36
Geotechnical News September 2011
GEO-INTEREST
After grain damage progresses and
the resisting soil-structure’s volume
becomes effectively smaller, I think it
inevitable that the inside of the mem-
brane becomes increasingly more
in contact with specimen water that
with specimen solids. I should stress
here that what I mean by specimen
solids are only those solids compris-
ing the soil-structure, and not includ-
ing the dust from broken asperities.
As the balance of water forces across
the membrane depends on the summa-
tion of the membrane areas resting on
either phase, it seems obvious that the
opportunity exists for water pressure
inside the membrane to escalate. And
this coming about without the need to
invoke soil-structure weakening, but
rather because of reduced effective lat-
eral confinement. To this we might per-
haps add the possible membrane bal-
looning because of inside temperatures
rising from energy expended (work
done) by the deviator load repetitions.
I realize this is heretical thinking,
but I gained some confidence in this
position by the encouragement offered
when, towards the end of 1998, Ralph
B. Peck wrote me saying: “I share your
feeling that much of what we think we
know about liquefaction is an artifact
of our tests.”
Surface Waves and Soil
Extension
Just after the Loma Prieta earthquake
in October 1989 I went down to
California to look at the evidence of
damage. Apart from the well reported
details, two things struck me as odd
and requiring an explanation which
didn’t seem to fit with the current
way of looking at the propagation of
energy from the epicenter in the Santa
Cruz Mountains to structures in San
Francisco about 100 km away.
First was the liquefaction of the
dredged sand foundations in the Ma-
rina District. As far as I knew attenu-
ation of shear waves were supposed to
leave them with little residual energy
once they had travelled about 30 km, so
how could such destruction be wrought
96 km away? The answer is I believe
that it wasn’t the shear waves that
caused liquefaction, it was the Ray-
leigh component of a surface wave that
did it. Rayleigh waves can travel great
distances - they are the geotechnical
equivalent of a tsunami. Also, this sur-
face wave, because it causes the ground
level to be temporarily super-elevated
as it passes would result in elongation
of the soil column (ground profile) thus
producing a stress path similar to the
one replicated by the Vaid triaxial ex-
tension work. And as we saw above,
soil extension is a most effective way
of precipitating liquefaction.
The second situation which made
me wonder was what might have gone
on in the ground under the Cypress sec-
tion of the Nimitz Freeway (I-880) to
make the upper deck fall as it did. In his
geological narrative
Assembling Cali-
fornia
, John McPhee describes it thus:
“The under road is northbound, and
so is disaster. One after the last, the
slabs of the upper roadway are fall-
ing . . . Aman in another car guns his
engine, keeps his foot to the floor,
and races the slabs that are succes-
sively falling behind him.”
It is very tempting for me to believe
this lucky individual was racing the
surface wave radiating out from Loma
Prieta. The orientation of the freeway is
consistent with this idea, so in Figure
12 I’ve drawn a cartoon of a possible
mechanism which seems consistent
with the above evidence. The red
bump moving to the left (towards
San Francisco) represents the surface
deformation due to the seismic wave.
Unfortunately, I find the math-physics
necessary to calculate the speed of the
Rayleigh wave in this particular soil
column too intimidating to attempt, and
therefore can’t say if a car could stay
ahead of the wave for a while in this
surficial geology.
The point I’m trying to make is this:
It is far easier to explain these events
by looking at the surface wave and soil-
structure extension rather than by ex-
amining the damage as if it were caused
by shear waves and cyclic loading.
in the Next Article
It is now time to move beyond single
particles and approach more closely
the practical goal of this series, and that
is the generation of pore water pressure
in real soils. The next necessary move
is to account for the magnification
effect due to the crowding of particles
inherent in an aggregation of grains
packed closely together in a mass. So,
in the next article I will develop what I
call the K-factor.
W.E.Hodge, Geotechnical Engineer,
P.Eng., M.ASCE,
(250) 307-4357,
Figure 12. Speculative failure mode for I-880 upper deck.
