Geotechnical News June 2011
45
WASTE GEOTECHNICS
successful,” says Lahaie. “We have a
large data set overlaid by many years’
worth of information. David can man-
age and work amongst all the data and
simplify it into useful metrics. On top
of that, he has a lot of practical knowl-
edge. He marries the two in a very utili-
tarian way.”
Carrier is currently working on
accelerated dewatering methods at
Syncrude’s Mildred Lake project near
Fort McMurray with Jack Seto of BGC
Engineering Inc. and Murray Fredlund
of SoilVision Systems Ltd. Carrier,
who conducted notable rim ditching
research in Florida, believes Terzaghi’s
effective stress principle can be applied
to the soft, yoghurt-like fine tailings.
“We needed to change the framework
and see this as a mechanistic process
rather than a chemical one,” says Car-
rier.
The combination of rim ditching
on the perimeter of the deposit with
evaporation on the top of the deposit
will increase the effective stress, creat-
ing a landform with greater trafficable
strength. According to Carrier, the key
is to drain the water off so that it can
evaporate and crack. Water draining
and surface cracking will lead to the
lowering of the water table, which ap-
plies a larger load on the layers under-
neath. “You leave it for a year and then
come back to cut a bit deeper around
the perimeter. Then leave it again for
year. You keep doing this until the soil
is dense enough that you can reclaim it
and pour sand on it or plant vegetation
on it,” he says.
Though the methodology is rela-
tively simple, Carrier says the primary
concern is getting the produced tail-
ings into a manageable volume. Then,
the next question is how to make it
stronger. Carrier believes all the tech-
nologies that the industry is looking at
now, such as centrifuging and thin-lift
deposition, will result in denser, more
manageable, more environmentally
friendly and sustainable material. Yet
he is quick to point out that dewater-
ing is not without its challenges. “The
more densification we do, the more
water we produce within the system,”
he says. “The long-term challenge for
the industry is reducing the volume of
water stored and ensuring the reclama-
tion of it.”
Carrier’s experiences prove that the
sky is certainly not the limit when it
comes to tackling tailings issues. The
rim ditching research done by Car-
rier and his Florida colleagues has
resulted in the reclamation of 50,000
hectares (500 km
2
) of phosphatic clay
ponds. Currently there are 426 km
2
of
disturbed land in Alberta’s Athabasca
region. In an effort to conduct collab-
orative research on the early reclama-
tion of tailings ponds in Alberta, the
Canadian Oil Sands Network for Re-
search and Development (CONRAD)
Carrier working at a test pit with baux-
ite tailings in the Amazon jungle of
Brazil.
Digging Towards Success
David Carrier is standing on crusted oil
sand flocculated fluid fine tailings after
one year of dewatering at the Syncrude
Canada Ltd. Flocculated Fluid Fine
Tailings Perimeter Ditch Dewatering
Pilot Project at its Base Lease
Operations north of Fort McMurray,
Alberta.
Carrier is standing on the edge of
flocculated fluid fine tailings (a “silty
clay” flocculated with polymers) that
have been deposited in an impound-
ment approximately 80 m by 80 m by
10 m deep and is dewatering mostly
by surface evaporation and water flow
to a progressively deepened perimeter
ditch that is connected to a drain. The
perimeter ditch concept is patterned af-
ter similar work that Carrier has been
involved in Florida with dewatering
clays from the phosphate industry.
Carrier is wearing a life jacket and is
being very careful not to break through
the crust that he is standing on and into
underlying fluid-like tailings below the
crust that has not yet dewatered. Where
he is standing, there is approximately a
20 cm crust on about 1 m depth of fluid
fine tailings.
Note the surface-cracking network
that has developed due to shrinkage of
the material as it dewaters. The ponded
water behind him in the center of the
Pilot deposit is from recent rains. This
water has not yet either evaporated
or found its way through the surface-
cracking network and perimeter ditch
to a drain where it is removed from the
Pilot.
The floating dock way and geo-
technical instrumentation poles (pi-
ezometers, thermistors, and settlement
gauges) behind Carrier are in the center
of the Pilot. The fence around the pe-
rimeter of the Pilot keeps animals and
people from wandering into the Pilot.
— Geoff Halferdahl, Syncrude Canada
Ltd.
