Geotechnical News • March 2012
39
WASTE GEOTECHNICS
It has long been known that MFT
exhibits a thixotropic behavior. This
behavior is captured in Figure 3. Data
from Miller et al. (2011a,b) on strength
gain of MFT after 365 days at rest
and combined data sets from Banas
(1991) and Suthaker and Scott (1997)
on strength gain after 680 days at rest
(displayed as “Banas 680 day peak”)
is compared to the remolded Su of
typical MFT in Figure 3. Upon shear-
ing of this thixotropic material (Banas
680 day residual), the Su collapses to
the remolded MFT Su line (open box
symbols). Also included in Figure 3 are
summaries of Su versus LI for various
dewatering techniques (PT and ILTT)
as reported in the literature (Jeeravi-
poolvarn, 2010; Masala and Matthews,
2010). For ease of comparison, typical
MFT is represented using the Locat
and Demers (1988) relationship.
Directive 074 requires Su values
in the range of 5-10 kPa or greater.
Therefore, the available correspond-
ing data within the range of LI from
0 to 3 is presented in Figure 4. It is
evident that strength gain of the fine
tailings with polymer/chemical addi-
tion to achieve the required Su values
of 5-10 kPa is possible, but at much
higher water contents. However, this
has implications on storage efficiency
of the fines deposit (kg fines/m
3
). At
higher water contents, a chemically
modified deposit will have lower
storage efficiencies and thus require
larger disposal areas (as compared to
untreated fines). For mining operations
that have limited lease space, there
may be operational challenges with
managing the larger volumes. Figure 4
also contains lines representing sensi-
tivity (S) as calculated by S = Su peak/
Su remolded. The sensitivity lines are
based on the remolded Su relationship
of Locat and Demers (1988). From
Figure 4, it can be deduced that the
chemically modified fine materials
(PT and ILTT) may exhibit sensi-
tive behavior based on their reported
strengths. Mitchell and Soga (2005)
would classify deposits with S = 4-8
as very sensitive and S = 8-16 as
slightly quick clays. The dewatering
techniques currently under investiga-
tion may create potentially metastable
and liquefiable deposits. Implications
of a sensitive, metastable deposit
could mean significant containment
is required for these deposits, even
though they meet the performance
criteria of Directive 074.
Conclusions
Bitumen has been extracted from the
oil sands deposits in northern Alberta
for several decades. Although tech-
nological advances have improved
mining and extraction efficiencies,
the industry still faces challenges in
finding practical methods to control
and reduce the formation of fluid fine
tailings. It was shown these deposits
of “MFT” behave like natural clay
slurries and can be represented by
Locat and Demers’ (1988) LI versus
remolded Su relationship. In response
to the ERCB’s Directive 074 in 2009,
the oil sands industry has conducted
considerable research on polymer
flocculation to augment dewatering
and strength gain of the fine tailings
stream. However, the chemically
amended fines deposits may exhibit
sensitive, metastable behavior upon
deposition, based on the reported data.
There is a significant need to under-
stand and conduct research regarding
the sensitivity and long-term behav-
iour of the flocculated, dewatered fine
tailings to best achieve Directive 074’s
objectives.
References
Banas, L. 1991. Thixotropic behavior
of oil sands tailings sludge. M.Sc.
thesis, Department of Civil Engi-
neering, University of Alberta.
Beier and Sego, 2008. The Oil Sands
Tailings Research Facility. Geo-
technical News, June 2008, 72-77.
Hyndman, A. and Sobkowicz, J. 2010.
Oil Sands Tailings: Reclamation
Goals and the State of Technol-
ogy. 63
rd
Canadian Geotechnical
Conference, Sept 12 -16, 2010,
Calgary, Alberta. pp 642 – 655.
Locat, J. and Demers, D. 1988.
Viscocity, yield stress, remolded
strength, and liquidity index
relationships for sensitive clays.
Canadian Geotechnical Journal,
25:799-806.
Masala S. and Matthews, J. 2010. Pre-
dicting development of undrained
shear strength in soft oil sand tail-
ings. 2
nd
International Oil Sands
Tailings Conference, December 5
-8, 2010, Edmonton, Alberta. pp
31-40.
Miller, W.G., Scott, J.D. and Sego,
D.C., 2011a. Influence of extrac-
tion process of oil sands fine tail-
ings. CIM Journal 1(2): 93-112.
Miller, W.G., Scott, J.D. and Sego,
D.C., 2011b. Influence of extrac-
tion process and coagulant addi-
tion on thixotropic strength of oil
sands fine tailings. CIM Journal
1(3): 197-205.
Mitchell, J.K. and Soga, K. 2005.
Fundamentals of Soil Behaviour.
John Wiley and Sons, Hoboken,
New Jersey.
Jeeravipoolvarn, S. 2010. Geotechni-
cal behavior of in-line thickened
oil sands tailings. PhD Thesis,
Department of Civil and Environ-
mental Engineering, University of
Alberta.
Suthaker, N. and Scott, J.D., 1997.
Thixotropic strength measurement
of oil sand fine tailings. Canadian
Geotechnical Journal, 34:974-984.
Nicholas Beier
PhD Candidate, Department of
Civil & Environmental Engineering,
University of Alberta, Edmonton,
Canada. E:
Amarebh Sorta
PhD Candidate, Department of
Civil & Environmental Engineering,
University of Alberta, Edmonton,
Canada.
Ward Wilson
Professor, Department of Civil & En-
vironmental Engineering, University
of Alberta, Edmonton, Canada.
David Sego
Professor Emeritus, Department of
Civil & Environmental Engineering,
University of Alberta, Edmonton,
Canada.
