Soil Types & Classification

Soil is not a single material — it is a complex mixture of mineral particles, organic matter, water, and air. The behavior of a soil deposit depends on the sizes and shapes of its particles, the minerals present, the amount of water in the pore spaces, and how the soil was deposited and loaded over geological time. Two sites separated by only a few hundred feet can have dramatically different soil conditions, which is why soil classification is so important to construction.

This lesson teaches you to identify and classify soils using both field techniques and standardized laboratory systems. You will learn the major soil types, how grain size and plasticity define soil behavior, and how the two most widely used classification systems — the Unified Soil Classification System (USCS) and the AASHTO system — categorize soils for engineering purposes.

All soils are composed of particles derived from the weathering of rock. These particles are classified by size into four primary groups:

Gravel

Gravel consists of coarse particles ranging from 4.75 mm (retained on a No. 4 sieve) to 75 mm (3 inches) in diameter. Gravels are individual rock fragments that are easily visible and distinguishable. Key characteristics include:

• High bearing capacity: Gravel is among the best foundation soils because it distributes loads well and resists compression.
• Excellent drainage: The large pore spaces between gravel particles allow water to pass through freely. Gravel is virtually unaffected by moisture changes.
• Low compressibility: Gravel settles very little under load, making it ideal for foundations and base courses.
• Non-cohesive: Gravel particles do not stick together — they derive their strength from friction between particles and interlocking.
• Easy to compact: Gravel responds well to vibratory compaction.

Common gravel types include river gravel (rounded, from water erosion), crushed stone (angular, from quarry operations), and pit-run gravel (a naturally occurring mix of gravel and sand).

Sand

Sand particles range from 0.075 mm (passing a No. 200 sieve) to 4.75 mm (retained on a No. 4 sieve). Sand is visible to the naked eye and feels gritty when rubbed between your fingers. Characteristics include:

• Good bearing capacity: Clean sand is a good foundation soil, though not as strong as gravel.
• Good drainage: Sand drains well, though not as rapidly as gravel. Fine sands drain more slowly than coarse sands.
• Non-cohesive: Like gravel, sand derives its strength from inter-particle friction. Dry sand has no cohesion — it cannot be molded into a shape (unless slightly moist, which creates temporary apparent cohesion from capillary forces).
• Susceptible to erosion: Sand is easily moved by flowing water and wind.
• Potential for liquefaction: Loose, saturated sands can lose all strength during seismic events — a phenomenon called liquefaction where the sand temporarily behaves like a liquid.

Sand is subdivided into coarse sand (2.0–4.75 mm), medium sand (0.425–2.0 mm), and fine sand (0.075–0.425 mm).

Silt

Silt particles range from 0.002 mm to 0.075 mm — too small to see individually with the naked eye, but you can feel a slight grittiness. When dry, silt feels smooth like flour. Key characteristics include:

• Moderate bearing capacity: Silt can support light to moderate loads but is significantly weaker than sand or gravel.
• Poor drainage: Silt drains slowly because the small particle size creates small pore spaces.
• Low plasticity: Silt has some cohesion when moist but crumbles easily when dry. It does not exhibit the strong plastic behavior of clay.
• Highly erodible: Silt is the most erosion-prone soil type — easily carried by water and wind.
• Frost susceptible: Silt's capillary action draws water upward, making it highly susceptible to frost heave. In cold climates, silty soils are among the most problematic for foundations and pavements.
• Sensitive to moisture: Silt's properties change dramatically with water content. When saturated, silt can become very soft and weak (a property called sensitivity).

The dilatancy test is a quick field identification method for silt: place a pat of moist soil in your palm and shake it horizontally. Water will quickly appear on the surface (the sample "weeps"), and when you squeeze the sample, the water disappears back into the soil. Clay does not exhibit this behavior.

Clay

Clay particles are smaller than 0.002 mm — invisible to the naked eye and visible only under electron microscopes. Clay is fundamentally different from the coarser soil types because its behavior is controlled by surface chemistry rather than particle friction. Characteristics include:

• Variable bearing capacity: Stiff, dry clay can have good bearing capacity, but soft, saturated clay is extremely weak. Clay's strength depends heavily on moisture content and loading history.
• Very poor drainage: Clay is essentially impervious to water flow. Its permeability is typically 10,000 to 1,000,000 times lower than sand.
• High plasticity: Clay can be molded into shapes that hold their form when wet. This is due to the electrochemical bonds between clay particles and the thin films of water attracted to clay mineral surfaces.
• Cohesive: Clay particles are held together by electrochemical forces, giving clay significant cohesion (internal stickiness). Clay can stand in vertical cuts — at least temporarily — because of this cohesion.
• Shrink-swell behavior: Many clays expand significantly when they absorb water and shrink when they dry out. Expansive clays (particularly those containing the mineral montmorillonite) can exert tremendous swelling pressures — enough to crack foundations, lift slabs, and break retaining walls. Expansive clays cause more property damage in the United States annually than floods, earthquakes, and tornados combined.
• Consolidation: When loaded, clay compresses slowly over time as water is squeezed out of the pore spaces. This process, called consolidation, can continue for months or years, causing long-term settlement of structures.

The ribbon test is a field identification method for clay: roll moist soil between your palms into a thread about 1/8 inch in diameter. If the thread holds together and can be formed into a ribbon that extends several inches without breaking, the soil has significant clay content.

Organic soils contain decomposed plant matter (humus) mixed with mineral particles. They are identified by their dark color, spongy texture, and organic odor (especially when heated). Organic soils are never suitable for supporting foundations because:

• They are highly compressible — they settle excessively under load
• They continue to decompose over time, causing additional settlement
• Their properties are erratic and unpredictable
• They have very low shear strength

Common organic soils include peat (partially decomposed plant material, found in bogs and wetlands) and muck (fully decomposed organic material). When organic soils are encountered at a construction site, they must be removed entirely and replaced with suitable fill material.

The distribution of particle sizes in a soil sample is determined by a sieve analysis (also called a gradation test). The soil is dried, weighed, and passed through a series of sieves with progressively smaller openings. The weight of soil retained on each sieve is recorded, and the results are plotted on a grain size distribution curve — a graph with particle size on the horizontal axis (log scale) and the percentage of soil finer than each size on the vertical axis.

Key terms from the gradation curve:

• D10 (Effective Size): The particle size at which 10% of the soil is finer. This value controls the soil's permeability (hydraulic conductivity).
• D30: The particle size at which 30% of the soil is finer.
• D60: The particle size at which 60% of the soil is finer.
• Coefficient of Uniformity (Cu): Cu = D60 / D10. A measure of the range of particle sizes. Cu > 4 (for gravels) or Cu > 6 (for sands) indicates a wide range of sizes — a well-graded soil.
• Coefficient of Curvature (Cc): Cc = (D30)² / (D10 × D60). A measure of the shape of the gradation curve. Values between 1 and 3 indicate a well-graded soil.

Well-graded soils contain a good distribution of particle sizes — small particles fill the voids between larger particles, resulting in a dense, strong soil with low permeability. Well-graded soils are excellent for construction because they compact well and have high bearing capacity.

Poorly graded (uniform) soils have particles that are all roughly the same size. They have large void spaces between particles, lower density, and lower bearing capacity than well-graded soils.

Gap-graded soils are missing particles of certain intermediate sizes. They have specific intermediate sizes absent from the gradation.

For particles finer than the No. 200 sieve (0.075 mm) — silts and clays — a sieve analysis cannot differentiate between them. Instead, a hydrometer analysis is performed, which measures the rate at which particles settle in a water suspension (larger particles settle faster per Stokes' Law).

For fine-grained soils (silts and clays), particle size alone does not adequately describe behavior. The Atterberg limits define the boundaries between different states of soil consistency based on moisture content:

• Shrinkage Limit (SL): The moisture content below which further drying does not cause additional volume decrease. Below this point, the soil is a solid.
• Plastic Limit (PL): The moisture content at which the soil transitions from semi-solid to plastic behavior. Determined by rolling a thread of soil to 1/8-inch diameter — the moisture content at which the thread crumbles is the PL. Below the PL, the soil is semi-solid and crumbles when deformed.
• Liquid Limit (LL): The moisture content at which the soil transitions from plastic to liquid behavior. Determined using the Casagrande apparatus — a brass cup with soil grooved by a standard tool, dropped repeatedly until the groove closes over a half-inch length in 25 blows. Above the LL, the soil flows like a viscous liquid.
• Plasticity Index (PI): PI = LL – PL. This is the range of moisture content over which the soil is plastic (moldable). A high PI indicates a highly plastic clay; a low PI indicates silt or a low-plasticity clay.

PI Range	Soil Description	Construction Implications
0–3	Non-plastic (silt)	Low cohesion, frost susceptible
4–7	Low plasticity	Moderate workability
8–20	Medium plasticity	May be expansive; moderate risk
21–40	High plasticity	Likely expansive; high shrink-swell risk
> 40	Very high plasticity	Very expansive; severe foundation risk

The USCS (ASTM D2487) is the most widely used classification system in geotechnical engineering and construction. It classifies soils into groups based on grain size distribution and Atterberg limits using a two-letter symbol:

First letter — Primary soil type:

• G — Gravel (more than 50% of coarse fraction retained on No. 4 sieve)
• S — Sand (more than 50% of coarse fraction passes No. 4 sieve)
• M — Silt (inorganic fine-grained soil, plots below the A-line on the plasticity chart)
• C — Clay (inorganic fine-grained soil, plots above the A-line)
• O — Organic soil
• Pt — Peat

Second letter — Subdivision:

• W — Well-graded (for coarse-grained soils: meets both Cu and Cc criteria)
• P — Poorly graded (for coarse-grained soils: does not meet gradation criteria)
• H — High plasticity (for fine-grained soils: LL ≥ 50)
• L — Low plasticity (for fine-grained soils: LL < 50)

Common USCS classifications and their construction suitability:

USCS Symbol	Description	Suitability for Foundations
GW	Well-graded gravel	Excellent
GP	Poorly graded gravel	Good to excellent
GM	Silty gravel	Good
GC	Clayey gravel	Good
SW	Well-graded sand	Good
SP	Poorly graded sand	Fair to good
SM	Silty sand	Fair
SC	Clayey sand	Fair
ML	Low-plasticity silt	Fair to poor
CL	Low-plasticity clay	Fair to poor
MH	High-plasticity silt	Poor
CH	High-plasticity clay	Poor to very poor
OL/OH	Organic silt/clay	Very poor — unsuitable
Pt	Peat	Unsuitable — must be removed

The USCS also uses a plasticity chart (Casagrande chart) to classify fine-grained soils. The chart plots Plasticity Index (PI) on the vertical axis against Liquid Limit (LL) on the horizontal axis. The A-line (PI = 0.73 × (LL – 20)) separates clays (above) from silts (below).

The AASHTO system (American Association of State Highway and Transportation Officials, AASHTO M145) is used primarily for highway and pavement design. It classifies soils into groups A-1 through A-7:

Group	Description	Subgrade Rating
A-1	Stone fragments, gravel, sand	Excellent
A-2	Gravelly/sandy soils with fines	Good to excellent
A-3	Fine sand	Fair to good
A-4	Silty soils (LL < 40, PI < 10)	Fair to poor
A-5	Silty soils (LL > 40, PI < 10)	Poor
A-6	Clayey soils (LL < 40, PI > 10)	Poor
A-7	Clayey soils (LL > 40, PI > 10)	Very poor

The AASHTO system also assigns a Group Index (GI) — a number from 0 to 20+ that increases with the percentage of fines and plasticity. A higher GI means a poorer subgrade material.

Before laboratory results are available, construction professionals use several field tests to estimate soil type:

1. Visual inspection: Examine particle size, color, and texture. Gravel and coarse sand are easily visible; silt looks smooth; clay is sticky.
2. Dilatancy (shaking) test: Described above — distinguishes silt from clay.
3. Ribbon test: Also described above — measures plasticity (clay content).
4. Dry strength test: Mold a pat of soil and let it dry. High dry strength indicates clay; low dry strength that crumbles easily indicates silt.
5. Odor test: Organic soils have a distinctive musty or decaying odor, especially when heated.
6. Bite test: Place a small amount of soil between your teeth. Sand feels gritty; silt feels smooth like flour; clay feels sticky.