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

Chapter 2 SAMPLING AND SAMPLING
DISTURBANCE
shear strength, although it may be
difficult to determine the true in situ value of this. In situ stiffness
can then be recovered
if p’o(in situ) or cu(in situ) be estimated. In lightly overconsolidated natural
clay Clayton et al. (1992) have shown, however, that even the careful reestablishment of in situ
effective stress levels before shearing cannot fully recover the undisturbed stiffness behaviour of the
soil.
A 60% reduction in E u / p’ o (measured locally,
and after re-establishment of in situ
stresses) was
obtained for the Bothkennar clay following tube sampling strain excursions of ±2% %, for example.
The results of a literature survey
by Hopper (1992) are shown in Fig. 2.2. Here the very severe
effects of tube sampling (including
the effects of borehole disturbance, and obtained
by comparing
test results from tube samples with
those on block samples in the same soil type) can be seen.
Figure 2.2 Influence of Tube
Sampling Disturbance on
Undrained Strength and Stiffness
(From a Survey by Hopper 1992).
March 2009
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