Geotechnical Engineering

DHARM

SOIL EXPLORATION 733

18.4.1Types of Samples

Broadly speaking, samples of soil taken out of natural deposits for testing may be classified as:

Disturbed samples, and undisturbed samples, depending upon the degree of disturbance
caused during sampling operations.

A disturbed sample is that in which the natural structure of the soil gets modified partly
or fully during sampling, while an undisturbed sample is that in which the natural structure
and other physical properties remain preserved. ‘Undisturbed’, in this context, is a purely
relative term, since a truly undisturbed sample can perhaps be never obtained as some little
degree of disturbance is absolutely inevitable even in the best method of sampling devised till
date.

Disturbed samples may be further subdivided as: (i) Non-representative samples, and
(ii) Representative samples. Non-representative samples consist of mixture of materials from
various soil or rock strata or are samples from which some mineral constituents have been lost
or got mixed up.

Soil samples obtained from auger borings and wash borings are non-representative sam-
ples. These are suitable only for providing qualitative information such as major changes in
subsurface strata.
Representative samples contain all the mineral constituents of the soil, but the struc-
ture of the soil may be significantly disturbed. The water content may also have changed. They
are suitable for identification and for the determination of certain physical properties such as
Atterberg limits and grain specific gravity.
Undisturbed samples may be defined as those in which the material has been subjected
to minimum disturbance so that the samples are suitable for strength tests and consolidation
tests. Tube samples and chunk samples are considered to fall in this category.

Besides using a suitable tube sampler for the purpose, undisturbed samples may be
obtained as ‘chunks’ from the bottom of test pits, provided the soil possesses at least some
cohesion.
The soil at the bottom of the pit is trimmed as a chunk to the required shape and size
approximately. A cylindrical container open at both ends is placed carefully over this chunk
after covering the top with paraffin wax. The bottom is scooped with a steel spatula and trimmed
after reversing the box along with the sample. Paraffin wax is again used to seal the face and
any gaps in the sides, before transporting it to the laboratory. The procedure will become
obvious from Fig. 18.4.

Open-ended cylindrical box

Paraffin wax

Soil

chunk

Spatula

Fig. 18.4 Obtaining a chunk sample