Geotechnical Engineering

DHARM

36 GEOTECHNICAL ENGINEERING

3.5.2 Pycnometer Method

This method may be used when the specific gravity of solids is known. This is a relatively quick
method and is considered suitable for coarse-grained soils only.

The following are the steps involved:

(i) The weight of the empty pycnometer (Fig. 3.2) with its cap and washer is found

(W 1 ).

(ii) The wet soil sample is placed in the pycnometer (upto about 1/4 to 1/3 of the volume)

and its weight is obtained (W 2 ).

(iii) The pycnometer is gradually filled with water, stirring and mixing thoroughly with

a glass rod, such that water comes flush with the hole in the conical cap. The

pycnometer is dried on the outside with a cloth and its weight is obtained (W 3 ).

(iv) The pycnometer is emptied and cleaned thoroughly; it is filled with water upto the

hole in the conical cap, and its weight is obtained (W 4 ).

The water content of the soil sample may be calculated as follows:

w =

()

()

WW

WW

G

G

21

34

− (^11)
−
F −
HG
I
KJ
−
L
N
M
O
Q
P × 100% ...(Eq. 3.6)
This can be easily derived from the schematic phase diagrams shown in Fig. 3.4:
If the solids from (iii) are replaced with water, we W 4 of (iv).
Volume of solids = W
G
s
(a) Empty
pycnometer wt. W 1
(b) Pycnometer + wet
soil wt. W 2
(c) Pycnometer + wet
soil + water wt. W 3
(d) Pycnometer +
water wt. W 4
Water Water
Water
Solids Solids
Fig. 3.4 Determination of water content
W 4 = W 3 – Ws +
W
G
s
Ws 1
F −^1
HG
I
GKJ = W^3 – W^4
∴ Ws = (W 3 – W 4 ) [G/(G – 1)]
Weight of water Ww in the soil sample is given by:
Ww = (W 2 – W 1 ) – Ws
Water content, w =
W
W
w
s