Geotechnical Engineering

DHARM

68 GEOTECHNICAL ENGINEERING

coated with a thin layer of vaseline. The dish is then filled with the prepared soil paste in
instalments. Gentle tapping is given to the hard surface to eliminate entrapped air. The excess
soil is removed with the aid of a straight edge and any soil adhering to the outside of the dish
is wiped off. The weight of the wet soil pat of known volume is found (Wi). The dish is then
placed in an oven and the soil pat is allowed to dry up. The weight of the dry soil pat can be
found by weighing (Wd).

120° 120°

30 pitch circle dia.

Glass plate 75 × 75 × 3

Brass pin secured firmly

3 f 3

5 15

Glass plate with prongs

Before shrinkage After shrinkage

Wet soil Shrinkage

Dish

Dry soil

Mercury

Dry soil pat

Obtaining displaced mercury

Glass plate

with prongs

Ground surface

of top of glass cup

Evaporating dish

Glass cup

Mercury displaced

by soil pat

All dimensions are in millimetres
Fig. 3.21 Apparatus for determining volume-change in the shrinkage limit test
The glass cup is filled with mercury and excess is removed by pressing the glass plate
with three prongs firmly over the top. The dry soil pat is placed on the surface of the mercury
in the cup and carefully pressed by means of the glass plate with prongs. The weight of the
displaced mercury is found and divided by its unit weight to get the volume of the dry soil pat
(Vd). The shrinkage limit may then be obtained by Eq. 3.47 or 3.48.
Alternative approach: Shrinkage limit may also be determined by an alternative ap-
proach if the specific gravity of the soil solids has already been determined.

From Fig. 3.20 (iii),

ws =

()VV

W

d sw

d

−γ

× 100

=

V w

W

d

d

s

w

d

−

F

HG

I

γ KJ

γ

× 100

∴ ws =

V

W

dw G

d

F γ −

HG

I

KJ

1/ × 100 ...(Eq. 3.49)

This may also be written as

ws = [(γw/γd) – 1/G] × 100 ...(Eq. 3.50)

where γd = dry unit weight based on the minimum or dry volume.