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

Chapter 1 PLANNING AND SCOPE
through sand and gravel. Disturbed samples however, should be taken in all kinds of borings at 1.5m
intervals and at each change of stratum.
Accordingly, the sampling routine should be aimed at:
i. Providing sufficient samples to classify the soil into broad soil groups, on the basis of particle
size and compressibility;
ii. Assessing the variability of the soil;
iii. Providing soil specimens of suitable quality for strength and compressibility testing; and
iv. Providing specimens of soil and groundwater for chemical testing.
In soft clays or for special conditions, continuous sampling may be necessary. Excessive use of water
to advance borings in clays should be avoided and, before a sample is taken, the bottom of the
borehole should be carefully cleaned out.
Undisturbed samples should be kept sealed with wax. Bulk samples are usually stored in heavy-duty
polythene bags tied up tightly with string. Small disturbed samples, usually taken from the cutting
shoe of an open-ended sampler or from the split-spoon sampler used in the standard penetration
test, are kept in jars, tins or small polythene bags. Water samples should be taken whenever water
is encountered during drilling. Samples are stored in jars whose lids are sealed by dipping them in
paraffin wax.
All samples must be clearly labelled, with labels both inside and outside the containers, and must be
carefully transported and stored. Once they are no longer required for inspection or testing, samples
may be discarded. However, care should be taken that they are not discarded too soon and all the
people who may wish to make use of the samples should be informed before they are disposed of.
In situ testing is carried out when:
i. Good quality sampling is impossible (for example, in granular soils, in fractured rock masses, in
very soft or sensitive clays, or in stoney soils);
ii. The parameter required cannot be obtained from laboratory tests (for example, in situ
horizontal stress);
iii. When in situ tests are cheap and quick, relative to the process of sampling and laboratory
testing (for example, the use of the spt in clay, to determine undrained shear strength); and
most importantly,
iv. For profiling and classification of soils (for example, with the cone test, or with dynamic
penetration tests).
The most commonly used test is the Standard Penetration Test (SPT), which is routinely used at 1.5
m intervals within boreholes in granular soils, stoney soils, and weak rock. Other common in situ
tests include the field vane (used only in soft and very soft cohesive soils), the plate test (used in
granular soils and fractured weak rocks), and permeability tests (used in most ground, to determine
the coefficient of permeability).
The primary decision will be whether to test in the laboratory or in situ. Table 1.3 gives the relative
merits of these options.
March 2009 1-13