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USCS classification, 75 micron 39%

Short answer

39% finer than 75µm makes it coarse-grained with >12% fines. The fines have PI=15% and plot above A-line for LL=33%, so clayey; assuming sand predominates, symbol SC.

Classify the following soil as per the unified soil classification system: soil passing from 75 µ sieve = 39%, liquid limit = 33%, plastic limit = 18%.

Given

Passing 75 micron =39%= 39\%; wL=33%w_L = 33\%; wP=18%w_P = 18\%.

Ip=33−18=15%I_p = 33 - 18 = 15\%
  • Only 39% is finer than 75 micron (less than 50%), so the soil is coarse-grained, with more than 12% fines.
  • Fines classification: A-line at wL=33w_L = 33: Ip=0.73(33−20)=9.5%I_p = 0.73(33-20) = 9.5\%. Ip=15I_p = 15 lies above the A-line and is greater than 7, so the fines are clayey (C).
  • The gravel/sand split is not given. Assuming the coarse fraction is mainly sand (the usual case), the symbol is SC; if gravel dominates it would be GC.

Answer: SC, clayey sand (GC if gravel exceeds sand).

✓ Answer checked by Claude Opus 5.5 (high effort)

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