DHARM50 GEOTECHNICAL ENGINEERINGwould remove calcium compounds. Later, a deflocculating or a dispersing agent, such as so-
dium hexameterphosphate is added to the solution. (Further details regarding the prepara-
tion of the sample may be obtained from IS: 2720 (Part IV)–1985 and its revised versions). The
mixture is shaken thoroughly by means of a mechanical stirrer and the test is started, keeping
the jar vertical. The soil particles are assumed to be uniformly distributed throughout the
suspension, at the instant of commencement of the test. After the lapse time t, only
those particles which have settled less than depth H would remain in suspension. The size of
the particles, finer than those which have settled to depth H or more at this instant, can be
found from Eqs. 3.21 and 3.22, Hence, sampling at different time intervals (by pipette), or
determining the specific gravity of the suspension (by hydrometer), at this sampling depth,
would provide the means of determining the content of particles of different sizes. (The logic
would become much clearer if all particles are considered to be of the same size). Since, the soil
particles are dispersed uniformly throughout the suspension, and according to Stoke’s law,
particles of the same size settle at the same rate, particles of a given size, wherever they exist,
have the same degree of concentration as at the commencement of the test. As such, particles
smaller than a given size will be present in the same degree of concentration as at the start,
and particles larger than this size would have settled already below the sampling depth, and
hence are not present at that depth. The percentage of particles finer than a specified size may
be got by determining their concentration at that depth at different times either with the aid
of a pipette or of a hydrometer.
The limitations of sedimentation analysis, based on Stokes’ law, or the assumptions are
as follows:
(i) The finer soil particles are never perfectly spherical. Their shape is flake-like or
needle-like. However, the particles are assumed to be spheres, with equivalent
diameters, the basis of equivalence being the attainment of the same terminal velocity
as that in the case of a perfect sphere.
(ii) Stokes’ law is applicable to a sphere falling freely without any interference, in an
infinite liquid medium. The sedimentation analysis is conducted in a one-litre jar,
the depth being finite; the walls of the jar could provide a source of interference to
the free fall of particles near it. The fall of any particle may be affected by the presence
of adjacent particles; thus, the fall may not be really free.
However, it is assumed that the effect of these sources of interference is insignificant
if suspension is prepared with about 50 g of soil per litre of water.
(iii) All the soil grains may not have the same specific gravity. However, an average
value is considered all right, since the variation may be insignificant in the case of
particles constituting the fine fraction.
(iv) Particles constituting to fine soil fraction may carry surface electric charges, which
have a tendency to create ‘flocs’. Unless these floces are broken, the sizes calculated
may be those of the flocs. Flocs can be a source of erroneous results.
A deflocculating agent, such as sodium silicate, sodium oxalate, or sodium hexa-
metaphosphate, is used to get over this difficulty.Pipette Analysis
The sedimentation analysis may be conducted with the aid of a pipette in the labora-
tory. A pipette, sedimentation jar, and a number of sampling bottles are necessary for the test.