CELE Geotechnical Engineering — Soil ClassificationSummary

From a sieve analysis or hydrometer test, the grain-size distribution curve is plotted with particle diameter on the x-axis and percent finer (cumulative passing percentage) on the y-axis. Three critical diameters are read: D₁₀ (the diameter at 10% passing, also called the effective size), D₃₀ (diameter at 30% passing), and D₆₀ (diameter at 60% passing). The effective size D₁₀ governs permeability and capillary rise; fine-grained soils have smaller D₁₀ values. Reading these values accurately from the gradation curve is essential because both Cu and Cc depend directly on D₁₀.

Cu measures the spread of the grain-size distribution. A high Cu indicates a wide range of particle sizes (uniform mixture across the spectrum), whereas a low Cu indicates most particles are similar in size (uniform in the sense of clustering). For coarse-grained soils, Cu ≥ 4 (for gravel) or Cu ≥ 6 (for sand) is required to be considered well-graded. However, Cu alone is insufficient; the curvature coefficient Cc must also satisfy 1 ≤ Cc ≤ 3. Common board mistake: students assume high Cu automatically means well-graded without checking Cc.

Cc (or sometimes Cc is written as the coefficient of concavity) describes whether the grain-size curve is smooth and well-distributed. For a soil to be well-graded, it must satisfy both Cu and Cc criteria. A Cc value between 1 and 3 indicates the gradation curve is gently curved (good interlocking of particles); values outside this range indicate gaps in the particle-size spectrum (poorly graded). For example, a bimodal distribution with two distinct particle-size peaks may have adequate Cu but poor Cc, resulting in poor-graded classification despite high uniformity.

Atterberg limits define the moisture contents at which a fine-grained soil transitions between states. The liquid limit (LL) is the minimum moisture content at which the soil flows as a liquid under a standard test (e.g., Casagrande fall-cone or penetrometer method). The plastic limit (PL) is the moisture content below which the soil cannot be deformed plastically and crumbles. The shrinkage limit (SL) is the minimum moisture content below which the soil no longer shrinks on drying. For laboratory reporting in the Philippines, both LL and PL are typically reported to the nearest whole percent. These values are determined using standardized procedures (ASTM D4318 or equivalent Philippine standards) and form the basis for fine-grained soil classification.

PI represents the range of moisture contents over which the soil exhibits plastic behavior. A high PI indicates the soil remains plastic over a wide moisture range, suggesting high clay content and potential for large volume changes during wetting and drying. Low PI soils (PI < 7) are less sensitive to moisture fluctuations. For fine-grained soils: if PI < 4, the soil may be classified as sandy or silty with low plasticity; if PI > 20, the soil has high plasticity and is prone to swelling and shrinkage. The plasticity index is one of the critical inputs for the plasticity chart and AASHTO classification.

The plasticity chart plots plasticity index (y-axis) against liquid limit (x-axis). The A-line, with equation PI = 0.73(LL − 20), divides clays (soils that plot above the A-line, labeled 'C') from silts and other materials (below, labeled 'M'). A horizontal line at LL = 50 separates low-plasticity (L) soils from high-plasticity (H) soils. Using the plasticity chart: (1) calculate or measure LL and PI; (2) plot the point on the chart; (3) determine position relative to A-line and LL = 50 line to assign the soil type (CL, CH, ML, MH, etc.). Organic clays (OL, OH) plot below the A-line but have LL values that increase markedly upon drying, distinguishing them from inorganic silts.

The USCS assigns a two-letter symbol to every soil based on grain-size distribution and plasticity. Coarse-grained soils (more than 50% retained on the #200 sieve, 0.075 mm): classified as G (gravel) or S (sand), followed by a second letter: W (well-graded), P (poorly graded), M (silty), or C (clayey). Fine-grained soils (at least 50% passing #200 sieve): classified as M (silt), C (clay), or O (organic), followed by L (low plasticity, LL < 50) or H (high plasticity, LL ≥ 50). Examples: SW (well-graded sand), SP (poorly graded sand), SM (silty sand), SC (clayey sand), CL (low-plasticity clay), CH (high-plasticity clay), ML (low-plasticity silt), MH (high-plasticity silt). The USCS is the primary classification system referenced in the NSCP 2015 and required for PRC examinations.

The AASHTO system categorizes soils from A-1 (excellent for highway subgrade) to A-8 (peat, unsuitable). The group index (GI) further refines the rating within each category. GI is calculated using: GI = (F − 35)[0.2 + 0.005(LL − 40)] + 0.01(F − 15)(PI − 10), where F is the percent passing the #200 sieve (expressed as a whole number, e.g., 60, not 0.60). Each term is capped: (F − 35) ≤ 40, (F − 15) ≤ 40, (LL − 40) ≤ 20, and (PI − 10) ≤ 20. The GI is rounded to the nearest integer and floored at 0 (never reported as negative). Higher GI indicates poorer subgrade performance; A-1-a soils (GI = 0) are best, while A-8 and soils with GI > 20 are unsuitable without treatment. AASHTO classification is critical for highway and pavement design in the Philippines.

A soil is classified as well-graded (W suffix in USCS) if BOTH conditions are met: (1) Cu ≥ 4 for gravel or Cu ≥ 6 for sand, AND (2) 1 ≤ Cc ≤ 3. If either condition fails, the soil is poorly graded (P suffix). Well-graded soils have a continuous range of particle sizes, allowing better particle interlocking and higher maximum dry density with lower void ratio. Poorly graded soils—even those with high Cu—may have gaps in the particle-size distribution or undesirable shapes, reducing compaction efficiency. The distinction directly affects compaction specifications and settlement predictions in foundation design.

The #200 sieve (0.075 mm opening) is the critical dividing line in soil classification. Coarse-grained soils are defined as those with more than 50% by weight retained on the #200 sieve, while fine-grained soils have at least 50% passing the #200 sieve. Within coarse-grained soils, the #4 sieve (4.75 mm) further divides gravel from sand. Below #200, particles are classified by hydrometer analysis into silt (0.002–0.075 mm) and clay (< 0.002 mm) based on particle size alone. However, in the USCS, fine-grained soils are actually classified by plasticity characteristics (position on the plasticity chart) rather than clay–silt particle-size distinction, which is a common source of confusion.

The values of D₁₀, D₃₀, and D₆₀ extracted from the gradation curve feed directly into Cu = D₆₀/D₁₀ and Cc = D₃₀²/(D₁₀·D₆₀). Both parameters must satisfy their respective thresholds to classify a coarse-grained soil as well-graded (W) or poorly graded (P). This relationship is foundational to USCS classification of coarse-grained materials.

The liquid limit (LL) and plastic limit (PL) are measured or reported from laboratory tests. Their difference, PI = LL − PL, is then plotted on the plasticity chart along with LL. The position of the (LL, PI) point relative to the A-line (PI = 0.73(LL − 20)) and the LL = 50 threshold determines whether the fine-grained soil is clay (C) or silt (M) and whether it has low (L) or high (H) plasticity, completing the two-letter USCS symbol.

The percent passing the #200 sieve determines the primary pathway: if >50% retained (coarse-grained), the soil is classified as G or S and requires gradation parameters (Cu, Cc) for the second letter. If ≥50% passing (fine-grained), the soil is classified using Atterberg limits and the plasticity chart (M, C, or O with L or H suffix). The percent passing #200 is also used in the AASHTO group index calculation.

While USCS and AASHTO are separate systems, the percent passing #200 sieve and Atterberg limits (LL and PI) are common inputs to both. AASHTO classification yields a letter rating (A-1 through A-8) and a group index (GI) that refines the subgrade suitability. In Philippine highway design (per NSCP 2015 and Department of Public Works and Highways standards), both systems are used concurrently: USCS for general soil characterization and communication, AASHTO for subgrade performance and pavement design.

Well-graded soils, by virtue of their continuous range of particle sizes, achieve higher dry densities and lower void ratios when compacted. This leads to reduced permeability, lower settlement, and better bearing capacity. In contrast, poorly graded soils, even with adequate total particle range, may have gaps that prevent optimal interlocking and result in lower dry density, higher permeability, and greater deformation potential. This relationship directly influences foundation design and embankment specifications.

Soils with high PI are prone to large volume changes upon wetting and drying, particularly in expansive clays common in certain Philippine regions. High PI correlates with higher liquid and plastic limits and suggests greater clay content. Foundation design for high-PI soils requires special attention to moisture control and often deeper footings or alternative systems. This relationship is critical in Philippine construction where seasonal moisture variations are significant.

During site investigation for building construction, boreholes are drilled and soil samples are collected. Geotechnical engineers perform sieve analysis (for coarse-grained soils) and hydrometer tests, measure Atterberg limits, and plot gradation curves. Classification is then documented in the geotechnical report using both USCS and AASHTO symbols. For example, if a sample is classified as 'CL – A-6', engineers immediately understand it is a low-plasticity clay with fair-to-poor subgrade performance, influencing foundation depth, design approach, and construction specifications. This classification drives all downstream foundation analysis and design decisions.

Soil classification informs immediate bearing capacity estimates and settlement predictions. A well-graded sand (SW) will typically have higher bearing capacity and lower settlement compared to poorly graded sand (SP) or clay (CL/CH). Engineers use the soil classification to select appropriate foundation types: shallow foundations (footings) for competent soils, deep foundations (piles) for soft clays or significant settlement concerns. In the Philippines, where many regions are underlain by recent alluvial deposits and old alluvium of varying grain size and plasticity, correct soil classification is essential for safe and economical foundation design.

For road embankments, dams, and building fill, specification of suitable materials depends on soil classification. Well-graded coarse-grained soils (SW, GW) are preferred for structural fills and base courses. Poorly graded soils, fine-grained clays, and organic materials are unsuitable without treatment. Engineers specify that fill materials must meet certain gradation criteria (e.g., Cu and Cc ranges, LL limits) and define compaction requirements (percent of maximum dry density, moisture content tolerance). During quality control, field technicians perform classification checks on incoming fill to ensure compliance with specifications. This application directly reduces risk of embankment failure and extends pavement life.

In highway design, the AASHTO group index determines the required thickness of asphalt and base courses. Soils with GI = 0–2 (excellent to good, typically A-1 and A-2 soils) require thinner pavements; soils with GI > 10 (poor, typically A-6 and A-7) require thicker pavements or stabilization. For example, if a subgrade soil is classified as A-7-6 (GI = 15), engineers must design a thicker pavement or consider soil stabilization (cement or asphalt binder treatment) to reduce GI and improve performance. In the Philippines, adherence to Department of Public Works and Highways (DPWH) standards, which rely on AASHTO classification, is mandatory for public roads. This application ensures pavement durability and cost-effectiveness.

Soil classification indicates permeability trends. Coarse-grained, well-graded soils with small D₁₀ values are highly permeable and require adequate drainage design to prevent pore pressure buildup. Fine-grained clays (high plasticity, e.g., CH) have very low permeability and may trap water, requiring controlled drainage or dewatering during construction. In the Philippine tropics with high rainfall, understanding soil permeability is critical for groundwater control, slope stability, and foundation drainage. Classification guides the selection of drainage materials and methods (e.g., geotextile type, aggregate gradation for drainage layers).

Soils with high plasticity index (PI > 20) and high liquid limit (LL > 50, classified as CH or MH) are prone to swelling upon moisture absorption. In the Philippines, such soils exist in some lowland areas. Early identification through classification allows engineers to design foundations that accommodate or prevent swelling: deep footings below the active zone, moisture-barrier systems, or reinforced concrete slabs designed for differential movement. Classification thus enables proactive management of construction risk in regions with expansive clays.

When natural soils do not meet project specifications (e.g., a poorly graded sand or high-PI clay for a structural fill), engineers design soil improvement measures. Classification determines the most appropriate method: blending of poorly graded soils with better-graded materials, chemical stabilization of high-PI clays with cement or lime, or use of geosynthetics for reinforcement. For instance, if a clay is classified as CH (high-plasticity clay) with GI = 18, stabilization with 4–6% cement by weight may reduce GI to acceptable levels. Classification guides these decisions and provides baseline metrics for performance evaluation post-improvement.