Civil Engineering Reference
In-Depth Information
Fig. 1.4
Changes in total volume against water content.
at values of water content below this level the soil is partially saturated. In other words, below the shrink-
age limit the volume of the soil remains constant with further drying, but the weight of the soil decreases
until the soil is fully dried.
In Fig.
1.4
the variation of the total volume of a soil with its water content is plotted, showing the posi-
tions of the liquid, plastic and shrinkage limits.
Determination of liquid and plastic limits
Liquid limit test
BS 1377: Part 2 specifies the following three methods for determining the liquid limit of soil.
(1)
Cone penetrometer method (definitive method)
Details of the apparatus are shown in Fig.
1.5.
The soil to be tested is air dried and thoroughly mixed.
At least 200 g of the soil is sieved through a 425
μ
m sieve and placed on a glass plate. The soil is then
mixed with distilled water into a paste.
A metal cup, approximately 55 mm in diameter and 40 mm deep, is filled with the paste and the
surface struck off level. The cone, of mass 80 g, is next placed at the centre of the smoothed soil
surface and level with it. The cone is released so that it penetrates into the soil and the amount of
penetration, over a time period of 5 seconds, is measured.
The test is now repeated by lifting the cone clear, cleaning it and filling up the depression in the
surface of the soil by adding a little more of the wet soil.
If the difference between the two measured penetrations is less than 0.5 mm then the tests are
considered valid. The average penetration is noted and a water content determination is carried out
on the soil tested.
The procedure is repeated at least four times with increasing water contents. The amount of water
used throughout should be such that the penetrations obtained lie within a range of 15 to 25 mm.
To obtain the liquid limit the variation of cone penetration is plotted against water content and the
best straight line is drawn through the experimental points. The liquid limit is taken to be the water
content corresponding to a cone penetration of 20 mm (expressed as a whole number).
