Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 51

A boiling tube of 500 ml capacity kept in a constant temperature bath may also be used in

place of a sedimenation jar. The capacity of the sampling pipette is usually 10 ml.

The method consists in drawing off 10 ml samples of soil suspension by means of the

sampling pipette from a standard depth of 10 cm at various time intervals after the start of

sedimentation. The soil-water suspension should have been prepared as has been mentioned

earlier. The usual total time intervals at which the samples are drawn are 30 s, 1 min., 2 min.,

4 min., 8 min., 5 min., 30 min., 1 h, 2h, and 4 h from the start of sedimentation. The pipette

should be inserted about 20 seconds prior to the chosen instant and the process of sucking

should not take more than 20 seconds. Each of the samples taken is transferred to a sampling

bottle and dried in an oven. The weight of solids, WD in the suspension, finer than a certain

size D, related to the time of sampling, may be found by careful weighing, from the concentra-

tion of these solids in the pipette sample. Let Ws be the weight of soil (fine fraction) used in the

suspension of volume V, and WD be the weight of soil particles finer than size D in the entire

suspension. Also, let Wp be the weight of solids in the pipette sample of volume Vp.

Then, by the argument presented in the general procedure for sedimentation analysis,

W

V

W

V

D p

p

= ...(Eq. 3.23)

or WD =

W

V

VW

V

V

p

p

p

p

. =

F

HG

I

KJ

...(Eq. 3.24)

The calculation will be somewhat as follows:
From Equations (3.21) and (3.22), for the known values of H and t, we obtain the size D.
Let the weight of solids per ml in the pipette sample be multiplied be multiplied by the
total volume of the suspension; this would give WD as defined in Eq. 3.24.

Percentage of particles finer than the size D, in the fine-fraction, Nf, is given by:

Nf =

W

W

D

s

× 100 ...(Eq. 3.25)

Substituting for WD from Eq. (3.24),

Nf =

W

W

V

V

p

sp

F

HG

I

KJ

F

HG

I

KJ

× 100 ...(Eqn. 3.26)

This is to be corrected if a dispersing agent is added. If w is the weight of the dispersing

agent added,

Nf =

()Ww

W

D

s

−

× 100 ...(Eq. 3.27)

For a combined sieve and sedimentation analysis, if W is the total dry weight of the soil

originally taken, the over-all percentage, N, of particles, finer than D, is given by:

N = Nf ×

W

W

f ...(Eq. 3.28)

where Wf = Weight of fine soil fraction out of the total weight of a soil sample, W, taken for the
combined sieve and sedimentation analysis.