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Atterberg Limits: Soil Mechanics Laboratory

General
background information

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 cohesive soils.

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.

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.

Error considerations

Water content inaccuracies - small samples and scale precision.

Procedural errors for the LL

- width of grove
- 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

Check textbook references below and course notes for additional background information

 

Apparatus
Get a bigger hammer

 

Liquid Limit (LL)

Liquid Limit Device with Casagrande grooving tool
Mixing bowl
Spatula
Squirt bottle
Water content cups and microwave
Balance sensitive to 0.01 gm
Drying lamp and fan if material is too wet

Plastic Limit (PL)

(in addition to LL equipment above)
Large glass plate
Water content can large enough to hold a water content cup

 

Procedure
Get a disk!

 

 

Procedure / Data Sheets [right-click | "save target as" xls]

Also see Figure 2.10 on p. 42 of Holtz, Kovaks and Sheahan (2011), section 2.3

References

Holtz, Kovaks and Sheahan (2011), section 2.7

Holtz and Kovaks (1981), pages 34-41.

Das (1998), sections 2.6 and 2.7, starting on page 55.

Gorrill (1998), pages 5-8.

Bowles (1986), pages 15-25.

ASTM D4318

AASHTO T89, T90

[full citations]

crestlogotiny.jpg (2k) Manion, William P. (wmanion@(nospam)maine.edu "Soil Mechanics Laboratory Course CIE 366." University of Maine, Civil and Environmental Engineering Department, Orono, Maine.  04 January 2011 02:33 PM.  http://www.civil.maine.edu/cie366/.