USCS Classification (Cu, Cc, PI)

C_{u} = \frac{D _{60}}{D _{10}}, C_{c} = \frac{D _{30}^{2}}{D _{10} D _{60}}, P I = LL - P L

Coarse soils: well-graded gravel GW if Cu≥4 and 1≤Cc≤3 (sands SW: Cu≥6 and 1≤Cc≤3). Fines by Casagrande: CL vs ML vs CH vs MH via A-line PI=0.73(LL−20). Organic O, peat Pt.

$C u$: Uniformity coefficient
$C c$: Coefficient of curvature
$P I$: Plasticity index

Step-by-step solved example

Sand, <5% fines, D10=0.15, D30=0.40, D60=1.20 mm. SW or SP?

Grain-size curve D10, D30, D60 and Atterberg PI.

1. Cu, Cc
Cu=1.20/0.15=8.0 ≥6. Cc=0.16/(0.15×1.20)=0.89.
2. Grade
Cc=0.89 not in [1,3] → SP (poorly graded sand).

Answer: SP (Cu OK but Cc < 1)

10 practice questions

0/10 correct

1.A-line equation is

2.CH is

3.GW requires Cu≥

4.More than 12% fines on a sand makes it

5.D10 is

6.LL=40, PL=22: PI=

7.Pt is

8.Cc=2, Cu=8, clean sand →

9.AASHTO classification uses

10.Gap-graded soils often fail