Geotechnical News September 2011
33
GEO - INTEREST
the Water in the Soil – Part 4
Bill Hodge
What I want to do in this article is tidy
up a few things about liquefaction
before I move on to trying to sort out
pore pressure generation within a
saturated aggregation of soil particles.
First, I’ll suggest why silts do not
seem as prone to liquefaction as sands.
Then, I will look at some good triaxial
testing to see if there is any support, or
conflict, between this hypothesis and
those laboratory findings. After that I’ll
touch on the possible different effects
earthquake induced shear waves
and surface waves might have on
liquefaction behaviour and structural
responses.
Can Silts Liquefy?
It is in attempting to answer questions
such as this: “Can silts liquefy ?”, that
the utility of a new physical model
of two-phase soil behavior can be
evaluated. So here I’ll attempt to use
the L-factor and the “soil” components
of the drag force to see if I can explain
what’s special about silts when it
comes to situations where sands would
be expected to liquefy and silts seem
not to. With this in mind I’ll use the
pieces put in place already in earlier
articles to see how this question might
be answered.
In Figure 6 (Part 2) I suggested that
particles of a size which can reach their
v
T
[Terminal velocity] within a fall
distance of less than 29% of their di-
ameter are inherently vulnerable to liq-
uefaction. The reasoning behind this is
that this is the distance uniform spheres
move downward while changing from
the loosest packing arrangement to
the densest. Silt sizes are all well be-
low this red line, so the implication is
that silts are extremely likely to liquefy
when going from a loose to a dense
packing.
As can be seen in Figure 8 (Part
3), for velocities at v
T
, the L-factor is
zero over the full silt size range. That
means, according to this model, that
the Pressure component of the Drag
force plays no part in silts under lique-
faction conditions, and hydrodynamic
resistance to relative velocity [particle
movement] is fully accounted for by
the Bearing component alone. It im-
plies that when a silt size particle falls
enough to have liquefied it does not
result in the generation of pore water
pressure, as a sand in similar circum-
stances would, but instead results in a
viscous response which I have equated
to bearing resistance of a cohesive soil.
So, rather than liquefying, a loose silt
deposit would tend to consolidate. And
since there would be no generation of
excess pore water pressure to produce
a critical gradient, there could be no
manifestation of concentrated venting
through local weakness at ground level.
Along this line of reasoning one
might wonder if the same way of look-
ing at the behaviour of fine particles
might have some involvement in per-
meability, consolidation and creep.
Before leaving silt there is a point I
want to make: The abrupt discontinu-
ity I have shown for the L-factor at Re
= 0.6 doesn’t seem quite right to me.
There isn’t the gradual change from
one mode of behaviour to the other that
I expect to see in physics where there
aren’t very different material proper-
ties across the boundary. What I’m in-
clined to think is that this discontinuity
is perhaps because our colleagues in
Fluid Mechanics didn’t, or couldn’t,
define the values accurately in this area
of conjunction. But, as I don’t have the
ability to come up with my own val-
ues, I will settle for theirs and trust that
someone else may follow up on this. I
suppose I’m basically expecting to see
the gradual sort of changes we get in
silt and sand sizes as we cross the 200
mesh sieve – nothing startling.
A final word about silt liquefaction:
It has been reported that the loess flow
slides which resulted form the great
1920 Kansu earthquake in China in-
volved silt liquefied in air. But with
no water present, “liquefied” is hardly
the word for it – perhaps “fluidized”.
In any event I think this behaviour is
consistent with the above understand-
ing since the viscosity of air is less
than one fiftieth that of water. In con-
sequence, the air would not have the
bearing capacity to prevent the coarser
silts from falling downwards, thereby
leading to generation of increased pres-
sure in the pore air beneath them. And
I imagine that if such pneumatically
charged air were entrapped within the
body of the sliding mass it would add
mobility to the motion.
Triaxial Testing
James K. Mitchell
Sometime after Professor Mitchell
moved from Berkeley to Virginia
Tech he was kind enough to write me
with a critique of some of these ideas.
While he was encouraging about my
bearing capacity analogy for small
particles he was troubled about what
