Geotechnical Engineering

Soil Classification Helper

Screen a preliminary USCS soil group using gradation percentages and Atterberg limits.

Engineering schematic

Gradation and plasticity screening

Static symbolic figure. Enter project values in the calculator fields below.

Combined gravel-sand-fines composition bar and simplified Casagrande plasticity chart with LL, PI, the A-line equation, and a generic test point. — Symbols
LL
liquid limit
PI
plasticity index
A-line
PI = 0.73(LL − 20)

Result

Preliminary USCS group: SC

Coarse fraction

Fines fraction

A-line PI

16.1

USCS symbol

Step-by-step calculation

Coarse fraction
$P_{\mathrm{coarse}}=P_{\mathrm{gravel}}+P_{\mathrm{sand}}=65\%$
A-line plasticity index
$PI_A=0.73(42-20)=16.1$
Measured plasticity index
$PI=18$

What this means

Use this helper as a screening aid only. Full USCS classification requires gradation curve details and lab procedures.

Assumptions

Percentages are dry mass percentages
Organic soils and dual symbols require additional checks
Borderline symbols may apply near classification boundaries

Formula Used

PI_A=0.73\left(LL-20\right)

Step-by-Step Example

A soil with 20% gravel, 45% sand, 35% fines, LL=42, and PI=18 screens as SC.

Engineering Notes

Check units, assumptions, boundary conditions, groundwater, drainage, and whether the simplified method is appropriate before using a result in any project decision.

FAQ

Is this a lab classification?

No. It is a screening helper and does not replace ASTM or project lab procedures.