Soil Classification

What is Soil?

Testing the Soil

Bearing Capacity

Settling Characteristics

Drainage

Site Planning

Function of Structure

Site Profile (Contour Map)

Geological Information

Soils Sample Data

Soil Samples

Samples Represent the Site

Taken about 1’-2’ below surface

Boring samples are taken at various locations and depths below the surface (subsurface) for deep foundations.

Unified Soil Classification System

Coarse-Grained Soils

Fine Grained Soils

Highly Organic Soils

Professional Development Lesson ID Code: 8006

Unified Soil Classification System

Sieve Sizes

Silt and clay can pass through the #200 sieve.

Gravels range from 3” down to the size of peas.

Grain Size

Click here to open the Grain Size Chart.

Gravel Identification

Sand Identification

Graded SoilWell-Graded Soils-have a good representation of all particle sizes from the largest to smallest

Poorly-Graded Soils- two typesUniformly-has soil particles nearly the same size.Gap-Graded- contains both large and small particles but the graduation

continuity is broken by the absence of some particle sizes.

Clean vs. Dirty Graded Soil

Clean: contains little or no fines (<5%)

Dirty: contains an appreciable amount of fines ( >12%)

Clay or Silt?

Dry Strength – crushing characteristics

Dilatancy – reaction to shaking

Toughness – consistency near plastic limit

Grain Shape

Soil Color* Can vary with moisture content* May indicate the presence of certain chemical

or impurities* Dark brown /black may indicate organic

material.* Gray, olive green indicate inorganic soils.* Gray-blue or gray-yellow indicates poor

drainage.* Red or yellow may indicate iron oxides.* White to pink may indicate considerable

silica, calcium carbonate, or aluminum compounds.

PlasticityAtterberg Limits:

Plastic Limit-lowest moisture content at which soils can be rolled into 1/8” diameter

thread without breaking.

Liquid Limit- minimum moisture content at which soil will flow when a small shear or cutting force is applied.

Soil Groups

Soil Type Gradation Liquid Limit

Gravel- G

Sand-S

Silt-M

Clay-C

Organic O

Peat- Pt

Well Graded- W

Poorly Graded - P

LL over 50- H

LL under 50 -L