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USCS versus AASHTO

Short answer

Both use grain size and Atterberg limits. USCS uses 50% passing 75 micron boundary and letter symbols; AASHTO uses 35% boundary and A-1 to A-7 with group index. USCS uses plasticity chart; AASHTO does not and rates subgrade quality.

Point out similarities and differences between the USCS system and the AASHTO system of soil classification.

Both classify soil using grain size distribution and Atterberg limits.

Similarities

  • Both use sieve analysis and liquid and plastic limits.
  • Both are based on the fraction of soil finer than 75 mm.
  • Both separate coarse-grained from fine-grained soil using the 75 micron sieve, though at different percentages.
  • Both are used widely in engineering practice.

Differences

BasisUSCSAASHTO
PurposeGeneral engineeringHighway subgrade
Coarse/fine boundary50% passing 75 micron35% passing 75 micron
SymbolsLetters (GW, SC, CH)A-1 to A-7 with group index
Plasticity chartUsed (A-line)Not used
Organic soilSeparate OL, OH, PtNo separate group
Group indexNot usedUsed to rate subgrade
RatingOrder of groups not numericalSubgrade quality worsens from A-1 to A-7

✓ Answer checked by Claude Opus 5.5 (high effort)

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