Geoscience Reference
In-Depth Information
Tests and theory (Fig 16.17) reveal that the susceptibility of a loose sand slope to
initiate a flow slide decreases with density, and increasing with slope steepness and
height (Stoutjesdijk, de Groot and Lindenberg). Hydraulic fill progresses usually
with local slides and induced high pore pressures (Fig 16.18). For a natural slope
severe rains may cause instability (see application 7.1), and in unfortunate
circumstances a dangerous mud flow may be triggered.
The liquefaction of a (steep) slope of dense sand follows a different process. The
density has to decrease (dilation) to the critical density before sliding of a thin zone
at the surface occurs. In fact the slope is moving into the sand. The corresponding
negative excess pore pressure keeps the sand inside temporarily stable. This
process can be described by a moving storage equation (van Os, Verruijt). The
velocity of the slope motion
v
is expressed by
1
n
tan
v
k
(
G
1
cos
(
1
(16.30)
|
n
|
tan
where
k
is the permeability, |
n
| is the porosity change from actual density to
critical density,
G
s
is the specific solid gravity, and
is the slope angle. Note that
for
<
no flowing sand zone at the surface of a dense sand slope occurs (natural
slope at
cv
). The velocity of the slope motion (16.30) characterises the
speed of breach growth and is essential in the estimation of dregding production.
=
=
Cyclic liquefaction
In the situation of a horizontal layer of loosely packed sand a cyclic dynamic
trigger (waves, earthquake) may cause excess pore pressure build up and
eventually liquefaction (cyclic liquefaction). If a coastal structure is positioned on
such a layer the available shear strength (on the edges) will decrease and this may
be the onset of collapse. For this situation a simple liquefaction model is presented.
Under cyclic loading, residual deformations may change the stiffness, the
strength and the permeability of sand. The corresponding change in density is also
referred to as preloading or preshearing. If the cycles are fast enough the pore
water in a drained soil can only partly dissipate and pore pressures will rise. This
pore-pressure rise
u
can be characterised by four parameters:
u
0
,
0
,
and
?
. The
first one
u
0
is the initial set-up (see Fig 16.19), the second one
0
represents the so-
called liquefaction potential (capacity of pore pressure rise under undrained
conditions), the third one
is related to drainage capacity (related to dissipation of
excess pore pressures) and the fourth one
to preshearing or densification (the
liquefaction is less likely when the soil gradually densifies).
Three situations can be described by these parameters (Fig 16.19a): undrained
(A), drained (B), and presheared (C). For a one-dimensional (vertical) situation the
average excess pore pressure
u
satisfies
?
u
= u
0
exp(
t
)
+
0
(exp(
?
t
)
exp(
t
))/(
?
)
(16.31a)
Here, averaging has been applied over the loose-sand layer height. The moment
t
m
of maximum pore pressure becomes (Fig 16.19b)
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