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Geotechnical News • March 2012
GEOTECHNICAL INSTRUMENTATION NEWS
GEO-INTEREST
the Water in the Soil - Part 6
Bill Hodge
Now that I am writing the last
article in this series I find myself
wondering where on earth these ideas
might have started out.
Maybe it was back in the 60’s when
Arrow Dam (now Keenelyside) was
been built on the Columbia, and I was
there as the junior engineer looking
after earthworks and instrumentation.
I remember at one stage the glacial till
core earthfill was responding to roller
compaction by making waves, as is
inclined to happen when such material
is placed too wet of optimum. As it
happened, Arthur Casagrande was due
to make one of his routine consulting
visits just about then, so I installed a
piezometer about 10 feet below grade
and attached it to a pressure gauge.
Then, as he watched, I had loaded
dump trucks pass over the spot where
the piezometer was buried. I wanted
to see what he would say to the fact
that the pressure on the gauge rose
as the truck moved over the spot and
then dropped back to zero as the truck
moved away. Although he looked for a
good while, sad to say, he went away
without telling me what he thought
about it. But now, half a century later,
I think that observation might have
done it for me.
Excess pore water pressure
As a geotechnical engineer work-
ing in design and construction I was
acutely conscious of being obliged to
deal with soil behaviour at only one or
other of two extremes: Fully drained,
or no drainage at all. The real world
was always somewhere in between
- but inaccessible. This wasn’t all
that bad until earthquakes entered
the scene. Then I felt our work was
degraded to following some quasi-
mystical beliefs set down by univer-
sity diktat, and coming from the same
place as the earthquakes - California.
All strangely reminiscent of, and
perhaps symptomatic of, times of on-
campus student unrest. What forced
us into that “soil-dynamics religion”
was the absence of a clear understand-
ing of the mechanics of pore pressure
generation. And the devil in the mix
was the undrained triaxial apparatus
which while defending the “estab-
lished truths” went about its business
of mutilating entrapped sand in a
manner reminiscent of what was done
to nonconformists during the Inquisi-
tion. To get out of that mindset, and
progress, it was necessary to become a
geotechnical heretic.
The first step was to walk away from
orthodox belief in the interpretation of
what went on inside the membrane of
the undrained triaxial machines. What
actually happens within this sealed
environment is that the vertically
moving plunger results in either dila-
tive or contractive deformation of the
soil-structure, and that in turn results
in either more or less solid area being
pushed into physical contact with the
membrane. Let’s take the case of a
contractive soil-structure. As specimen
straining continues the volume of the
soil-structure diminishes, and with it
the proportion of the solid phase in
contact with the membrane. Because
of this, the load carried by the water
inside the membrane will have to
increase accordingly in order to main-
tain radial/horizontal force equilibrium
as the solid phase retreats more and
more from membrane contact. In
consequence the water pressure in the
specimen goes up. And at the same
time the effective intergranular normal
Figure 15. Contractive triaxial specimen.
