Geoscience Reference
In-Depth Information
The sensitivity to pore pressure generation under cyclic loading is measured and
represented in a graph, normalised with respect to the applied initial effective stress
'
0
and the cycle number
N
l
for which liquefaction is observed;
N
l
values found in test
1 to 4 are 200, 35, 200, 150, respectively. For wave generation over a loose seabed
sand layer based on a relatively stiff substratum, a two-dimensional fully-coupled
two-phase numerical analysis shows that the stress ratio
^
/
'
0
is then about constant
(see also the discussion under equation (16.13)). It implies that
N
l
measured in the test
can be applied in the field at any depth in the sand bed.
61
In most sands the hydrodynamic period (consolidation time) is relatively short,
except when the drainage path is long due to a specific geological situation, such as
a sand layer partly sealed off by a clay layer. It is suggested in that to use case
?
=
0. The average excess pore-pressure generation then becomes for
t <<
1
u = u
0
exp[
t
]
+
0
(1
exp[
t
])/
= u
0
+
0
t
(16.32)
This complies well with case A (undrained, see Fig 16.19a). When the initial
pore pressure and cyclic pore pressure rise together and have reached a value equal
to the total stress, the corresponding effective stress will be zero, and then the soil
has lost its structure. It is in a state of liquefaction, like a mud. Any shear resistance
will be completely lost. Also during the process of pore pressure rise, shear
resistance is already decaying. This phenomenon is important for land reclamation,
landslides, and coastal and offshore foundations.
Liquefaction of the seafloor
Because of morphological dynamics the top few metres of a sandy seafloor is
usually loose, and in the sand cyclic pore pressure increase or liquefaction may
occur due to wave or earthquake agitation. An example is worked out for the slope
stability of a large breakwater during a storm. Waves cause small shear
deformation which in loose sand (contractancy) gradually result in excess pore
pressures. Local effective stresses will decrease, accordingly. In Fig 16.20 the
situation is shown for the critical moment of a design wave on the slope. The effect
of the momentary actual water pressures on and in the structure and the seabed,
excluding or including the effect of gradual excess pore pressure increase (using
16.31), is reflected in the slope stability factor (slip circle approach) as shown in
Table 16.1.
TABLE 16.1 EFFECT OF EXCESS PORE PRESSURES ON THE SLOPE STABILITY
excess
u
Not included
Yes included
factor
F
1
1.21
1.03
factor
F
2
1.47
0.94
61
If no information on
N
l
is available it may be determined from the shear stress ratio.
Experiments imply that the relation between
N
l
and
^
/
'
0
can be expressed by an empirical
^
^
/
'
0
-
^
0
/
'
0
)
a
for
^
>
^
relation, including a threshold shear amplitude
0
. The
empirical constants
a
and
b
are dependent on the type of sand and its density. For the 4 tests in
Fig 16.12c a constant value:
a =
0.13 was found and
b
varied with density (porosity).
0
:
N
l
= b
(
Search WWH ::
Custom Search