46
Geotechnical News • March 2012
gering one billion. I used spheres of
10mm, 0.3mm, and 0.01mm diam-
eter, at a relative density of 50% to
calculate these ballpark figures. Now,
since it takes a million ccs to make
a cubic metre, then, whether they be
silt or gravel, there is obviously no
way of dealing with real soils in the
field other than statistically. Even in
the laboratory, determining the size
and shape of each particle in a small
specimen of sand is utterly impracti-
cal. And once it came to dealing with
soil grains theoretically I knew I had
no option but to simplify the shape to
the extreme.
Spheres have the great benefit that
their shape, no matter what way you
approach them, is exactly the same.
And their geometry is entirely defined
by one dimension – diameter. The
next simplest shape is a cube, again
definable by a single length, but a cube
looks different depending on the view
point. Also, a cube has kinematic char-
acteristics which are absent in a sphere
and so difficult that my theoretical
work is constrained to spheres.
The need to have a simple geometry
which is amenable to mathemati-
cal treatment is more of a scientific
necessity than an engineering one.
It is a fundamental tenet of scien-
tific advancement that propositions
describing natural phenomena be
expressed in mathematical terms.
That way the reasoning can be fully
and continuously traced through the
mathematical formulation: It makes
the proposition more amenable to
falsification, and allows it to be either
dismissed or subsequently built upon
by others. As this is a new idea I’m
presenting here, it needs to leave
an uninterrupted mathematical trail
behind.
Before ending this series I would
like to draw attention to something
I’ve being watching myself with
some amusement while writing these
articles. And that is the difference
between the pore water pressure equa-
tion I suggested in Part 1 and what I
am offering now. There’s a substantial
difference. Even before Part 1 went to
print I knew it could be improved on.
But I decided to leave it alone, and let
it stand. I wanted it to be a benchmark
for myself, to see how much my ideas
would change over the year and a half
it took to get to where I find myself at
now: The end of this series of articles.
Acknowledgements
It is quite necessary for me at this
juncture to mention my good old
friend Nigel Skermer. He has willingly
read the drafts of each of these articles
and I am most grateful to him for
guiding me towards some measure of
logical continuity, and we hope, avoid-
ing serious lapses in reason. Thank
you, Nigel.
Thanks also to John Gadsby for
agreeing to publish this series, and of
course, to Lynn Pugh for keeping me
in line throughout the effort to put the
ideas into print.
W.E. Hodge, P.Eng, M.ASCE
604 565 7175
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603 – 5411 Vine Street
Vancouver, BC
V6M 3Z7
Pacific Northwest Cascadia Fault
One of the most dangerous faults in
North America is the Pacific North-
west’s Cascadia fault – an offshore,
subduction zone fault capable of
producing a magnitude 9 earthquake
that would damage Portland, Tacoma,
Seattle, and Victoria, British Colum-
bia, and generate a large tsunami. Yet
there are currently no instruments
installed offshore, directly above the
fault, for measuring the strain that is
currently building up along the fault.
But a recent $1 million grant from the
W. M. Keck Foundation to scientists at
the Woods Hole Oceanographic Insti-
tution (WHOI) will change that. An
interdisciplinary project led by WHOI
geologist Jeff McGuire, an expert in
global earthquake seismology and
geodesy, and John Collins, director of
WHOI’s Ocean Bottom Seismometer
Lab, will build and install the first
seafloor geodesy observatory above
the expected rupture zone of the next
great Cascadia earthquake.
Scientists agree there will likely be
another magnitude 9 earthquake off
Oregon and Washington. Information
that is critically important for model-
ing how much the fault will slip – and
hence how much the ground will
shake – and for predicting the maxi-
mum height of the tsunami that could
be generated.
The real-time data flowing from the
fault on the seafloor will not only
advance our understanding of earth-
quakes but can help city planners and
emergency response managers.
The Cascadia subduction zone is a
very long sloping fault that stretches
from mid-Vancouver Island to North-
ern California. It separates the Juan de
Fuca and North American plates. For
many years, according to conventional
