Textural Identification Properties of Soil

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Visual Classification System

• Based on visual identification of soils• Soils divided into three categories:

1. Granular2. Non-granular (Clays and Silts)3. Organic Soils

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Granular Soils

• Proportion • Gradation• Shape of gravel and course sand grains

noted.–Rounded, subrounded, subangular,

angular

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Size Ranges of Soil ComponentGranular Soils

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Non Granular Soils• Material passing No. 200 sieve.• Degree of plasticity –controlling factor–Clays (plastic)–Silts (non plastic)

• Occurs in combination with granular.• For granular size soils with clay sizes combine granular and

cohesive systems.

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Terms to Describe Plasticity

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Clays vs. SiltsIdentification Tests

• Differentiate between non-plastic cohesionless silt and plastic clay soils.

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1. Dilatancy Test

• Shake, squeeze and release wet soil.–Silt material when squeezed •Dry dull surface

–When released and shaken•Glassy wet surface

• Decreases with increasing clay content.

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2. Dry Strength Test• Portion of soil is dried completely.• Press material between fingers.• Clays– High Strength– Doesn’t break easily– Predominantly clay materials

• Silts– Very low strength– Breaks apart easily

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3. Stickiness

• Clays - High degree of stickiness and smooth in the natural state.

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4. Soil Thread Test• Estimates degree of plasticity. • Differentiates between organic and inorganic

materials.• Take sample and add water.• Roll out threads 1/8” in diameter.• Fold and repeat until thread crumbles.• Similar to Plastic Limit test

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5. Thread Diameter Test

• Adjust water content: –For ball to form & flatten when dropped 2

feet.• Roll smallest non-crumbled thread• The thinner the thread the more plastic

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Relationships for Identification

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6. Dispersion Test

• Determine the settling rate–Shake sample in a water filled test tube.–Allow particles to settle:

a) Sand and Gravel settle out in seconds to minutesb) Pure silts settle out in less than 1 hourc) Clays remain suspended for hours to days

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How Soil Feels Matters

• Tough–highly plastic, inorganic fat clay

• Weak–Leaner sandy, or silty clays.

• Spongy or elastic–Organic and inorganic

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Organic Soils

• Contain organic matter• Affect the engineering properties of soil.• Examples-Topsoil, peat, and organic silt.

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Clay Soils

• Any color• More plastic means

more effort to pull apart.

• High plasticity – Smear looks shiny

and waxy.

Organic Soils

• Dark gray, black, and brown

• Have a Hydrogen sulfide smell (stink).

• Smear looks dull and silty.

• Low shear strength• Respond to dilatancy

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Identification of fine-grained soils

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Describing Soil

–Color–Predominant Soil Type–Grain Size–Depositional Features–Degree of Saturation/Relative Moisture–Density (granular soils) or consistency

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Describing Soil

• Major Constituent is upper Case• Second (20-50%) constituent is adjective

modifying major.–Example (70% gravel, 30 % sand)=

sandy GRAVEL

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Constituents 10-20%

• Come after the main constituent with a hyphen.–Example (55% gravel, 30% sand, 15%

silt)= sandy GRAVEL-trace to some silt.

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Constituents 10% or less

• Indicated by “trace” and come after grain size.–Example (50% Gravel, 30% sand, 12%

silt, & 8% clay)= sandy GRAVEL, trace to some silt, trace clay.

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Two Components=20-35%

• Exist as secondary components• “Some” is used for one–Example (52% sand, 27% clay, and 21%

silt)= clayey SAND-some silt

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Descriptive TermsDepositional Features

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Descriptive TermsSoil Color

• Confined to a few basic terms.• Terms can be combined to give pairs.• Useful descriptive terms are helpful.–Example-Rusty or reddish brown.

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Descriptive TermsDensity – Granular Soils

• Based on Standard Penetration Test (SPT) results performed in test borings.–N-Value - Number of blows of a falling

weight required to drive a sample spoon a given distance into soil.

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Descriptive TermsConsistency – Non Granular

• Based on Unconfined Compression Test–Axially loading a soil sample under a

constant strain rate until failure is obtained. (Also can be related to pocket penetrometer testing.)

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Other Descriptive Terms

• Topsoil: Organic soil at surface • Peat: Soft organic soils –Lots of decaying vegetable matter.

• Varved: Alternating thin layers of silt and clay.

• Glacial Till: Dense with a variety of grain sizes.

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Other Descriptive Terms

• Interbedded: Hard to differentiate between various soil types.

• Loess: Wind blown deposits of silt. – immediately under the topsoil.

• Fill: Artificially placed material.

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Classification Procedure

• Granular Soils–Visual estimation of the % of fraction

passing through sieves by wet sieving.• Cohesive Soils–Use 1 or more of the identification tests.

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