In the past 10 years or so, specialised firms
including consulting companies have
developed enough experience to model
seismic behaviour of clayey and sandy
soils. Recording of earthquake records has
also improved, helping provide a proper
engineering basis to the exciting work that
can now be done.
Importantly, this improved technological
capacity is also giving us a better
understanding of static liquefaction and
progressive failure in tailings dams. While
dynamic liquefaction of a soil structure takes
place as a result of a sufficiently high seismic
perturbation or disturbance, static liquefaction
occurs due only to a small perturbation. There
are several factors which could lead to static
liquefaction; modern computers and software
— leveraging the results of laboratory and
field testing — are now capable of modelling
the behaviour of tailings dams for these static
liquefaction triggers.
The same applies to the problem of
progressive failure in soil mechanics, which
is the phenomenon behind some of the case
histories of TSF failures involving failed
foundations.
Progressive failure usually starts off very
slowly but, with time, the rate of deformation
increases rapidly. In several case histories,
the failure starts as an effective stress site
condition; then, when the rate of deformation
is high enough, the mode of failure changes
from effective stress to undrained or total
stress behaviour. Once again, computer
technology’s ability to use past data can be
employed to model the behaviour of tailings
dams for these progressive failure triggers.
The face of displacement
One back-analysed model is represented in
Figure 1. This shows the overall picture of
displacement in a return water dam, with
time steps modelled. This dam showed no
signs of distress for more than 20 years and
then failed dramatically within one week,
with more than 2.5m of displacement at the
crest of the water dam.
If a set of shear strength properties are
selected, such as an average effective stress
friction angle of 25 degrees, then the model
cannot be made to fail — even after a long
time of modelling. If an average effective
stress friction angle of 24.7 degrees is selected,
it can be seen that — for a long time — the
model behaves in a meta-stable manner;
when sufficient loss of effective stress strength
occurs, the failure develops very quickly,
as was observed on site. This is the class A
prediction of the actual behaviour of this
strain softening clay material from site. If,
however, the actual average shear strength of
the clay is taken as an effective stress angle
of 22 degrees, it can be observed that the
modelled failure occurs a short while after
the end of construction, which does not
correspond with the observed site conditions
where it took 20 years for the failure to be
initiated. The other two examples modelled,
as shown on Figure 1, are located between
the limits of 22 degrees and 24.7 degrees
and show the impact of friction angle (shear
www.miningmirror.co.za
Mining in focus
Figure 1
Figure 1 shows — on the vertical axis — the vertical displacement of the crest of the dam at
the location of failure. The horizontal axis shows the time steps modelled. Note the following:
• The change in the time increment related to the time steps modelled when the effective
stress behaviour was changed to undrained behaviour;
• The effective stress properties changed when the inception of failure commenced and the
rate of failure was about to accelerate; and
• When the rate of failure was increasing, the stiffness of the clay soils was changed from a
Poisson ratio of 0.2 to a Poisson value of 0.45 (close to 0.5).
The key point to be understood is this: The time increment related to the time steps for the
effective stress portion of the graph is much longer than for the time increments related to the
time steps for undrained behaviour modelling. This means that the horizontal axis for the time
period before failure commences for the 24.7-degrees case should be 20 years, and the time
from inception of failure to the end of failure should be 10 days.
strength) on the inception and manner of
progressive failure.
Evolution of technology
Another important contribution to the TSF
modelling and design tools at our disposal
today come from other technologies. Satellite-
type technologies, for example, can be used
to measure three-dimensional displacement
profiles. In one case, satellite imagery has been
used to measure the extent to which sinkholes
below a tailings dam were allowing settlement
of the TSF over a few years.
Technological developments in milling
will also have an impact on how we design
TSFs going forward — in particular the
implications of finer grind. While 45%
of tailings before 1980 was finer than 75
microns, this had risen to 90% by 2010. In
new mining projects, the expectation is that
80% of material will be finer than 53 microns.
The significance of this cannot be
overstated. This will mark the first time that
rock-flour tailings from hard rock mines
will become too fine to be self-supporting
for normal rates of rise of about one metre
a year. The change in the permeability of
tailings will require new tailings deposition
techniques and TSF designs. This might
include solutions such as impoundment
walls and suitably sized buttresses of various
materials. Mines will soon be considering the
use of waste rock, filtration layers, and filtered
tailings as conventional construction material
in their TSF strategies.
The good news is that computer
technology and continuous software
improvements have given the sector valuable
tools with which to leverage our considerable
experience in TSF management. That said,
innovation is not always quick or easy; but
the work has begun and needs continued
commitment from all who are affected.
About SRK
SRK Consulting — a global network of
engineers and scientists — earned much
of its early reputation from its work on
tailings storage facilities, working closely
with mining companies to develop
science-based innovations to make tailings
dams safer and more environmentally
sound. Today, SRK is a multidisciplinary
operation with a depth of expertise relevant
to mining, infrastructure, environment,
energy, and water.
APRIL 2019 MINING MIRROR [23]