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relative to those imposed during consolidation, and relatively low undrained
strength results. Active and passive states may even differ more due to (induced)
anisotropy. Along slip surfaces of failed embankments active states occur under the
crest and passive states near the toe. An average value of
s
u
/
p
'
= 0.22 is often
assumed for the entire slip surface where
p
'
is the oedometric yield stress of the
soil. More complex relationships such as
s
u
=
s
u,nc
OCR
0.8
are sometimes used.
Correlations of
s
u
with CPT penetration resistance and SPT blow count also exist.
A new development is to use so-called full-flow penetrometers (T-bar or ball)
around which the soil closes back. This removes the overburden pressure as a
source of penetration resistance and allows for more accurate correlations.
Because the undrained shear strength may vary with depth and highly depends
on the mode of shear and time to failure, the choice of a proper value requires
experience. One must recognise that the undrained shear strength
s
u
is not an
intrinsic soil parameter.
F
THE
A
AND
B
FACTOR
In saturated soil the pore pressure plays an essential role. It affects the normal
effective stress, not the shear stress. Because the pore pressure is related to volume
change, which equals the pore volume change when assuming rigid solid particles,
the following holds for isotropic conditions
v
= -
V/V = -
(
V
w
+V
s
)
/V = -
V
w
/V = - n
(
V
w
/V
w
)
= n
)
u
(7.14)
u
. For
undrained conditions the soil fabric (granular matrix) undergoes the same volume
change
36
, which is expressed by
where
)
is the pore water compressibility, defined by
)
=
V
w
/V
w
v
=
'/K
(7.15)
where
'
is the isotropic effective stress and
K
the bulk modulus. Using (4.1b):
=
' + u
, leads to
v
=
(
-
u
)/
K
, and with (7.14) one obtains
u =
/(1+
n
)
K
)
= B
(7.16)
where
B
is the undrained pore pressure factor (Skempton). For incompressible pore
water (
= 0) this factor becomes
B
= 1, expressing that the response of an
undrained soil sample under isotropic loading is letting the pore fluid carry the
entire load, so that the corresponding effective stress change is zero. Pore water
may contain a minor air fraction, which has however a significant influence on the
compressibility, and subsequently
B
< 1. In test procedures, one should incorporate
B
properly.
)
36
Undrained conditions imply no relative motion between pore fluid and grains. Hence,
there is volumetric consistency for the pore fluid and the soil matrix.
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