provides a numerical refinement. A higher GI indicates worse subgrade performance. The Group Index is calculated using the formula: **GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10)** Where: • F = percentage of soil passing the #200 sieve (expressed as a whole number, 0–100) • LL = liquid limit • PI = plasticity index **Calculation Rules:** 1. All terms in brackets are independent; calculate each separately 2. If a calculated term is negative, use zero 3. Each bracketed term in the formula has a maximum value: - (F − 35) ≤ 40, so use 40 if F > 75 - (LL − 40) ≤ 20, so use 20 if LL > 60 - (F − 15) ≤ 40, so use 40 if F > 55 - (PI − 10) ≤ 20, so use 20 if PI > 30 4. Round the final GI to the nearest whole number (0.5 rounds up) 5. The minimum value is 0 (negative results are reported as 0) **Interpretation:** The Group Index modifies the AASHTO rating: - GI = 0: Best subgrade material in that AASHTO class - GI = 20: Worst subgrade material in that AASHTO class For example: A-4-3 means AASHTO A-4 (silt) with Group Index 3; A-6(12) means A-6 (clay) with Group Index 12. The higher the GI within a class, the more problematic the material for subgrade use. **Use in Pavement Design:** In pavement thickness design (per AASHTO pavement design methods), the Group Index directly affects the required thickness of asphalt and base courses. A soil with A-1-a(0) requires minimal pavement thickness, while A-7-6(18) requires substantially thicker pavements due to poor load-bearing capacity and high volume change potential. **Relationship to USCS:** While AASHTO and USCS measure different aspects, there is general correspondence: - AASHTO A-1: Corresponds to USCS GW, SW - AASHTO A-2: Corresponds to USCS GM, SM, GC, SC - AASHTO A-3: Corresponds to USCS SP - AASHTO A-4: Corresponds to USCS ML - AASHTO A-5: Corresponds to USCS MH - AASHTO A-6: Corresponds to USCS CL - AASHTO A-7: Corresponds to USCS CH A soil can have one USCS classification and one AASHTO classification; they address different engineering questions.

Effective soil classification requires a systematic approach to laboratory testing and data interpretation. This section synthesizes the concepts from previous sections and provides a workflow that mirrors board examination questions. **Step 1: Obtain Basic Soil Information** Begin with: • Representative soil sample (minimum 200–500 g for sieve analysis) • Visual description (color, smell, texture, visible particles) • Site location and depth • Previous boring data if available **Step 2: Perform Preliminary Visual Classification** Before detailed testing, make preliminary observations: • **Grain size:** Rub between fingers; sand feels gritty, silt feels like flour, clay sticks to fingers • **Mica content:** Glints in sunlight indicate silt or fine sand • **Organic material:** Distinctive smell, dark color, fibrous texture suggest OL, OH, or Pt • **Coarse fraction:** Estimate gravel vs. sand visually **Step 3: Perform Sieve Analysis (Coarse-Grained Soils)** If preliminary assessment suggests coarse-grained soil (>50% retained on #200): 1. Dry the sample (oven at 105°C until constant mass) 2. Pass through standard sieves (#4, #10, #40, #200) 3. Record percent retained on each sieve 4. Calculate percent passing #200 (if >50%, soil is actually fine-grained) 5. Determine grain-size curve and read D₁₀, D₃₀, D₆₀ 6. Calculate Cu and Cc 7. Determine if gravel (>50% coarse > 4.75 mm) or sand 8. Classify as well-graded (W) or poorly graded (P) based on Cu and Cc **Step 4: Perform Atterberg Limit Tests (Fine-Grained Soils)** If soil is fine-grained (≥50% passing #200): 1. **Liquid Limit (Casagrande cup method, ASTM D4318):** - Mix soil with distilled water to paste consistency - Place in cup and create groove with standard tool - Drop cup repeatedly; count blows to close groove at 1/2 inch - Typically 25 blows = LL; make multiple determinations - Plot moisture content vs. log(number of blows); LL is at 25 blows 2. **Plastic Limit:** - Roll small soil ball on glass plate into 3 mm diameter thread - Thread crumbles when moisture approaches PL - Record moisture content at crumbling; PL is average of multiple trials 3. **Calculate PI:** - PI = LL − PL - If PL cannot be determined (non-plastic), report PI = 0 **Step 5: Classify Fine-Grained Soil Using Plasticity Chart** With LL and PI values: 1. Locate point (LL, PI) on chart 2. Determine if above or below A-line: PI = 0.73(LL − 20) 3. Determine if LL < 50 or LL ≥ 50 4. Assign USCS classification: - Above A-line, LL < 50 → CL - Above A-line, LL ≥ 50 → CH - Below A-line, LL < 50 → ML - Below A-line, LL ≥ 50 → MH 5. Check for organic material (visual inspection, LL reduction test) **Step 6: Determine Fines Type in Coarse-Grained Soils (If ≥5% Fines)** For coarse soils with 5–12% or >12% passing #200: 1. Perform Atterberg limits on the fines fraction (soaked for 10 minutes, allowed to settle) 2. Plot on plasticity chart to determine if silt (M) or clay (C) 3. Add second letter to coarse classification: - GM: gravel with silt - GC: gravel with clay - SM: sand with silt - SC: sand with clay **Step 7: Final USCS Classification** Assemble the two-letter code: - First letter: G (gravel), S (sand), M (silt), C (clay), O (organic), Pt (peat) - Second letter: W/P (well/poorly graded), M/C (silt/clay fines), L/H (low/high plasticity) **Step 8: Determine AASHTO Classification and Group Index** For highway projects: 1. Refer to AASHTO classification table (A-1 to A-8) based on grain size and plasticity 2. Calculate Group Index if LL and PI are available: - GI = (F−35)[0.2+0.005(LL−40)] + 0.01(F−15)(PI−10) - Apply all caps and floor at zero 3. Report as AASHTO A-X(GI) **Board Exam Strategy:** **For USCS Problems:** - Always check BOTH Cu and Cc criteria for well-graded classification - Plot on plasticity chart for fine soils; use A-line equation precisely - Watch for borderline cases (CL-ML); be prepared to justify - Distinguish organic soils by oven-drying LL test **For AASHTO Problems:** - Apply the GI formula mechanically; watch bracket caps - F is always a percentage (0–100), not a decimal - Round GI to nearest whole number; never report negative - Understand the engineering meaning: higher GI = worse subgrade **For Combined Problems:** - A soil can be GW-GC (USCS) AND A-1-a(2) (AASHTO) simultaneously - USCS addresses engineering design; AASHTO addresses highways - Show both classifications if both systems are mentioned **Common Board Exam Mistakes to Avoid:** 1. **Forgetting Cc in well-graded determination:** Cu ≥ 4/6 alone is insufficient 2. **Misreading D₁₀, D₃₀, D₆₀ from curve:** Use semilog graph correctly 3. **A-line slope error:** Always use 0.73(LL − 20), not approximations 4. **Ignoring fines in coarse soils:** 5% passing #200 changes the classification letter 5. **GI formula errors:** Calculate each term independently; apply caps 6. **Rounding GI incorrectly:** Round to nearest whole number, minimum 0 7. **Confusing LL and PI thresholds:** LL = 50 is the dividing line; PI = 30 is not a threshold in USCS 8. **Forgetting to classify organic material:** OL/OH classification changes with LL reduction test **Quick Reference: USCS Classification Decision Tree** • If >50% passing #200 → Fine-grained (M/C/O/Pt) - If below A-line → M (silt) - If above A-line → C (clay) - If LL < 50 → add L (low-plasticity) - If LL ≥ 50 → add H (high-plasticity) • If <50% passing #200 → Coarse-grained (G/S + W/P + M/C if fines) - If >50% coarse > 4.75 mm → G (gravel) - If ≤50% coarse > 4.75 mm → S (sand) - If Cu and Cc both satisfy criteria → W (well-graded) - Otherwise → P (poorly graded) - If >12% passing #200 → add fines type (M or C) **Practice Approach:** For each board-style problem: 1. Extract given data (sieve percentages, D₁₀, LL, PL, etc.) 2. Identify what system is being asked (USCS, AASHTO, or both) 3. Determine grain-size category first (coarse vs. fine) 4. Work through the decision criteria step-by-step 5. Write the classification clearly (e.g., "GW-GC" or "A-6(11)") 6. Provide brief engineering context if requested (e.g., "suitable for embankment core") 7. Double-check calculations; especially GI formula and curve reading