USCS classification from PSD curves

Reading the curves

For each soil, the larger percentage is taken as the percent passing the 4.75 mm sieve and the smaller as the percent passing the 0.075 mm sieve.

Soil A

Fines $= 55\% > 50\%$, so it is fine-grained.

$$I_{p,A\text{-line}} = 0.73(45 - 20) = 18.25\%$$

$I_p = 30 > 18.25$: above the A-line (clay); $w_L = 45 < 50$ (low plasticity).

Soil A: CL, sandy lean clay.

Soil B

Fines $= 13\%$, which is more than 12% but less than 50%, so it is coarse-grained with fines.

• Gravel $= 100 - 63 = 37\%$
• Sand $= 63 - 13 = 50\%$

Sand is more than half of the coarse fraction ($50/87 = 57\%$), so it is S.

$$I_{p,A\text{-line}} = 0.73(25 - 20) = 3.65\%$$

$I_p = 15 > 7$ and above the A-line: clayey fines.

Soil B: SC, clayey sand with gravel.

Plasticity chart

  Ip (%)
  35 |                    |      /   A-line
  30 |                  A |   /
  25 |                    | /
  20 |                   /|
  15 |          B     /   |
  10 |             /      |
   5 |           /        |
   0 |        /           |
     +------------------------------ wL (%)
      0       20 25        50

Point A (45, 30) and point B (25, 15) both lie above the A-line.