Geotechnical Engineering

DHARM

62 GEOTECHNICAL ENGINEERING

Table 3.5 Consistency classification

C.I. LI Consistency

1.00 to 0.75 0.00 to 0.25 Stiff

0.75 to 0.50 0.25 to 0.50 Medium-soft

0.50 to 0.25 0.50 to 0.75 Soft

0.25 to 0.00 0.75 to 1.00 Very soft

3.9.2 Laboratory Methods for the Determination of Consistency Limits and
Related Indices
The definitions of the consistency limits proposed by Atterberg are not, by themselves, adequate
for the determination of their numerical values in the laboratory, especially in view of the
arbitrary nature of these definitions. In view of this, Arthur Casagrade and others suggested
more practical definitions with special reference to the laboratory devices and methods developed
for the purpose of the determination of the consistency limits.

In this sub-section, the laboratory methods for determination of the liquid limit, plastic
limit, shrinkage limit, and other related concepts and indices will be studied, as standardized
and accepted by the Indian Standard Institution and incorporated in the codes or practice.

Determination of Liquid Limit

The liquid limit is determined in the laboratory with the aid of the standard mechanical liquid
limit device, designed by Arthur Casagrande and adopted by the ISI, as given in IS:2720 (Part
V)–1985. The apparatus required are the mechanical liquid limit device, grooving tool, porce-
lain evaporating dish, flat glass plate, spatula, palette knives, balance, oven wash bottle with
distilled water and containers. The soil sample should pass 425–μ IS Sieve. A sample of about
1.20 N should be taken. Two types of grooving tools—Type A (Casagrande type) and Type B
(ASTM type)—are used depending upon the nature of the soil. (Fig. 3.17).

The cam raises the brass cup to a specified height of 1 cm from where the cup drops
upon the block exerting a blow on the latter. The cranking is to be performed at a specified rate
of two rotations per second. The grooving tool is meant to cut a standard groove in the soil
sample just prior to giving blows.

Air-dried soil sample of 1.20 N passing 425–μ I.S. Sieve is taken and is mixed with water

and kneaded for achieving uniformity. The mixing time is specified as 5 to 10 min. by some

authorities. The soil paste is placed in the liquid limit cup, and levelled off with the help of the

spatula. A clean and sharp groove is cut in the middle by means of a grooving tool. The crank

is rotated at about 2 revolutions per second and the number of blows required to make the

halves of the soil pat separated by the groove meet for a length of about 12 mm is counted. The

soil cake before and after the test are shown in Fig. 3.17. The water content is determined from

a small quantity of the soil paste.

This operation is repeated a few more times at different consistencies or moisture contents.

The soil samples should be prepared at such consistencies that the number of blows or shocks

required to close the groove will be less and more than 25. The relationship between the number

of blows and corresponding moisture contents thus obtained are plotted on semi-logarithmic