Geotechnical News • June 2017
35
WASTE GEOTECHNICS
ground techniques are sometimes
employed where strengths are a
little less than 25 kPa – defor-
mations are usually large and a
stabilizing “bow wave” in the tail-
ings forms ahead of the advancing
lift. Operators “chase the wave”
from all sides to the centre of the
deposit.
• Standard earthworks techniques
using mining equipment is an
option where strengths are more
than about 100 kPa or where there
is a thick enough mechanically
or hydraulically placed cap. This
is the realm of traditional mine
reclamation: D9 dozers and 100
tonne trucks working to place cap-
ping lifts on firm to stiff tailings.
Given the high bearing pressures
of most haultrucks (approximately
equal to their tire pressures of 400
to 700 kPa), even stronger tailings
would be needed for direct traffic-
ability of the tailings surface. The
ultimate bearing capacity of soft
tailings is roughly five times its
undrained shear strength.
The capping boundaries in Figure 5
are approximate and will vary with
the sensitivity of the tailings and the
acceptable level of risk. The bound-
aries will continue to be refined as
additional commercial-scale experi-
ence is gathered. The boundaries will
also vary for different types of tailings
in different industries.
Techniques to improve tailings
to make capping easier
There are a number of techniques
employed to make soft-tailings cap-
ping easier by increasing the density
and strength, each with its own ben-
efits and challenges at mining scales.
Tailings processing for density
Commonly, mines employ tailings
processes that make denser, stronger
tailings. These processes can allow
water and reagents to be recovered
quickly and can reduce containment
requirements and risks of spills. Many
processes target pumpable tailings,
some of which result in deposits that
are soft, difficult to cap, and prone to
large settlements.
Crust management
Allowing or enhancing a dried tailings
crust aids trafficability of amphibi-
ous and light vehicles and provides
workers access on foot. However, a
crust on its own doesn’t provide traf-
ficability required to support mining
equipment because dozer tracks and
haultruck tires can punch through into
underlying soft tailings. Similarly,
embankment stability does not benefit
much from a thin tailings crust. Crust
management can be important but by
itself is seldom the full answer.
Frost
In cold regions, thick frost greatly aids
foot and light vehicle trafficability, and
when thick enough, can allow D6 doz-
ers to traffic soft tailings deposits. Ice-
bridge guidelines are used to roughly
estimate trafficability of frozen soft
tailings. Haultrucks and larger dozers
are heavy enough to crack and break
through the frost and can become
mired or sink.
Adding cement and flyash
Cement or flyash amendments can
increase tailings strengths dramati-
cally. The amount of amendment
controls the shear strength; concrete-
like strengths can be achieved. Such
treatment remains in the pilot stage
in oil sands largely due to cost and
logistics for the existing large and
deep deposits. Treating tailings during
discharge (at end of pipe) is being
studied. Costs and geoenvironmental
impacts of these amendments are key
parts of these studies.
Dewatering with wickdrains
Wickdrains (vertical band drains) are
used to accelerate consolidation of
natural soils for heavy-civil projects
and tailings deposits some metal
mines by shortening drainage paths.
For oil sands, they require installa-
tion at close spacing (1 to 2 m) due to
the low permeability of the tailings.
Large-scale field testing is ongoing
in oil sands tailings (eg Wells and
Caldwell 2009). Wickdrains can speed
Figure 6. The six-pack of soft tailings
capping methods.
