Method for predicting unknown Atterberg limits

ERDC TN-DOER-D1

August 2004

the plastic and liquid limits impact not only the classification but the behavior as well. As an

example, knowing the Atterberg limits enables prediction of the clumping capability in mechani-

cal dredging, which is needed for numerical models such as MDFATE. Laboratory testing

(ASTM 2000b) is required to determine the Atterberg limits. An expedient prediction method is

to conduct a single slump test and apply the following equations:

LL = 52.74 + 0.526W 59.97N

(2)

where

LL = liquid limit percent

W = water content percent

N = normalized slump = slump / cylinder height

Equation 2 is based on a multiple linear regression of 126 data points with R2 = 0.86.

LI = 1.601(W/LL) 0.612

(3)

where

LI = liquidity index

W/LL = water content percent / liquid limit percent

Equation 3 is based on a linear regression of 139 data points with R2 = 0.98.

( LI ) ( LL ) - W

PL =

(4)

LI - 1

where

PL = plastic limit percent

LI = liquidity index

LL = liquid limit percent

W = water content percent

Equation 4 is the standard soil mechanics textbook equation for liquidity index (Atkinson and

Bransby 1978) rearranged for determining the plastic limit. The plasticity index (PI) is then cal-

culated as the liquid limit minus the plastic limit, or

PI = LL PL

(5)

Thus by calculating the liquid limit using the slump test correlation (Equation 2), one may

determine the liquidity index, plastic limit, and the plasticity index. Although Equation 2 is based

on limited data to date with evident variance, there exists a strong correlation between water

content, normalized slump, and the liquid limit.

For a quick assessment of a dredged material's liquid limit consistency without using the stan-

dard test method, insert the sharpened tip of a common No. 2 pencil (with the tip shaved to an