Geotechnical News • March 2018
51
WASTE GEOTECHNICS
permanent features on the landscape.
There are over 40 tailings dams in
Alberta, many with extreme failure
consequences. Figures 1 and 2 show
examples of TSFs in Alberta, at a coal
and oil sands mine, respectively.
Dam operators must comply with
regulatory requirements to construct,
operate, and monitor active tailings
dams. As part of this, operators must
make resources and personnel avail-
able to manage and monitor the facili-
ties. Failure modes of active dams
are well understood, and contingency
plans are put in place in case the dam’s
performance deviates from what is
expected.
In Alberta, the safe operation of coal
and oil sands mines—and their tailings
dams and ponds—is regulated by the
Alberta Energy Regulator (AER). The
AER ensures that operators construct,
operate, monitor, and decommission
their tailings dams and ponds respon-
sibly; regulation and compliance are
enforced through enactments, regula-
tions, standards, and directives. For
oil sands tailings dams in Alberta in
particular, their design, construction,
operation, and management systems
are advanced and highly regarded
internationally (Morgenstern 2010).
Decommissioning tailings dams
Many of Alberta’s early tailings dams
were designed with little consideration
of their long-term behaviour (i.e.,
when the dam is no longer active), but
we now recognize that the post-mining
life of these structures is significantly
longer than their active life. Some
higher risk dams may prove difficult
to re-integrate into the surrounding
environment.
It is expected that a safely closed and
reclaimed tailings dam will no longer
function as a dam; that is, it will no
longer retain fluids or liquefiable
tailings, nor should there be unaccept-
able consequences if a failure occurs.
It is also expected that the reclaimed
structure will be compatible with
natural landforms in the region and
will be suitably resistant to processes
of landscape evolution, including
slope instabilities, surface erosion
from wind and rain, erosion due to
stream or river flow and floods, animal
burrowing, and internal seepage. The
operator must also identify and plan
for foreseeable future climate condi-
tions, land uses, and anthropogenic
activities.
Many institutions, geotechnical
conferences, and research facilities,
as well as organizations and programs
such as the Canadian Dam Associa-
tion, the International Committee on
Large Dams, and the United Nations
Environment Programme, have
advanced our knowledge about the
decommissioning and delicensing
process for tailings dams; however,
we continue to know very little about
the aging processes that tailings dams
undergo in perpetuity. This informa-
tion gap poses an unquantified long-
term environmental risk, public safety
risk, and financial liability to opera-
tors, the public, and the AER. Until we
fully understand the conditions leading
to post-operation risks, such as dam
failures, diligence and prudence are
required by the operator and regulator.
Kupper et al. (2013) proposed a risk-
based approach to delicensing oil
sands tailings dams that identified risk
considerations, defined assessment
criteria, and described how to apply a
risk assessment process. Eaton (2016)
reviewed a number of dam failures
that occurred between 1972 and 2015,
and related the failures to deficiencies
in the dam safety management system,
described elements of the AER Dam
Safety Program and Tailings Manage-
ment Framework, and discussed tail-
ings dam safety and decommissioning.
The Obed Dam failure and
creative sentence
An event in 2013 brought immediate
attention to the need for long-term risk
assessment and monitoring of tailings
dams through the decommissioning
and reclamation process. On Octo-
ber 31, 2013, a tailings dam failed at
the Obed Mountain coal mine, about
15 km northeast of Hinton, Alberta
(Figure 3). The failure occurred while
the mine was being monitored and
maintained after operations were sus-
pended in 2012 due to poor economic
conditions. About 670,000 m
3
of
waste water and 90,000 tonnes of fine
Figure 3. Plan view of the Obed Mountain coal mine showing the breach
location and flow path of tailings fluids to the Athabasca River. (Source:
Alberta Energy Regulator)
