Atterberg Limits and Plasticity Index

PI = LL - PL, LI = \frac{w - PL}{PI}

LL (liquid limit) and PL (plastic limit) are water contents at the Casagrande boundaries. PI = LL − PL measures plasticity; used on the Casagrande chart (A-line PI = 0.73(LL − 20)) to classify fine-grained soils. Liquidity index LI locates natural w between PL (LI = 0) and LL (LI = 1). Activity = PI / (% clay).

$PI$ (%): Plasticity index
$LL$ (%): Liquid limit
$PL$ (%): Plastic limit
$LI$: Liquidity index
$w$ (%): Natural water content

Step-by-step solved example

LL = 62, PL = 28, natural w = 40. Find PI and LI.

Water content moving the soil from solid to plastic to liquid.

1. PI
PI = 62 − 28 = 34.
$PI = 34$
2. LI
LI = (40 − 28)/34 = 12/34 = 0.353.
$LI = 0.353$

Answer: PI = 34, LI = 0.35

10 practice questions

0/10 correct

1.LL = 55, PL = 25. PI =

2.A soil with w = PL has LI =

3.A-line on the plasticity chart is

4.USCS: a fine soil above the A-line with LL = 60 is

5.w = 50, PL = 20, PI = 40. LI =

6.If natural w exceeds LL, the soil is

7.Shrinkage limit SL is the water content where

8.Nonplastic silt (NP) has

9.Activity = PI / (clay fraction). High activity (> 1.25) suggests

10.Liquidity index of a soil at the liquid limit is