Settlement and Collapse Behaviour of Coal Mine Spoil

SETTLEMENT AND COLLAPSE
BEHAVIOUR OF COAL MINE SPOIL
AND WASHERY WASTES
Ali Shokouhi, David Williams and Adrian Kho
TAILINGS AND MINE WASTE 2014
Conference Sponsors
AMEC Earth & Environmental
Knight Piésold and Co.
Ausenco
MWH
BASF Chemical
MineBridge Software, Inc.
CETCO
Paterson & Cooke
ConeTec
Robertson GeoConsultants, Inc.
DOWL HKM
SRK Consulting, Inc.
Engineering Analytics, Inc.
Tetra Tech, Inc.
Gannett Fleming
URS
Golder Associates, Inc.
Community Sponsor
CDM Smith
TAILINGS AND MINE WASTE 2014
Motivation
• Increasing demands are being placed on open
cut coal mine operators in Australia to seek
alternative methods of disposing of washery
wastes:
– Coarse-grained coarse reject (<50 mm)
– Fine-grained tailings (<1 mm)
– Attention is focusing on possible incorporation of
washery wastes (comprising 2 to 7% of total
wastes) within spoil (up to 2 m in size) piles
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Conventional Disposal of Washery
Wastes
• Coarse reject:
– Typically loose-dumped using haul trucks
– Facilitates oxygen ingress and rainfall infiltration
– May be prone to spon. com. and ARD
• Tailings:
– Thickened to ~25% solids by mass
– Pumped to a surface TSF or pit
– Difficult to dewater and rehabilitate
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Co-Disposal of Wastes
• Pumped co-disposal of combined washery
wastes:
– Typically at ~25% solids and high velocity
– Leads to a 1 in 10 coarse upper beach and
washout of fines at 1 in 100
• Possible co-disposal of washery wastes and
spoil, without or with dewatering of tailings:
– Co-placement, co-deposition (cells or layers) and
co-mingling (mixed)
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Laboratory Testing of Spoil and
Washery Wastes
• CR:T dry mass ratios of:
– Coarse reject only (at ~10 to 15% grav. mc)
– CR:T = 9:1
– CR:T = 6:1
– CR:T = 3:1
• Characterisation and compression testing of:
– Weakly cemented and cemented spoil (<19 mm)
– Mixtures of coarse reject and tailings
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Characterisation Testing
•
•
•
•
•
As-sampled gravimetric moisture content
Atterberg limits
Specific gravity
Dry and wet sieving for PSD
Standard compaction testing, as appropriate
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Compression Testing
• 76 mm diameter x 20 mm high oedometer on:
– Loose spoil specimens scalped to <2.36 mm
– Loose, wet-scalped coarse reject (<2.36 mm)
– Loose wet-scalped CR:T at dry mass ratios of 9:1,
6:1 and 3:1
• Coarse reject was added at its as-sampled
gravimetric moisture content of 14.5%
• Tailings were added at their settled grav. mc of
150% (40% solids)
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Results of Characterisation Testing
Jeebropilly
Weathered Rock
(Weaklycemented)
Mt Arthur
Sandstone
(Cemented)
Jeebropilly
Coarse Reject
Jeebropilly
Tailings
Grav. mc
14.9
3.1
14.5
150.0
Liquid limit (%)
71.0
26.3
Non-plastic
41.2
Plastic limit (%)
21.0
22.6
Non-plastic
17.2
Plasticity Index (%)
50.0
3.7
Non-plastic
24.0
Specific Gravity
2.60
2.79
2.53
1.90
SAMPLE
• Weakly-cemented Jeebropilly spoil is clay mineral-rich
• Cemented Mt Arthur spoil is almost non-plastic
• Jeebropilly coarse reject is non-plastic
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Particle Size Distributions for all-in
Spoil Samples
Similar for weakly-cemented
and cemented spoils!
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Particle Size Distributions for
Jeebropilly Weathered Rock Spoil
Breakdown on
wetting
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Particle Size Distributions for Mt
Arthur Sandstone Spoil
Little breakdown on wetting
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Particle Size Distributions for
Jeebropilly Coarse Reject & Tailings
Some breakdown of
coarse reject on wetting
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Summary Results of Standard
Compaction Testing
MAXIMUM DRY
DENSITY MDD
(t/m3)
OPTIMUM
MOISTUIRE
CONTENT OMC (%)
DEGREE OF
SATURATION AT
MDD & OMC (%)
Jeebropilly Weathered Rock
1.52
19.0
69
Mt Arthur Sandstone
1.96
11.5
67
Coarse reject only
1.45
18.8
64
CR:T = 9:1
1.42
19.3
65
CR:T = 6:1
1.36
16.2
50
CR:T = 3:1
1.31
18.5
55
SAMPLE
(Scalped to <19 mm)
• Weakly-cemented spoil has a much lower MDD & higher OMC than cemented spoil
• MDD of washery waste mixtures decreases with increasing tailings
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Compression Results for Spoil
(tested dry and wet)
• For weakly-cemented spoil:
‒ Greater compression than for cemented spoil
‒ Greater difference between dry and wet testing
• Similar compression for dry and wet testing of cemented spoil
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Compression Results for Washery
Waste Mixtures (all tested wet)
Increasing initial loose density and decreasing
compressibility (slope) with increasing tailings to 6:1
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Summary Results of Compression
Testing
SAMPLE
(Scalped to <19 mm)
COMPRESSION INDEX (Cc)
Dry
Wet
Jeebropilly Weathered Rock
0.41
0.27
Mt Arthur Sandstone
0.17
0.21
Coarse reject only
-
0.52
CR:T = 9:1
-
0.52
CR:T = 6:1
-
0.33
CR:T = 3:1
-
0.17
• Weakly-cemented spoil is far more compressible than cemented spoil, particularly dry
• Coarse washery waste mixtures (to 6:1) are far more compressible than spoil
• Washery waste mixtures become less compressible with increasing tailings filling voids
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Comparisons of Initial Loose and
Final Dry Densities Tested Wet
• All but cemented spoil are compressed to ~MDD or higher
• Intermediate CR:T mixtures compressed to >>MDD
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Comparisons of Final Wet Compression
Relative to Initial Loose Height
High CR:T mixtures, followed by weakly-cemented spoil,
compress more that cemented spoil and low CR:T mixtures
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Implications for Co-Disposal
• High CR:T mixtures compressed 40 to 50%
from their initial loose state under an applied
stress of only 1,000 kPa (45 to 60 m spoil
height)
• As tailings content increased, compression
decreases to ~20%, which is comparable to
that of cemented spoil
• Compression of weakly-cemented spoil was
intermediate at ~30%
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Implications for Co-Disposal
• Depending on CR:T ratio generated, codisposal of washery wastes in spoil piles
would be efficient in terms of optimising
available storage volume
• Given high dry densities achieved for
combined washery wastes, it is likely that their
shear strengths would also be acceptably high
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Practical Application
• In practice:
– Tailings need to be dewatered in plant, e.g. by beltpress filtering
– Coarse reject + filtered tailings can then be
conveyored and/or trucked
– Washery waste mixture can be placed in cells within
spoil and/or end-dumped
• Challenges include:
– Inconsistent washery wastes
– Stability on end-dumping washery waste mixture
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Field Trials by BMA, Involving Coarse
Reject + Filtered Tailings in a Spoil Cell
Coarse reject only
Filtered tailings only
End-dumped washery wastes
Large washery waste cell within spoil
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Acknowledgements
• Research was funded under Australian Coal
Association Re-search Program Project C19022
• Jeebropilly Coal and Mt Arthur Coal mines are
acknowledged for providing spoil samples
• Masters student Bharat Yadav, and
undergraduate students Morgan Midgley,
Mayla Martins, Nanae Kaneko and Nicholas
Smith, are acknowledged for their assistance
with testing
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