Geotechnical News March 2011
61
GEO - INTEREST
the Water in the Soil – Part 2
Bill Hodge
This is the second in a series of articles
in which I am proposing a way of
calculating the pore water pressure that
comes about within a saturated granular
soil while it is undergoing deformation.
In the previous article I began the
development and justification of this
idea by showing that when a particle
is falling through water there is a pres-
surized zone ahead of the particle,
and suggested that the magnitude of
this pressure front is somehow depen-
dent on how far the particle had fallen
through the water. Then I ended with a
prediction of what would be the rate of
generation of water pressure in front of
a solid sphere in a test to be carried out
in the research laboratory at UBC un-
der the kind auspices of Professor Vaid.
Now here are those results and my
interpretation of them. Afterwards I’ll
go on to suggest what these findings
say about liquefaction of saturated
soils.
Results of Test at UBC
[Note: Figures 1 – 4 were in Part 1 –
December 2010] In Figure 5 the ragged
blue line is a trace of the digitized
record of weight against time measured
at UBC for the fall of a 2 inch ball
bearing. The red curve is the weight
predicted earlier. The x-axis shows
time. The y-axis shows the system
weight, and where the line approaching
from the left is hovering around zero.
If you recall, the test setup that pro-
duced this trace (Part 1, Figure 3) has
the ball suspended from an electromag-
net below the water level in the cylin-
der; the load cell records the weight
of all the hardware (cylinder, water,
electromagnet and ball). So the trace
in Figure 5 is the weight of all compo-
nents measured before, and for about a
half second after the power to the elec-
tromagnet is
cut, resulting in the ball
being abruptly dropped to let it “free-
fall” through the water column.
Now, what the trace shows us is a
sudden weight drop into negative val-
ues, and then, a subsequent gradual
oscillating recovery of weight until,
at the end of the trace, the readings go
off-scale. The mechanical explanation
for the shape of the trace shape is as
follows.
Immediately the ball is set loose the
system records the complete loss of the
buoyant weight of the steel ball. Now
that it’s weight is no longer attached
to the side of the cylinder, the cylinder
itself which up to that point has
been
carrying that load in axial compres-
sion, reacts like a spring and begins
bouncing up and down. This vibration
Figure 5. Digital results of UBC test.
Figure 6. Weight transfer for fall distance.
