Unified Soil Classification System


Criteria Group Symbol Group Name
Gravels

> 50% retained on No. 4 Sieve

Clean Gravels

< 5% fines

Cu ≥ 4 & 1 ≤ Cc ≤ 3 GW Well-graded gravel
Cu < 4 and/or Cc < 1 or Cc > 3 GP Poorly graded gravel
Gravels with Fines

> 12% fines

Fines ML or MH GM Silty gravel
Fines CL or CH GC Clayey gravel
Sands

> 50% of coarse fraction passed No. 4 Sieve

Clean Sands

< 5% fines

Cu ≥ 6 and 1 ≤ Cc ≤ 3 SW Well-graded sand
Cu < 6 and/or Cc < 1 or Cc > 3 SP Poorly graded sand
Sands with Fines

> 12% fines

Fines ML or MH SM Silty sand
Fines CL or CH SC Clayey sand
Silts and Clays

LL < 50

Inorganic PI > 7 and plots on or above "A" line CL Lean clay
PI < 4 or plots below "A" line ML Silt
Organic PI ≥ 4 and plots on or above "A" line OL Organic clay
PI < 4 or plots below "A" line OL Organic silt
Silts and Clays

LL ≥ 50

Inorganic PI plots on or above "A" line CH Fat clay
PI plots below "A" line MH Elastic silt
Organic Plots on or above "A" line OH Organic clay
Plots below "A" line OH Organic silt
Highly Organic Soils PT Peat

Reference

ASTM D2487-06 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)

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