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Centrifuge Tests on Comixing of
Mine Tailings and Waste Rock
Downloaded from ascelibrary.org by UNIVERSITY OF NEW SOUTH WALES on 10/26/17. Copyright ASCE. For personal use only; all rights reserved.
Nonika Antonaki, Ph.D., A.M.ASCE 1; Tarek Abdoun, M.ASCE 2; and Inthuorn Sasanakul, P.E., A.M.ASCE 3
Abstract: A series of centrifuge tests were conducted to examine self-weight consolidation, dynamic response, liquefaction potential, and
slope stability of mine tailings alone and in mixtures with waste rock. Mixture ratio of waste rock to tailings by dry mass varied around the
region of a rock skeleton just-filled with liquefiable mine tailings (roughly 2.3∶1 in this case). The performance of three mixtures was compared to that of tailings in terms of pore pressure dissipation, shear wave velocity, soil acceleration, lateral displacement, pore pressure
buildup, liquefied depth, settlement, and slope stability. Comixing proved to be a promising alternative to conventional separate disposal.
Consolidation time and settlement were reduced significantly, and shear wave velocity increased with increasing rock content. The improvement in dynamic response was not pronounced when the tailings proportion was larger than required to fill the rock skeleton but became very
apparent when that ratio was reached or surpassed. DOI: 10.1061/(ASCE)GT.1943-5606.0001783. © 2017 American Society of Civil
Engineers.
Introduction
Vast quantities of mine waste are generated annually and disposed
of by the mining industry. The produced waste can typically be
divided into two categories: highly processed mine tailings and
coarse waste rock. The different types of waste are normally stored
separately (Blight 2009). Failures of tailings storages can pose an
enormous threat to human safety as well as the environment because of the large material volumes that may be released.
Earthquake-induced liquefaction is one of the major causes of
failure in seismically active areas (Rico et al. 2008; Blight
2009). Tailings from hard rock mines are often very loose,
saturated, nonplastic sandy silts and are thus vulnerable to liquefaction (Vick 1990). Their generally low permeability can result
in slow consolidation rates, and shear strength typically remains
low even after consolidation. Conversely, waste rock dumps
possess high shear strength and permeability and are mostly dry.
Yet water seepage through the sizeable voids can cause acid mine
drainage (AMD) and contamination, depending on the chemical
composition of the mine waste.
Comixing [initially proposed by Brawner (1978)] is defined as
the combination of tailings and waste rock prior to disposal
(Bussiere 2007). Taking advantage of the benefits of both waste
streams is a promising way of addressing the issues of conventional
separate disposal. Keeping the waste rock saturated reduces AMD,
and adding large frictional particles to the mine tailings can both
1
Geotechnical Engineer, Geotechnical and Tunneling, Technical
Excellence Center, WSP; 1 Penn Plaza, New York, NY 10119 (corresponding author). ORCID: https://orcid.org/0000-0003-4226-3231. E-mail:
nonika.a@live.com; antonakin@pbworld.com; nonika.antonaki@wsp.com
2
Professor, Dept. of Civil and Environmental Engineering, Rensselaer
Polytechnic Institute, 110 8th St., JEC 4049, Troy, NY 12180. E-mail:
abdout@rpi.edu
3
Assistant Professor, Dept. of Civil and Environmental Engineering,
Univ. of South Carolina, 300 Main St., C227, Columbia, SC 29208. E-mail:
sasanaku@cec.sc.edu
Note. This manuscript was submitted on July 26, 2016; approved on
May 18, 2017; published online on October 25, 2017. Discussion period
open until March 25, 2018; separate discussions must be submitted for
individual papers. This paper is part of the Journal of Geotechnical
and Geoenvironmental Engineering, © ASCE, ISSN 1090-0241.
© ASCE
reinforce and aid drainage. Using the voids of the waste rock to
store the tailings additionally serves to utilize otherwise unexploited space. Increased shear strength can lead to steeper and more
stable beach slopes in the impoundments and further conserve
space. A few key considerations are chemical compatibility of
the materials, degree of mixing, method of placement, and mixture
ratio of waste rock to mine tailings (Wickland et al. 2006). Ratios
between 0.5∶1 and 8∶1 (by volume) were considered, and ratios
higher than 4∶1 were suggested by Leduc and Smith (2003).
Wickland and Wilson (2005) performed a meso-scale column study
on the consolidation of mixtures at ratios of 4∶1 and higher, relating
waste rock to tailings by dry mass. That would translate to somewhat lower ratios of dry rock to wet tailings by volume. They
blended clay to sand-size tailings with rock particles smaller than
15 cm using a concrete transit mixer. Codisposal by combined
pumping at a ratio of 5.25∶1 has been tried by the Jeebropilly colliery in southeastern Queensland, Australia (Morris and Williams
1997). The authors did not clarify whether the ratio referred to volume or mass. According to Wickland et al. (2010), codisposal was
listed in the services of major Canadian geotechnical firms in 2003
and was considered as an option since 2007, even though the
concept had barely been applied or investigated. Bussiere (2007)
subdivided codisposal of tailings and waste rock into comixing,
layering codisposal and waste rock in impoundments, depending
on the degree of mixing of the two materials. Waste rock is commonly a coarse run-of-mine product created from the blasting of
hard rock. Leduc and Smith (2003) talked about practical issues
of mixing, such as the selected method, and said that it should
be evaluated for the specific site and mine waste. They specifically
mentioned three methods of blending: mixing at the face by placing
both tailings and waste rock near the crest of the active dump and
then pushing both over the face with a dozer; placing the tailings in
a haul truck preloaded with waste rock and then dumping the
un-mixed material at the dump face; and mixing the waste and
tailings together on a conveyor belt. Costs associated with these
new approaches can initially be higher than with traditional tailings
management methods, but the difference may be worth it when
considering site rehabilitation costs and long-term environmental
risks (Bussiere 2007).
04017099-1
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Centrifuge Testing Program and Description
A set of nine centrifuge tests was conducted to study the behavior
of fine mine tailings alone and in mixtures with waste rock.
Tests 1–6 were performed with tailings, and Tests 7–9 were
performed using mixtures. The first two tests were static selfweight consolidation tests, and the second two examined the dynamic response of partially consolidated mine tailings (Antonaki
et al. 2013, 2014). Only Tests 5–9 are presented herein for the
purpose of comparing the behavior of fully consolidated mine
tailings and waste rock–tailings mixtures in terms of their consolidation, as well as their response to dynamic loading. Dynamic loading was applied by the in-flight shaker installed on the centrifuge
basket. A one-dimensional stacked-ring laminar container was used
to allow soil profile deformation in the direction of dynamic loading as illustrated in the cross-sectional view of Fig. 1. The container
included rigid end walls that were removed before the model was
subjected to an earthquake. A centrifugal acceleration of 80 g was
deemed appropriate to simulate a deposit of small-to-medium
height, and all quantities, including the container dimensions in
Fig. 1, are reported in the prototype scale.
The mine tailings were shipped for study from the metallurgical
pilot plant for a planned mining project (by Golder Associates Ltd.)
located in the earthquake-prone Andean region of South America.
The material consisted of 60% fines and was classified as CL-ML
(LL of 22% and PL of 17%). A water content of 59% was selected
as the pumping water content at disposal and was applied to all
tests. The corresponding density was 1,650 kg=m3 . According
to Taylor (1995), water is most commonly used in geotechnical
centrifuge tests. The velocity of water is scaled up by scale factor
N, and water is sometimes replaced with fluid at a viscosity N times
higher than that of water. That, however, is generally not practically
applicable to fine-grained soils, and the slight increase in
permeability when using water in centrifuge tests is considered
acceptable. Because of the very low shear strength of the material,
special miniature plates had to be designed and glued to all sensors
to make instrumentation feasible. Excessive lateral movement of
the sensors during a calibration test led to the development of a
supplementary means of stabilization in the form of thin vertical
wires running through the sensor plates. The wires were loosely
tied to the crossbeams and the bottom of the container. Waste rock
particles with dimensions in the range of 20–40 cm were considered to be adequately representative of field conditions based on a
literature review (Wickland and Wilson 2005; Wickland 2006;
Wickland et al. 2010) and internal discussions with experienced
mining engineers (Golder Associates Ltd., personal communication, 2014). Crushed stone with an average size of 0.3 cm was purchased and sieved to achieve that range when a scaling factor of
approximately 80 was taken into account. The density and void
ratio of the dry waste rock were measured at 1,400 kg=m3 and
0.85, respectively. Using basic material properties and soil phase
relations, the mixture ratio to accomplish a just-filled rock skeleton
was calculated at approximately 2.3∶1 waste rock to mine tailings
by dry mass. Ratios between 2∶1 and 3∶1 were therefore used in
Tests 7–9. The two materials were blended using a power mixer,
as illustrated in Fig. 2(a). While the tailings’ initial water
content was equal to 59% for all tests, water content of mixtures
at disposal was between three and four times lower.
Models were built in four layers, at approximately 6–7.5-m
initial height per layer depending on the material. Layers were
numbered in the order that they were placed, as shown in Fig. 1.
Layer heights varied slightly between tests and testing phases.
Mine tailings models were built higher because they were expected
to settle more than mixtures. Brief consolidation at 20 g under the
material’s own weight followed deposition of every layer, whereas
drainage was allowed only at the surface. The following layer was
placed after fully instrumenting the previous one. Sensors
embedded in the soil model included miniature pore pressure
transducers, modified pore pressure transducers that served as
52.5 m
4%
26.6 m
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Evaluation of mine waste properties is undoubtedly essential,
but obtaining representative samples and performing laboratory
tests under realistic conditions is a challenge, particularly
because waste rock particles can reach dimensions larger than
3 m (Wickland et al. 2010). Centrifuge modeling allows scaling
of the rock particles and enables the simulation of layered disposal,
consolidation and earthquake loading of the materials under
prototype stress levels. In this study, tailings alone and uniformly
mixed deposits were constructed, instrumented, and tested in terms
of consolidation, liquefaction potential, and slope stability on the
centrifuge at the Center for Earthquake Engineering Simulation at
Rensselaer Polytechnic Institute in Troy, New York. Mixtures were
designed in terms of ratio of waste rock to mine tailings by dry
mass, aiming to create a rock skeleton “just-filled” with tailings.
Slow cyclic and postcyclic monotonic triaxial tests performed by
Wijewickreme et al. (2010) demonstrated that the rock skeleton
predominantly carried the loads in that configuration. Wickland
(2006) suggested that mixture ratio and tailings water content
are the most manageable mixture design variables. The latter
was not varied for the purposes of this research.
Layer 4
Layer 3
Layer 2
4.7 m
Layer 1
Direction of Dynamic Loading
Pore Pressure Transducer
Accelerometer
Settlement Gauge
LVDT
Pair of Bender Elements
Fig. 1. Key prototype dimensions and model configuration in stacked-ring rectangular container mounted on shaker prior to dynamic loading
© ASCE
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Fig. 2. (a) Comixing mine tailings and waste rock at specified ratio prior to disposal; (b) pair of bender elements attached to individual aluminum
mounts and embedded in the mixture during layer disposal and consolidation
settlement gauges (Antonaki et al. 2013, 2014), and accelerometers. Additional fixed-depth pore pressure transducers were
attached to a vertical ruler that was then loosely taped against
the middle of one of the long container walls, as depicted in Fig. 1.
Bender element pairs were installed in vertical aluminum columns
and glued to opposite sides of a rigid container during static Tests 1
and 2. Shear wave velocity measurements were thus obtained for
the mine tailings, and bender elements were not used again until
Tests 8 and 9. Because a deformable container was used in these
tests, bender elements had to be installed in individual mounts that
were embedded in the soil as exhibited in Fig. 2(b). As shown in
Fig. 1, bender elements were not placed in the top (fourth) layer
because the confinement was not sufficient to guarantee reliable
data. The 2∶1 mixture was not stiff enough to hold the mounts
in place, and shear wave velocity data is not available for
Test 7. After construction was complete, a centrifugal acceleration
of 80 g was applied to all models to simulate the desired height and
corresponding stress level. Consolidation was monitored until
completion, and models were subsequently spun down. Consolidation duration was in the range of 1.3–3.4 years, or a few hours on
the centrifuge. Table 1 summarizes test parameters including
consolidation time for each of the presented tests. As expected,
mine tailings exhibited slower consolidation rates than mixtures.
The necessary steps before dynamic loading included stopping
the centrifuge, draining the water that collected at the surface,
recording the observed settlement, removing surficial material
for a 4% slope excavation, detaching the rigid end walls to allow
deformation, and placing additional instrumentation. In practice,
thickened tailings can be deposited to form a gentle slope (between
2 and 6%) (Sanin et al. 2012). Beach slopes typically range between 0.2 and 1.5% when tailings are not highly thickened and
between 2 and 10% for high-density thickened tailings (HDTT)
and paste (Engels 2002). A view of the mild slope created at
the surface of a layered mixture model after consolidation is given
in Fig. S1(a). Sensors added included accelerometers and LVDTs
glued on opposite ends of the sliding rings that constituted the container. The final configuration is presented in Fig. 1. Additionally,
four targets were placed at the slope surface and monitored via
video cameras for slope displacement. A harmonic motion consisting of 50 cycles at a frequency of 0.9 Hz was selected as the base
excitation. Acceleration amplitude increased during the first
10 cycles, was maintained constant for the next 30, and then decreased during the last 10 cycles. Maximum acceleration could be
adjusted, and two values were targeted in this testing sequence:
roughly 0.10 and 0.25 g. The motion adjusted to approximately
0.10 g will henceforth be referred to as Motion 1, and the second
© ASCE
Table 1. List of Presented Centrifuge Tests and Main Parameters
Test number
5
6
7
8
9
Material
Consolidation
time (years)
Applied
motions
amax (g)
Mine tailings
Mine tailings
2.0∶1 mixture
3.0∶1 mixture
2.4∶1 mixture
3.3
3.4
1.3
1.3
1.3
1
2
1 and 2
1 and 2
1 and 2
0.10
0.26
0.07 and 0.24
0.1 and 0.21
0.13 and 0.24
and stronger motion will be referred to as Motion 2. Time was allotted for pore pressure dissipation before and after the motions
were applied. Because of low permeability and shear strength, only
one motion was applied to each mine tailings model, whereas both
motions were applied to all three mixed models, as stated in Table 1.
Slight discrepancies in acceleration amplitude were attributed to
small differences in model mass and stiffness as well as expected
variation in shaker response. Fig. S1(b) depicts a vertically excavated mixture deposit after a test was concluded. The bottom pore
pressure transducer was revealed as the excavation progressed.
Results
Results from the consolidation and dynamic loading phases of
Tests 5–9 are presented in prototype scale and discussed in the
following sections.
Consolidation
All deposits consolidated until excess pore pressure dissipated,
although some evaporation was still taking place as the centrifuge
was spinning. Pore pressure transducers fixed at the bottom of the
container and on the vertically placed ruler were used to assess the
end of consolidation. As stated in Table 1, consolidation of mine
tailings lasted approximately 3.4 years, whereas consolidation of
mixed deposits ended markedly sooner, approximately 1.3 years
after construction was concluded. Waiting time on the centrifuge
was kept the same for all three mixtures, even though mixtures with
higher waste rock content consolidated slightly faster. The final
dissipation rate was close to 0.01 kPa=day at the bottom of all
deposits. Fig. 3 compares the pore pressure dissipation rate of a
mine tailings deposit and three mixed deposits at two different
depths. Pore pressure has been normalized with its initial value
to facilitate the comparison between different materials and model
heights. Fig. 3(a) shows pore pressure measured by a fixed sensor
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J. Geotech. Geoenviron. Eng.
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Fig. 4. Shear wave velocity profiles of mine tailings and 2∶1, 2.4∶1, and
3∶1 mixtures after 1.25 years of consolidation (prototype scale)
Fig. 3. Pore pressure dissipation normalized with maximum value
during consolidation of mine tailings and 2.4∶1, 3∶1 mixtures at depth
of (a) Layer 3; (b) Layer 2 (prototype scale)
in the proximity of the third layer surface, which corresponded to
approximately 7 m from the surface of all deposits with the
exception of the 2∶1 mixture. The thickness of the 2∶1 deposit was
a few meters less than that of the other mixtures, and thus the sensor
was located 4.1 m from the surface. All normalized curves start at
100%, but different rates and proportions of dissipation were experienced by each material. Similarly, Fig. 3(b) shows pore pressure measured at the level of the second layer, or 12 m from the
surface. The depth of the same sensor was 9.3 m in the 2∶1 mixture.
Some initial dissipation that occurred in the 2∶1 deposit was not
captured because of a technical issue; hence, the 2∶1 mixture
experienced somewhat more dissipation at the beginning than
could be plotted, which would move the curve slightly lower than
shown but would not markedly alter the observed trends. Pore pressure in the mine tailings dissipated 34–35% of its starting value at
both presented depths [Figs. 3(a and b)]. The corresponding value
was 7–8% for the 2∶1 mixture and approximately 7% for the 2.4∶1
mixture. The 3∶1 mixture data was corrected for evaporation because the top of the deposit was partially saturated by the end
of the consolidation phase. Dissipation was within the range of
4–6% after the correction was applied. Similar findings were derived at other depths in terms of dissipation amounts as well as
general trends. Mixtures consolidated rapidly because of lower
fines and water content, which led to higher permeability and less
development of excess pore pressure. There appears to be a shift in
behavior when the mixture ratio rises above the just-filled value.
Both the 2∶1 and 2.4∶1 mixture curves look qualitatively similar
to those corresponding to tailings. They all consist of an initial
steep fragment and a noticeably less steep second fragment. The
3∶1 mixture curves, however, are moderately sloped and resemble
a straight line near parallel to the final part of the tailings dissipation
© ASCE
curves. The behavior approaches that of a coarse rock with very
little pore pressure to dissipate. Consolidation that occurred while
the centrifuge was accelerating was most likely a substantial portion of the total dissipation for the 3∶1 mixture. Therefore, the dissipation rate was already very low when centrifugal acceleration
finally became constant. The 2∶1 and 2.4∶1 curves are enclosed
by the tailings and 3∶1 curves but lie in closer proximity to the
3∶1 curves.
Shear wave velocity was evaluated from bender element
readings, and a comparison between tailings alone and mixtures
at approximately 1.25 years after construction is presented in Fig. 4.
Data is unavailable up to a 5–6-m depth below ground surface level
(GSL) because of low confining stress and shear strength. Even
though the tailings deposit continued to consolidate, shear wave
velocity more or less remained constant after the first year. Shear
wave velocity was observed to increase with depth in all three
cases. The rate of increase with depth was more pronounced in
the 3∶1 mixture. Only two data points were available for the
just-filled 2.4∶1 deposit because the top bender pair was found
slightly rotated and measurements were disregarded. Shear wave
velocity for that mixture was almost exactly equal to the average
of the other two near the bottom and closer to the shear wave velocity profile of the 3∶1 mixture at shallower depths. The mine tailings
profile started at 118 m=s and increased to 160 m=s between 7 and
18 m from the surface, whereas the V s range for the 3∶1 mixture
was 153–268 m=s between 6 and 18 m. The available range for the
2.4∶1 deposit was 194–208 m=s between 12 and 18 m. The 2∶1
shear wave velocity profile would presumably lie between the mine
tailings and 2.4∶1 curves. Fig. 4 confirms the expected increase in
stiffness with the addition of waste rock to the tailings.
Dynamic Loading
The dynamic response of the tailings and mixed deposits to
Motions 1 and 2 is explored in terms of soil acceleration, lateral
displacement, and pore pressure buildup with depth. Plotted in
Fig. 5 are soil accelerations measured at the surface of the deposits
during Motion 1. Base excitation is included in the graphs to
demonstrate the modification of the acceleration amplitude and
frequency content as it traveled through the deposits. Waste rock
04017099-4
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Fig. 5. Motion 1: soil acceleration at a small depth from the surface
plotted with base excitation for (a) mine tailings; (b) 2∶1 mixture;
(c) 2.4∶1 mixture; (d) 3∶1 mixture (prototype scale)
content increases from top to bottom, with the uppermost graph of
Fig. 5 corresponding to tailings alone and the bottom graph corresponding to the 3∶1 mixture. Mine tailings were highly liquefiable,
as is indicated by the decrease in soil acceleration and the period
elongation captured in Fig. 5(a). In fact, the slope liquefied before
the maximum base acceleration was reached. It is visible that the
soil (in black) stopped being able to follow the base movement
(in gray) after only a few cycles, both in terms of acceleration
amplitude and frequency of oscillation, because loss of strength
and stiffness were caused by liquefaction. The response changed
drastically with the rise in mixture ratio. The 2∶1 mixture exhibited
asymmetrical dilative spikes in the direction toward the top of the
slope, as seen in the second graph of Fig. 5. At the same time, it
experienced a rapid decrease in acceleration and concurrent period
elongation, but only after the maximum base acceleration was
applied and amplified by the soil. The 2.4∶1 mixture behaved
similarly, but the base excitation amplitude was 86% higher than
that applied to the 2∶1 mixture (Table 1). That led to sooner and
higher amplification of the base motion, followed by deamplification as the surficial material started liquefying. A slower, more
gradual decrease in acceleration amplitude paired with lower simultaneous spikes asymmetrically pointing upslope indicated lower
strains than those experienced by the 2∶1 mixture. Signs of dilative
behavior were present in the acceleration of the 3∶1 deposit in the
© ASCE
Fig. 6. Motion 1: soil acceleration at the depth of the first layer plotted
with base excitation for (a) mine tailings; (b) 2∶1 mixture; (c) 2.4∶1
mixture; (d) 3∶1 mixture (prototype scale)
form of increasingly pronounced spikes (Zeghal and Elgamal
1994). High amplification occurred through the soil profile, and
soil acceleration amplitude continued to rise during the entire motion. The behavior was evidently dominated by the rock skeleton
once the tailings became less than enough to fill the voids.
Plotted in Fig. 6 is soil acceleration deeper within the deposits
during Motion 1. Base excitation is included in these graphs as
well. Prior to dynamic loading, all deposits had an average height
between 18.5 and 19 m, with the exception of the 2∶1 deposit that
had an average height of 17.4 m. At a depth of 13–14 m from the
surface, which corresponded to Layer 1, all deposits were capable
of sustaining the applied base excitation and only presented small
amplification of the motion. It should be taken into account that the
highest base acceleration was applied to the 2.4∶1 mixture and the
lowest to the 2∶1 mixture (Table 1). They are, however, considered
comparable for the purposes of this study.
Figs. 8 and S2 analogously compare the response of the four
deposits when subjected to the stronger Motion 2. The improvement in dynamic response because of comixing with waste rock
became more evident when a stronger earthquake was applied.
At 1.9 m from the surface, the mine tailings liquefied after four
loading cycles, whereas period elongation was noticeable even before the soil acceleration started decreasing (Fig. S2). The material
at that depth completely lost its strength and stiffness, as was indicated by its inability to transmit any acceleration. At 3 m from its
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Fig. 7. Motion 2: soil acceleration at the depth of the second layer
plotted with base excitation for (a) mine tailings; (b) 2∶1 mixture,
and at the depth of the first layer for (c) 2.4∶1 mixture; (d) 3∶1 mixture
(prototype scale)
surface, the 2∶1 mixture started exhibiting dilative spikes associated
with simultaneous pore-pressure drops (Zeghal et al. 1996; Zeghal
and Elgamal 1994) after approximately five acceleration cycles.
The spikes were asymmetrically higher toward the top of the slope,
indicating that the main direction of soil deformation was downslope. Only a few cycles later, this material also liquefied completely, and the acceleration reading became zero. Spikes
appeared in the 2.4∶1 acceleration record at 1.4 m from the surface
at approximately the same time as in the 2∶1, but the soil required
more loading cycles to substantially lose strength. Period elongation became visible in both mixtures as their stiffness started to
drop. Comparably to Motion 1, the 2.4∶1 mixture exhibited lower
asymmetrical spikes, which were present until the end of the base
excitation, indicating some residual stiffness as the deposit continued to accumulate shear strain. Conversely to the deterioration in
the response of the first three deposits because of the increase in
acceleration amplitude, the 3∶1 deposit behaved practically identically to Motion 1 at shallow depths. In Fig. 7, soil acceleration at a
larger depth is plotted with Motion 2 for the four deposits. The
response of the tailings deposit appears only slightly improved
compared to Fig. S2 because it was able to sustain two more loading cycles at this depth. Soil acceleration dropped to zero, suggesting that liquefaction reached a minimum depth of 10.7 m.
A small improvement is also visible in the 2∶1 acceleration, during
which the material required a few more cycles to completely
© ASCE
liquefy at 9.6 m from the surface. The closest available accelerometers located in the 2.4∶1 and 3∶1 deposits were somewhat deeper,
but that does not affect the conclusions that can be drawn from the
comparison. The 2.4∶1 acceleration record at 12 m shows mostly
dilative behavior of the material at that depth. Some loss of strength
and stiffness occurred near the end of dynamic loading, and the
spikes remained asymmetrical toward the top of the slope. Acceleration measured at 7.2 m showed high spikes earlier on, followed
by zero positive acceleration with negative spikes near the end of
the motion, indicating that liquefaction reached a depth of 7.2 m in
the 2.4∶1 mixture. The 3∶1 mixture essentially followed the base
movement at 13.2 m from its surface. The depth is not directly
comparable to that of the plotted tailings and 2∶1 records, but
the next available record was located at a depth of 5 m and looked
identical to the bottom graph of Fig. 7, if slightly amplified. It can
be concluded that the 3∶1 mixture did not liquefy at any depth,
irrespective of the base excitation intensity.
Fig. 8 illustrates the performance of the four materials in terms
of residual horizontal displacement in the direction of dynamic
loading. Data was obtained from horizontal LVDTs glued on the
container rings. Final deformed profiles after application of Motion
1 are plotted in Fig. 8(a). Positive displacement denotes movement
toward the toe of the slope. Maximum final displacements are quite
small, ranging from 15 cm downslope experienced by the tailings
deposit to 31 cm upslope experienced by the 2.4∶1 mixture. All
mixtures permanently deformed toward the head of the slope,
despite the fact that the slope surface moved toward the toe in
all cases. Video tracking of surface targets using TEMA software
gave an estimate of horizontal slope displacement, which was in the
region of 1 m for the mine tailings and the 2∶1 and 2.4∶1 mixtures,
and in the region of 10 cm for the 3∶1 mixture. These values are
approximate but give a good estimate of the slope surface displacement as opposed to the profile deformation, which was measured
by LVDTs along the container rings and did not reflect the displacement of surficial material during slope failure. Four targets were
typically used, and average displacement is reported. From the
shape of the deformed profiles in Fig. 8, the top 6–7 m of the
tailings deposit and the top 2–3 m of the 2∶1 deposit appeared
to be liquefied and isolated from the deposit below that depth.
A similar observation cannot be made for the 2.4∶1 and 3∶1 deposits, which presented a more uniform distribution of lateral displacement. Deformed profiles only due to application of Motion 2 are
plotted in Fig. 8(b). The mine tailings deposit visibly deformed
downslope, with a maximum value of 68 cm at the surface. The
2∶1 mixture deformed in the same direction with a maximum value
of 40 cm. The top few meters appear isolated from the rest of the
deposit in both cases, but displacements are sizeable below as well.
The 2.4∶1 mixture only slightly deformed during Motion 2, whereas
the 3∶1 mixture exhibited an almost identical response as to Motion
1. Slope surface displacements obtained from video tracking were
in the region of 1 m for tailings and the 2∶1 mixture, 50 cm for the
2.4∶1, and 20 cm for the 3∶1.
Motions 1 and 2 were applied as a sequence to the mixed deposits, and cumulative displacement profiles are displayed in Fig. 9.
Mixture response was comparable in terms of maximum values, but
the existence of the 4% slope was only sufficient to bias the 2∶1
deformation direction. The deformed profiles of tailings for
Motions 1 and 2 were not combined because they came from
two different tests (Tests 5 and 6), but if added, the maximum
displacement would be 83 cm as opposed to the mixture range
of 24–27 cm. Accumulated soil surface displacements obtained
from video tracking were in the range of 2 m for the tailings
and 2∶1 deposits, 1.5 m for the 2.4∶1, and 30 cm for the 3∶1 deposit,
all toward the toe of the slope.
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Fig. 8. Horizontal displacement profiles for mine tailings for 2∶1 mixture, 2.4∶1 mixture, and 3∶1 mixture due to (a) Motion 1;
(b) Motion 2 (prototype scale)
Fig. 9. Final horizontal displacement profiles for 2∶1 mixture, 2.4∶1
mixture, and 3∶1 mixture (prototype scale)
Figs. 10 and S3 illustrate the development of excess pore pressure with depth during application of Motions 1 and 2, respectively.
Waste rock content increases from top to bottom in the figure.
Figs. 10(a) and S3(a) corresponds to mine tailings and Figs. 10(d)
and S3(d) corresponds to the 3∶1 mixture ratio. Markers denote data
points obtained from floating pore pressure transducers, and corresponding curves denote continuous pore-pressure buildup profiles that were estimated using the displayed points. The black
solid line denotes buildup at the end of dynamic loading. Vertical
effective stress at rest is included in the graphs to enable evaluation
of liquefied depth. Unit weight and water level varied to some extent between materials as well as with time, and effective stress was
assessed separately for every deposit and motion.
As shown in Fig. 10(a), pore pressure gradually built up in the
tailings deposit, and by the end of Motion 1, the top 6 m or so had
© ASCE
liquefied. This observation is consistent with the deformed profile
of Fig. 8(a). The same is plotted in Fig. 10(b) for the 2∶1 mixture,
the top 4 m of which experienced complete liquefaction. The top
2–3 m appeared separated in Fig. 8(a), but the next available LVDT
reading was located at a depth of 8 m, which prevented a more
accurate estimation of the liquefied depth from the deformed
profile. Considerable pore pressure built up below the liquefied
depth. In this case, most of the buildup happened within the first
15 s, and the maximum occurred at approximately 50 s. Because of
an increased permeability of the mixture compared to tailings, pore
pressure started dissipating as soon as acceleration amplitude
started decreasing. The shape of the profiles in Fig. 10(c) looks
quite different, which can be attributed to the more permeable
continuous arrangement of rock particles in the just-filled case
of the 2.4∶1 mixture ratio. Most of the buildup transpired within
the first 10–15 s, and it appeared almost as if there was double
drainage, at the top and the bottom of the deposit. After the first
15 s of Motion 1, the bottom half of the deposit started to catch up,
and maximum buildup occurred at 50 s, when dissipation was initiated. The material did not liquefy, although the top half came
close to zero effective stress conditions. The surface acceleration
depicted in Fig. 5 did exhibit signs of liquefaction. Fig. 10(d)
clearly displays that the 3∶1 mixture did not come close to losing
its strength at any depth. Pore pressure buildup did transpire but
was much lower than in the other deposits. It mostly took place
within the first 10–15 s, and the maximum occurred at 15–30 s.
The top 14.5 m—at least—of the tailings deposit liquefied during Motion 2, as shown in Fig. S3(a). The top half of the deposit
liquefied during the first 15 s, and the overall maximum occurred at
50–60 s. The response of the 2∶1 mixture, as captured in Fig. S3(b),
looks very similar. The top 14 m at a minimum liquefied. Most of
the buildup took place within the first 30 s, and the maximum occurred at 50 s. As when subjected to Motion 1, pore pressure in the
2∶1 deposit started dissipating slowly when acceleration amplitude
started decreasing. Fig. S3(c) depicts pore pressure buildup in the
2.4∶1 deposit during Motion 2. The shape of the profiles was similar
to that during Motion 1, resembling double drainage conditions;
however, pore pressure built up more steadily until complete liquefaction of the top half took place at approximately 50 s. In this case,
04017099-7
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Fig. 10. Motion 1 (0.10 g): evolution of pore pressure buildup plotted with effective stress profile for (a) mine tailings; (b) 2∶1 mixture; (c) 2.4∶1
mixture; (d) 3∶1 mixture (prototype scale)
dissipation did not initiate before the end of dynamic loading, and
maximum pore pressure buildup values occurred at 60 s. Fig. S3(d)
demonstrates that, consistently with measured accelerations and
displacements, the 3∶1 mixture behaved almost identically when
subjected to Motions 1 and 2. Some buildup was measured but remained far from liquefaction throughout dynamic loading. The
maximum occurred at 50 s, and some dissipation became noticeable by the end of the motion.
The four materials are compared with respect to settlement
corresponding to each phase of the centrifuge tests in Table 2.
Settlement was normalized with deposit height during each phase
to allow for meaningful comparisons between deposits. The primary observation is that consolidation settlement decreased when
waste rock content increased. The difference was very noticeable
between mine tailings and the 2∶1 mixture, from 33 to 15%,
respectively, but not as large when more waste rock was added
Table 2. Settlement Normalized with Model Height for Every Testing Phase and Material
Consolidation
Dynamic loading
Motion 1
Material
Per layer
(20 g) (%)
Complete
(80 g) (%)
Total (%)
Head (%)
Tailings
2.0∶1
2.4∶1
3.0∶1
20
11
7
4
13
4
2
2
33
15
9
5
4
3
5
4
© ASCE
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Motion 2
Toe (%)
2
−1
0
3
Head (%)
7
3
3
4
Toe (%)
−3
−1
0
3
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to the mix. This remark is consistent with pore pressure dissipation
trends previously discussed and shown in Fig. 3. The three mixtures
exhibited comparable behavior, whereas the difference with mine
tailings alone was more distinct. Most consolidation-induced settlement took place during layer consolidation at 20 g. Specifically,
61% of the total settlement occurred during layer consolidation of
mine tailings. That proportion increased to 73% for a 2∶1 mixture
ratio, and to 78 and 80% for 2.4∶1 and 3∶1, respectively. That was to
be expected because the mixture permeability and consolidation
rate rose with an increasing mixture ratio.
The bottom half of Table 2 lists the settlements that occurred
during dynamic loading at the head and the toe of the slopes.
The differences between material performances in terms of
earthquake-induced settlement might appear negligible, but they
were notable when considering stability of a 4% slope. Negative
settlement denotes apparent rise in elevation occurring at the toe
of the slope when some of the surficial material moved downslope.
During both motions, settlement at the head of the slope was larger
than at the toe in all deposits, denoting some reduction of the slope
inclination. Tailings settlement at the head was 4% for Motion 1
and 7% for Motion 2. Settlement at the toe of the slope was 2%
because of Motion 1, whereas a 3% uplift was caused by
Motion 2, resulting in 2.8 and 0.6% final inclinations, respectively.
The 2∶1 deposit experienced 3% settlement at the head and 1% uplift at the toe during both motions. Inclination was approximately
equal to 2.8% after Motion 1 and 1.4% after Motion 2. The second
motion was applied to the same mixed deposits after pore pressure
buildup caused by Motion 1 dissipated and was hence applied to a
2.8% slope in this case. The 2.4∶1 deposit settled 5 and 3% at the
head of the slope for Motions 1 and 2, correspondingly, whereas the
toe did not settle or elevate for either motion. The aforementioned
settlement resulted in 3.4 and 2.4% inclination, respectively. The
3∶1 deposit responded similarly to both motions in terms of settlement, experiencing 4% at the head and 3% at the toe and settling to
3.5 and 3.1% inclinations. Slope stability undoubtedly showed
considerable improvement as waste rock content increased.
Conclusions
Comixing of waste rock and tailings was found to be beneficial
both in terms of consolidation rate and settlement as well as
dynamic response and slope stability. The mixture ratio of waste
rock to tailings by dry mass to achieve contact between rock
particles by simply filling the voids with tailings was estimated
at 2.3∶1. Consolidation time and settlement were reduced by approximately 2.5 times when comixing at a ratio of 2∶1 was applied.
At higher ratios, mixtures consolidated even faster, and settlement
was further reduced. At 1.25 years, shear wave velocity of the 3∶1
mixture was 1.3–1.6 times higher than that of the tailings within the
same depth range (6–18 m), whereas the 2.4∶1 mixture lied in between. Further increase with time was not measurable, even though
consolidation continued to occur.
The improvement in dynamic response was not pronounced
when the tailings proportion was larger than required to fill the rock
skeleton. That highlighted the importance of accurately estimating
the theoretically optimum mixture ratio (just-filled), particularly
when the objectives include both shear strength increase and
saturation of the waste rock for AMD reduction. Mine tailings
and three mixed deposits were subjected to two levels of harmonic
base excitation, with average acceleration amplitudes of 0.10 and
0.24 g. The tailings and 2∶1 mixture liquefied in both cases. The
liquefied depth was at least 25–30% of the deposit depth for Motion
1 and roughly 80% for Motion 2. The 2∶1 mixture exhibited some
© ASCE
signs of dilation in the acceleration data. Only the surface of the
2.4∶1 deposit showed signs of liquefaction for Motion 1, whereas
45% of the profile liquefied during Motion 2. The 3∶1 deposit exhibited strictly dilative behavior regardless of base excitation intensity. Residual horizontal container displacement induced by Motion
1 was low for all deposits, and the improvement with rise in mixture
ratio became substantial only when base acceleration was higher.
Video tracking showed that all slopes moved toward the toe, but
deformation of the 3∶1 slope was practically negligible. Slope
performance was only satisfactory for mixtures 2.4∶1 and 3∶1 when
subjected to Motion 1 but markedly superior for the 3∶1 deposit
when subjected to Motion 2. The 3∶1 ratio, however, does not
satisfy saturation of the waste rock skeleton, thus preventing water
seepage and AMD.
The possibility of partially or completely eliminating the retaining structure when tailings are mixed with waste rock at a just-filled
configuration is intriguing because it would be very advantageous
to the mining industry. Additional centrifuge tests are planned to
investigate the feasibility of free-standing or partially supported
mixtures deposited at their angle of repose.
Acknowledgments
The authors would like to acknowledge Golder Associates Ltd. for
partial funding of this research.
Supplemental Data
Figs. S1–S3 are available online in the ASCE Library (www.
ascelibrary.org).
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© ASCE
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