USCS classification using D10, D30, D60

Given

Passing 75 micron (No. 200) $= 40\%$; retained on 4.75 mm (No. 4) $= 55\%$; $D_{10} = 1.2$ mm, $D_{30} = 2.6$ mm, $D_{60} = 3.8$ mm.

Step 1: Coarse or fine?

60% is retained on the 75 micron sieve, so the soil is coarse-grained.

Step 2: Gravel or sand?

• Gravel $= 55\%$
• Sand $= 100 - 55 - 40 = 5\%$

Gravel is more than half of the coarse fraction, so the soil is gravel (G).

Step 3: Grading parameters

$$C_u = \frac{D_{60}}{D_{10}} = \frac{3.8}{1.2} = 3.17, \qquad C_c = \frac{D_{30}^2}{D_{10}D_{60}} = \frac{2.6^2}{1.2 \times 3.8} = 1.48$$

The gravel fraction is poorly graded ($C_u < 4$), but $C_c$ is in the range 1 - 3.

Step 4: Fines content

The fines are 40%, which is more than 12%. So the grading is not used; the symbol depends on the type of fines (GM or GC) found from $w_L$ and $I_p$ using the A-line. The plasticity data are not given.

Answer: a gravel with fines, GM or GC (silty or clayey gravel). If the fines are silty the symbol is GM; if the fines plot above the A-line with $I_p > 7$ it is GC. The grading ($C_u = 3.17$, $C_c = 1.48$) of the coarse fraction is poor.