Soil Classification

Q: What does the A-line on the plasticity chart represent?

The A-line (PI = 0.73 x (LL - 20)) separates clays from silts on the Casagrande plasticity chart. Soils plotting above the A-line are classified as clay (C), while those below are silt (M). The vertical line at LL = 50 separates low-plasticity (L) from high-plasticity (H) soils. For example, CL is low-plasticity clay (above A-line, LL 50).

Free reference guide: Soil Classification

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About Soil Classification

The Soil Classification Reference is a searchable guide to the two primary soil classification systems used in geotechnical engineering: the Unified Soil Classification System (USCS) and the AASHTO classification system. For USCS, it provides complete specifications for all coarse-grained soil types (GW, GP, GM, GC, SW, SP, SM, SC) with particle size criteria, fines content thresholds, uniformity and curvature coefficient requirements, and corresponding engineering properties including bearing capacity, SPT N-values, and permeability coefficients.

The fine-grained USCS classifications (CL, CH, ML, MH, OL, OH, PT) are detailed with liquid limit boundaries, plasticity chart position relative to the A-line, compression indices, swelling potential, and frost heave susceptibility. Each classification includes typical bearing capacity ranges, SPT N-value ranges, and permeability values. The AASHTO system (A-1 through A-7) is covered with sieve analysis criteria, plasticity requirements, Group Index calculations, and USCS equivalents for cross-referencing.

Beyond classification, this reference includes essential laboratory and field test parameters: uniformity coefficient (Cu) and coefficient of curvature (Cc) for gradation analysis, liquid limit (LL) and plasticity index (PI) for Atterberg limits, SPT N-value interpretation for both sandy and clayey soils, and compaction test results (standard and modified Proctor) with typical OMC and maximum dry density values. Physical property reference data covers friction angle, bearing capacity, permeability, and unit weight ranges for all major soil types.

Key Features

Complete USCS coarse-grained classification (GW/GP/GM/GC/SW/SP/SM/SC) with Cu, Cc, and fines content criteria
USCS fine-grained classification (CL/CH/ML/MH/OL/OH/PT) with liquid limit, A-line position, and compression index
AASHTO classification (A-1 through A-7) with sieve criteria, Group Index formula, and USCS cross-reference
Atterberg limits reference including liquid limit ranges, plasticity index interpretation, and activity calculations
SPT N-value interpretation tables for relative density (sandy) and consistency (clayey) with bearing capacity correlations
Standard and Modified Proctor compaction test reference with typical OMC and maximum dry density by soil type
Friction angle ranges for loose to dense sand and effective/undrained conditions for cohesive soils
Permeability coefficient ranges from clean gravel (10^0 cm/s) to clay (10^-9 cm/s) with test method recommendations

Frequently Asked Questions

What is the difference between USCS and AASHTO soil classification?

USCS (Unified Soil Classification System) uses a two-letter symbol (e.g., SW, CL) based on grain size distribution and Atterberg limits, primarily for general geotechnical engineering. AASHTO classification (A-1 through A-7) was developed specifically for highway and pavement engineering, using Group Index values to rate soil suitability as subgrade material. The two systems can be cross-referenced: for example, GW/GP/SW/SP in USCS typically correspond to A-1 in AASHTO.

How do I determine if gravel or sand is well-graded (W) or poorly-graded (P)?

Calculate the uniformity coefficient Cu = D60/D10 and coefficient of curvature Cc = (D30)^2/(D10 x D60). For gravel, well-graded requires Cu > 4 and 1 < Cc < 3. For sand, well-graded requires Cu > 6 and 1 < Cc < 3. Both conditions must be met simultaneously. If either criterion fails, the soil is classified as poorly-graded (GP or SP). A high Cu indicates a wide range of particle sizes, while Cc ensures a smooth gradation curve.

What does the A-line on the plasticity chart represent?

The A-line (PI = 0.73 x (LL - 20)) separates clays from silts on the Casagrande plasticity chart. Soils plotting above the A-line are classified as clay (C), while those below are silt (M). The vertical line at LL = 50 separates low-plasticity (L) from high-plasticity (H) soils. For example, CL is low-plasticity clay (above A-line, LL < 50), while MH is high-plasticity silt (below A-line, LL > 50).

How do I interpret SPT N-values for foundation design?

For sandy soils, N < 4 indicates very loose conditions, 10-30 is medium dense, and N > 50 is very dense. For clayey soils, N < 2 is very soft, 4-8 is medium, and N > 30 is hard. Approximate bearing capacity can be estimated as qa = N/4 kgf/cm2 for strip footings. N-values should be corrected for energy ratio (N60) using the equation N60 = (Em x CB x CS x CR / 60) x N before interpretation.

What is the typical bearing capacity range for different soil types?

Hard rock exceeds 4,000 kPa, dense gravel ranges 300-600 kPa, dense sand 200-400 kPa, stiff clay 100-200 kPa, medium clay 50-100 kPa, and soft clay only 25-50 kPa. Peat and organic soils have bearing capacity below 25 kPa and are generally unsuitable as foundation material. These are approximate values; actual design requires site-specific geotechnical investigation.

What is the difference between CH and CL clay classifications?

CL (low-plasticity clay) has a liquid limit below 50, plots above the A-line, with moderate compression (Cc = 0.2-0.4), SPT N-values of 5-15, and permeability of 10^-6 to 10^-8 cm/s. CH (high-plasticity clay) has LL > 50, higher compression (Cc = 0.4-1.0+), lower SPT N-values (2-8), and lower permeability (10^-7 to 10^-9 cm/s). CH clay has significantly higher swelling potential and develops desiccation cracks when dry.

How do I calculate the AASHTO Group Index?

Group Index GI = (F-35)[0.2 + 0.005(LL-40)] + 0.01(F-15)(PI-10), where F is the percentage passing the No. 200 sieve. A GI of 0 indicates excellent subgrade material (like A-1), while higher values indicate poorer performance. The GI ranges from 0 to about 20, with A-7-6 soils typically having the highest values. Negative partial terms are set to zero in the calculation.

What permeability values indicate soil is suitable for drainage versus liner applications?

For drainage applications (french drains, filter layers), soils need permeability above 10^-2 cm/s, which includes clean gravel (10^0 to 10^1 cm/s) and coarse sand (10^-1 to 10^0 cm/s). For liner or barrier applications (landfill liners, dam cores), soils need permeability below 10^-6 cm/s, which includes CL clay (10^-6 to 10^-8 cm/s) and CH clay (10^-7 to 10^-9 cm/s). Silt (10^-4 to 10^-6 cm/s) is generally too permeable for liners but too slow for drainage.