Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 45

Paraffin, being water-proof, prevents the entry of water into the soil sample, thus af-

fording a simple means to determine the volume of the sample,

Particle Size Distribution (Mechanical Analysis)

This classification test determines the range of sizes of particles in the soil and the percentage
of particles in each of these size ranges. This is also called ‘grain-size distribution’; ‘mechanical
analysis’ means the separation of a soil into its different size fractions.*
The particle-size distribution is found in two stages:
(i) Sieve analysis, for the coarse fraction.
(ii) Sedimentation analysis or wet analysis, for the fine fraction.
‘Sieving’ is the most direct method for determining particle sizes, but there are practical
lower limits to sieve openings that can be used for soils. This lower limit is approximately at
the smallest size attributed to sand particles (75μ or 0.075 mm).

Sieving is a screening process in which coarser fractions of soil are separated by means

of a series of graded mesh. Mechanical analysis is one of the oldest test methods for soils.

3.8.1 Nomenclature of Grain Sizes

Natural soils are mixtures of particles of various sizes and it is necessary to have a nomencla-

ture for the various fractions comprising particles lying between certain specified size limits.

Particle size is customarily expressed in terms of a single diameter. This is taken as the size of

the smallest square hole in a sieve, through which the particle will pass.

The Indian standard nomenclature is as follows:

Gravel ... 80 mm to 4.75 mm

Sand ... 4.75 mm to 0.075 mm

Silt ... 0.075 mm to 0.002 mm

Clay ... Less than 0.002 mm

3.8.2 Sieve Analysis

Certain sieve sizes have been standardised by certain Standard Organisations such as the

British Standards Organisation (B.S.), American Society for Testing Materials (A.S.T.M.), and

Indian Standards Institution (I.S.I.); the first two, in F.P.S. units and the third, in M.K.S.

units. Sieve designation is specified by the number of openings per inch in the B.S. and A.S.T.M.

standards, while it is specified by the size of the aperture in mm or microns in the I.S. stand-

ard. (IS: 460–1978 Revised).

*Determination of the textural composition of the soil is also known as ‘granulometry’.