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Geotechnical News • June 2012
WASTE GEOTECHNICS
techniques are needed for mining
under or adjacent to backfills where
strength may have been compromised
(e.g., due to rock bursting or nearby
blasting). The effect of creating
adjacent openings near existing back-
filled stopes is also of interest. Most
important, technology transfer needs
to continue so that research results
become incorporated into the mines’
best practices.
On the surface ensuring the stability
of tailings storage facilities remains a
priority for the industry. This critical
aspect is becoming even more chal-
lenging in this era of low-grade and
large-volume operations. In this
regard, the performance of tailings
deposits under earthquake loading is
one of the key engineering consider-
ations that influence the design of stor-
age facilities situated in seismic zones.
Due to the commonly used hydraulic
placement methods, as-placed tailings
generally exist as loose, saturated
deposits. These tailings are sig-
nificantly compressible, low in shear
strength, and generally susceptible to
liquefaction. Liquefaction essentially
involves a rapid loss in shear stiffness
and strength in loose saturated soils
due to the generation of excess pore
water pressures under shear loading
conditions. While static liquefaction
has been cited as being responsible
for a number of tailings dam failures,
liquefaction due to earthquakes is still
a major concern.
The seismic assessment of tailings
deposits involves addressing the fol-
lowing key concerns: (a) will liquefac-
tion be triggered in significant zones
within the tailings impoundment under
the design earthquake? (b) if so, are
the liquefied shear strengths adequate
to prevent a flow failure? and (c) if
so, are the displacements tolerable?
Answering the above questions is a
difficult task in engineering practice
due to many reasons, including the
highly variable and layered nature of
tailings deposits in terms of particle
size (i.e., coarse-grained or fine-
grained) as well as packing density
resulting from the type of discharge
strategy being operated during stor-
age. One example of complexity in
this regard is the void redistribution
that occurs during liquefaction in
layers with contrasting particle sizes
(and permeability) that is known to
introduce significant uncertainty in the
estimated liquefied shear strengths.
Another major concern is the current
limited understanding of the cyclic
shear behavior of tailings. For exam-
ple, the current practice is pivoted
Significant advances have been made in integrated instrumentation systems
to monitor underground mine backfill, as illustrated in the 3 photos above
for a recent field monitoring program.
