30
Geotechnical News • March 2017
WASTE GEOTECHNICS
sure – easily penetrated several inches
by thumb (12 to 25 kPa). And so on.
Once you reach firm (25 to 50 kPa)
to stiff (50 to 100 kPa) one reaches
the realm of trafficable materials. For
comparison, soils at their liquid limit
have shear strengths of 2 to 5 kPa.
Oil sands soft tailings (Figure 1) have
been studied by many people over
the past 50 years (see McKenna et al
2016b). Tailings vane strengths are
highly correlated to the geotechni-
cal moisture contents and also to
the liquidity index. High void ratio
tailings (e=3, 150% geotechnical
moisture) have strengths of less than
0.2 kPa. At a void ratio of 1.1 (43%
geotechnical moisture) the strength is
about 0.2 to 5 kPa. It’s not until you
get to about 25% moisture do you start
to get strengths more akin to those
in soil mechanics (5 to 50kPa). Low
strengths (<2 kPa remolded) are good
for processing and pumping but dif-
ficult to cap or reclaim terrestrially due
to low bearing capacity and very large
consolidation settlements.
The shear strength of soft foods is
not news to food process engineers
and technicians – they use strength
measurements every day for design
and quality control. There are about a
dozen common methods to measure
the shear strength (or yield strength)
of processed foods, including the vane
test.
Methods and results
Foods and household products were
selected to provide a broad range of
strengths and consistencies with an
attempt to cover a large range of com-
mon foods and condiments including
some iconic processed foods.
The vane test
The vane test followed ASTM D4648
as noted above. Following food
industry norms, foods were tested
undisturbed in their product container.
To minimize edge effects, the vane
was carefully inserted to middle of the
sample, with an insertion depth of at
least two vane heights while keeping
the vane at least one diameter away
from the edges of the container. In
some cases, these constraints could not
be met (for example the half-banana in
its skin) and a notation was made.
A high-resolution vane shear device
designed at the University of Alberta
to conduct field vane tests with a soft
ground rover (Olmedo and Lipsett
2016) was used (Figure 2). The design
consists of a DC motor, a gearbox,
three angular position sensors, and
a torsional spring. A DC motor was
selected to provide continuous rotation
of the vane while permitting pre-
cise speed control. A reconfigurable
gearbox was designed to control the
rotational speed of the vane. Most of
the measurements were made with
a 13 mm diameter (D) and 25 mm
high (H) vane. For foods with a high
moisture content and lower strength
(32 total samples), a larger vane was
required (up to 34 mm diameter and
69 mm high).
The vane was rotated at 60 degrees
per minute (as per ASTM D4648).
The angular displacement of the vane
was recorded along with the torque
and plotted on a computer in real time.
After the peak strength is reached,
the torque drops, then starts to level.
The vane was then rotated rapidly five
times through 360 degrees, then the
test restarted at 60 degrees per minute
again to determine the remolded shear
strength. Figure 3 presents the results.
The sensitivity is the ratio of the peak
vane strength to the remolded vane
strength. In nature, sensitive clays
have a sensitivity of 4 to 8. Quick
clays have sensitivities greater than
16. Most foods tested had a sensitivity
between 4 and 8. A few materials had
sensitivities greater than 16 (peanut
butter, banana, refrigerated butter and
margarine, car wax and candle wax).
Tailings had similar sensitivities.
Geotechnical moisture contents
Subsamples (Figure 4) were oven
dried at 110°C following ASTM
D2216. Moisture testing didn’t prove
that insightful as many of the foods
contained large quantities of oils that
didn’t evaporate; many samples didn’t
change in appearance even after a
few days in the oven. A Dean-Stark
extraction test would have been more
appropriate in hindsight.
Will a spoon stand in it?
“You can stand a spoon in this tail-
ings!” The spoon test (Figure 5) is
an informal tailings test often used at
the lunchroom table during tailings
field pilots. A steel teaspoon (154 mm
long, 53 mm wide, 17 g mass) was
inserted vertically into the sample
and observed. If there was no move-
ment, the sample was tipped until the
spoon tilted. We found that the spoon
stood vertically where the peak vane
strengths were more than about 0.2
kPa. The spoon remained in position
horizontally where the samples had
strengths more than 0.4 to 0.8 kPa.
Strong to a process engineer con-
cerned with pumping this material,
nothing to celebrate for a geotechnical
engineer concerned with capping.
Will it stay in the cup if I tip it
over?
“You can put the treated tailings in
a cup and tip it over and it won’t
come out!” The pour test (Figure 6)
is another lunchroom tailings test.
Figure 4. Geotechnical moisture test
samples ready for the oven.
