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

Chapter 2 SAMPLING AND SAMPLING
DISTURBANCE
Figure 2.1 Effects of
Tube Sampling Disturbance of Lightly Overconsolidated Natural (‘Structured’)
Clay on: (a) Stress Path and Strength during Undrained Triaxial Compression
(b)
One-Dimensional Compressibility during Oedometer Testing
(II)
Compressibilit
ty and Stiffness
The effects of sampling on compressibility (as measured in the oedometer, for example) are difficult
to assesss because of bedding effects, particularly in heavily overconsolidated clays. The use of local
axial strain measurement on triaxial specimens during the past decade has produced new and more
reliable stiffness dataa than can normally be expected from
routine one-dimensional consolidation
tests, It is now known that the measured small-strain stiffnesses of clays, most relevant to many
geotechnical engineering problems, is for a given clay approximately
linearly proportional to the
mean effective stress at the time of measurement. This means that changes in effective stress as
a
result of disturbance are directly translated into proportional changes in measured soil stiffness.
Because of the growing appreciation of the influence of bedding and effective stress changes on
measured stiffness, it
has becomee common practice in the
UK to adopt
laboratory methods which
will avoid
these problems. In heavily overconsolidated clays, small-strain stiffness is often normalized
with respect to the mean effectivee stress at the start of shear (p’o=(σ’1+σ’2+σ’3)/3). Alternatively,
the stiffness of bonded soils is perhaps more appropriately normalized with respect to undrained
2-8
March 2009