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USCS classification, 75 micron 8%

Short answer

GP-GM, poorly graded gravel with silt and sand. Fines 8% require dual symbol; gravel is 63% of coarse fraction. Gravel grading fails Cc=4>3 despite Cu=6, and fines with Ip=4 are silt.

A soil sample on laboratory test gives the following results. Classify the soil and give its symbol as per the USCS classification system. Passing through 75-micron sieve = 8% Passing through 4.75 mm sieve = 42% Coefficient of uniformity = 6 Coefficient of curvature = 4 Plasticity index = 4

Given

Passing 75 micron =8%= 8\%; passing 4.75 mm =42%= 42\%; Cu=6C_u = 6; Cc=4C_c = 4; Ip=4I_p = 4.

Step 1: Coarse or fine?

92% is retained on the 75 micron sieve, so the soil is coarse-grained.

Step 2: Gravel or sand?

  • Retained on 4.75 mm (gravel) =100−42=58%= 100 - 42 = 58\%
  • Sand =42−8=34%= 42 - 8 = 34\%

Gravel is more than half of the coarse fraction (58/92=63%58/92 = 63\%), so the soil is gravel (G).

Step 3: Fines content

8% is between 5% and 12%, so a dual symbol is required.

Step 4: Grading

For well-graded gravel: Cu≥4C_u \ge 4 and 1≤Cc≤31 \le C_c \le 3.

  • Cu=6≥4C_u = 6 \ge 4: OK
  • Cc=4>3C_c = 4 > 3: fails

So the gravel is poorly graded (GP).

Step 5: Nature of fines

Ip=4I_p = 4 is only at the lower limit of the 4 - 7 band, and the liquid limit is not given. With IpI_p not above 4, the fines are classed as silt, so they are taken as silty (M).

Answer: GP-GM, poorly graded gravel with silt and sand.

✓ Answer checked by Claude Opus 5.5 (high effort)

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