The objective of the Atterberg limits test is
to obtain basic index information about the soil used to estimate strength
and settlement characteristics. It is the primary form of classification for
Fine-grained soil is tested to determine the liquid and plastic
limits, which are moisture contents that define boundaries between material consistency
states. These standardized tests produce comparable numbers used for soil
identification, classification and correlations to strength.
The liquid (LL) and plastic (PL) limits define the
water content boundaries between non-plastic, plastic and viscous fluid states. The
plasticity index (PI) defines the complete range of plastic state. Figure 1
illustrates it nicely.
Figure 1: Atterberg limits illustration.
Liquid Limit (LL)
The liquid limit defines the boundary between plastic and viscous fluid states.
It is determined using a standard "Liquid Limit Device," which drops a shallow
cupfull of soil 1 cm consistently. When a groove cut through the sample closes
1/2", the number of drops is recorded and a moisture content sample processed.
Repeating the procedure for a total of four drop-count ranges provides enough data to
plot on a semi-log scale. From the plot, the moisture content at 25 drops defines
the Liquid Limit.
Plastic Limit (PL)
The plastic limit defines the boundary between non-plastic and plastic states. It
is determined simply by rolling a thread of soil and adjusting the moisture content until
it breaks at 1/8 inch diameter.
Water content inaccuracies - small samples and scale
Procedural errors for the LL
- depth of grove
- soil not uniformly mixed
- handle turned too fast or too slow
- height of fall adjusted improperly
- length of closure not 1/2 in.
- air drying of soil between trials
Procedural errors for the PL
improper technique in rolling thread
- thread not 1/8 in. diameter
- moisture content sample too small
- air drying of soil before moisture content taken
references below and course notes for additional background information