Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 2 SAMPLING AND SAMPLING DISTURBANCE
2.1 INTRODUCTION
2 SAMPLING AND SAMPLING DISTURBANCE
Sampling is soil and rock is carried out for identification and description of soils strata and rock type
with depth, and to perform laboratory testing for determination of index, classification and
engineering properties. Laboratory tests typically consist of:
i. Index tests (for example, unconfined compressive strength tests on rock);
ii. Classification tests (for example, Atterberg limit tests on clays); and
iii. Tests to determine engineering design parameters (for example strength, compressibility,
and permeability).
Samples obtained either for description or testing should be representative of the ground from which
they are taken. They should be large enough to contain representative particle sizes, fabric, and
fissuring and fracturing. They should be taken in such a way that they have not lost fractions of the
in situ soil (for example, coarse or fine particles) and, where strength and compressibility tests are
planned, they should be subject to as little disturbance as possible.
2.2 SAMPLING METHODS
Generally, sampling during a soil investigation program can be grouped into two main categories.
1. Undisturbed sampling
2. Disturbed sampling
The methods of sampling adopted for a particular site investigation program is based on the type
and requirement of soil investigation data for design and construction. While a large number of
samplers and sampling methods are available, however, before a suitable technique can be selected
it is always necessary to consider whether the sample size will be adequate, and whether the most
suitable method of sampling has been selected, to ensure that sample disturbance is sufficiently
small.
2.2.1 Undistured Sample
Undisturbed samples are obtained with specialized equipment designed to minimize the disturbance
to the in-situ structure and moisture content of the soils. The term “undisturbed” soil sample refers
to the relative degree of disturbance to the soil’s in-situ properties. Specimens obtained by
undisturbed sampling methods are used to determine the strength, stratification, permeability,
density, consolidation, dynamic properties, and other engineering characteristics of soils.
Undisturbed samples are obtained in clay soil strata for use in laboratory testing to determine the
engineering properties of those soils. Undisturbed samples of granular soils can be obtained, but
often specialized procedures are required such as freezing or resin impregnation and block or core
type sampling. Common methods for obtaining undisturbed samples are summarized in Table 2.1
and briefly described below.
March 2009 2-1