Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 59

910 87 6 5 4 3 2 189 8 7 6 5 4 3 2 90.1 765432 0.01

100

80

60

40

20

10

0

Per cent finer (by wirght)

Grain size mm

0.72 0.12

D = 0.72 mm 60

D = 0.12 mm 10

(Effective size)

U = D /D = 6.0 60 10

(Uniformity coefficient)

Fig. 3.15 Effective size and uniformity coefficient from grain-size distribution curve

45°

Solids

Vs

Air

Vd

Vp

Volume of soil mass

Solid

state

Vl Semi

solid

state

Plastic

state Liquid state

Linear

change

(Assumed)

Curvilinear

change

(true)

OWs Wp Wl

Water content %

LL = W = Liquid limit

PL = W = Plastic limit

SL = W = Shrinkage limit

V = Volume of soil mass at LL

V = Volume of soil mass at PL

V = Volume of soil mass at SL

V = Volume of soilds

t p s t p d s

SL PL LL

Fig. 3.16 Variation of volume of soil mass with variation of water content
Initially intended for use in agricultural soil science, the concept was later adapted for
engineering use in classification of soils. Although the consistency limits have little direct
meaning in so far as engineering properties of soils are concerned, correlations between these
and engineering properties have been established since then.

‘Plasticity’ of a soil is defined as that property which allows it to be deformed, without
rupture and without elastic rebound, and without a noticeable change in volume. Also, a soil is
said to be in a plastic state when the water content is such that it can change its shape without
producing surface cracks. Plasticity is probably the most conspicuous property of clay.