Geotechnical Engineering

DHARM

58 GEOTECHNICAL ENGINEERING

Quantitatively speaking, the uniformity of a soil is defined by its “Coefficient of Uni-

formity” U:

U =

D

D

60

10

...(Eq. 3.35)

where D 60 = 60% finer size.

and D 10 = 10% finer size, or effective size.

These can be obtained from the grain-size distribution curve as shown in Fig. 3.15.

The soil is said to be very unifrom, if U < 5; it is said to be of medium uniformity, if well-

graded, U= 5 to 15; and it is said to be very non-uniform or well-graded, if U > 15.

Another parameter or index which represents the shape of the grain-size distribution

curve is known as the “Coefficient of Curvature”, Cc, defined as:

Cc =

()

.

D

DD

30

2

10 60

...(Eq. 3.36)

where D 30 = 30% finer size.
Cc should be 1 to 3 for a well-graded soil.
On the average,
for sands U = 10 to 20,
for silts U = 2 to 4, and
for clays U = 10 to 100 (Jumikis, 1962)

Consistency of Clay So4a

‘Consistency’ is that property of a material which is manifested by its resistance to flow. In this

sense, consistency of a soil refers to the resistance offered by it against forces that tend to

deform or rupture the soil aggregate; in other words, it represents the relative ease with which

the soil may be deformed. Consistency may also be looked upon as the degree of firmness of a

soil and is often directly related to strength. This is applicable specifically to clay soils and is

generally related to the water content.

Consistency is conventionally described as soft, medium stiff (or medium firm), stiff (or

firm), or hard. These terms are unfortunately relative and may convey different meaning to

different persons. In the case of in-situ or undisturbed clays, it is reasonable and practical to

relate consistency to strength, for purposes of standardisation. (A little more of this aspect will

be studied in one of the later sections).

In the remoulded state, the consistency of a clay soil varies with the water content,

which tends to destroy the cohesion exhibited by the particles of such a soil. As the water

content is reduced from a soil from the stage of almost a suspension, the soil passes through

various states of consistency, as shown in Fig. 3.16. A. Atterberg, a Swedish Soil Scientist, in

1911, formally distinguished the following stages of consistency–liquid, plastic, semi-solid,

and solid. The water contents at which the soil passes from one of these states to the next have

been arbitrarity designated as ‘consistency limits’–Liquid limit, Plastic limit and Shrinkage

limit, in that order. These are called ‘Atterberg limits’ in honour of the originator of the concept.