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P. Simms, S. Sivathayalan, F. Daliri
Carleton University
Desiccation in dewatering and strength development of high density hard rock tailings
Stress history of high density tailings stack layers : Effect of degree of desiccation
Settling o r s/w consolidation
Stress history of thickened tailings stack layers:
4
2
1
Geotehcnical Stability?Cost?Acid Rock Drainage?
Geotechnical Stability?
Set
tling
Sett
ling
ICOLD and U
NEP
(2001)
Dry
ing to
Shrinkage L
imit?
3
Effect of salts? Cracks?
Consolidation
Thickness of fresh-layerRheology
Site topography
Conceptual model of factors influencing “drying time”
Within a range, initial solids concentration does not affect initial degree of settling (column test data)
5
How we predict the rate of drying?
Hypothesis: We can use standard unsaturated flow codes, that are used in soil cover design, to predict rate of drying
Models such as SoilCover, SVFlux, Hydrus…
Complicating factors with tailings: salts, cracking, significant volume change
Matric suctiondata from a two-layer test
Initial period of hindered settling
Placement of Second layer
So we have found that this approach has a certain skill,what happens when we extrapolate to deep layers of tailings?
Hypothetical analyses performed to analyze what happens when a fresh layer is deposited over several (1m to 10 m) of previously desiccated tailings
We used a range of parameters (Water retention curves, saturated hydraulic conductivity) typical to many hard rock tailings (air-entry value from 50-500 kPa, saturated hydraulic conductivity between 10-6 and 10-8 m/s)
Potential evaporation rate varied between 2 and 10 mm/day.
Drainage and no-flow bottom boundary conditions applied
Influence of fresh layer thickness on drying rate
PE = 5 mm /day
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5 10 15 20 25 30 35
Mid
-laye
r gra
vim
etri
c wat
e co
nten
t
Time after deposition (Days)
Drying of 0.5 m layer over relatively dry tailings
Drying of 0.2 m layer over relatively dry tailings
Uniform drying for 0.5 m layer
Uniform drying for 0.2 m layerThese and similar predictions published in 2010 Paste and thickened tailings conference
Multilayer Deposition in a Drying Box
Volumetric Water Content Sensors
Tensiometers
Senix Distance Sensors
Scale
Desiccation Process in the Drying Box
Day 1 Day 2
Day 3 Day 4
Day 5 Day 6
Evolution of Cracks
Drying Time
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14 16
AE (m
m/d
ay)
Time (Days)
Layer 1 predicted
Layer 1 Measured
Layer 5 Predicted
Layer 5 Measured
Modelling of drying box
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00
Mid
-laye
r gra
vim
etric
wat
er co
nten
t
Time after deposition (Days)
Layer 5 data
Specific prediction for drying box
Modelling of drying box
Modelling of drying box
Generic predictions compared to field and drying box results
16
Predictions from Paste 2010 paper
Drying box – only 1 m of tailings
Monotonic Results of Desiccated/Rewetted Samples
0 2 4 6 8 10 120
5
10
15
20
25 Wd=30%, OCRd=1Wd=28%, OCRd=1Wd=23%, OCRd=1Wd=19%, OCRd=1.7Wd=17%, OCRd=2.6Wd=12%, OCRd=4.1Wd=4%, OCRd=8
Shear Strain %
Sh
ea
r S
tre
ss (
kPa
)
Settling &No desiccation
Vane Shear Results
The influence of stress history on shear strength
0 2 4 6 8 10 120
5
10
15
20
25 Wd=30%, OCRd=1
Wd=28%, OCRd=1
Wd=23%, OCRd=1
Wd=19%, OCRd=1.7
Wd=17%, OCRd=2.6
Wd=12%, OCRd=4.1
Wd=4%, OCRd=8
Shear Strain %
She
ar S
tres
s (k
Pa)
4
1
3
24
1
2
De
siccatio
n H
istory
Se
ttlin
g
3
If only desiccation to 25 % instead of 20% (SL) required, significant reduction in needed drying time
20
Important Outcomes:
Time to reach end of SW consolidation, for lifts up to 1 m and as low as 65% solids concentration, seems to be within 48 hours. Only small influence of initial solids concentration on final void ratio post-consolidation and post-drying
Samples experiencing settling without desiccation exhibited strain softening response. A relatively small degree of desiccation changes the response to strain hardening.
Though desiccation to SL and beyond increases the shear strength, it requires substantially longer drying time.
The degree of desiccation should be determined based on required site specific shear strength.
Acknowledgements
Former and current graduate students who have contributed to this work: Bereket Fisseha, Julio Henriquez, Rachel Bryan, Adedeji Dunmola, Shabnam Mizani, Adrian Manlagnit, Parviz Heidarian, Mary-Ellen Gleeson, and Farzad Daliri
Collaborator Siva Sivathayalan (co-supervisor of Farzad Daliri)
Thank you Golder Associates, Musselwhite Mine, and Barrick Gold
0
1
2
3
4
5
6
7
8
9
10
0 10 20 30 40 50 60 70
AE (m
m/d
ay)
Time (Days)
First Layer SecondLayer ThirdLayer
FourthLayer
First Resaturation
SecondResaturationFifthLayer
Actual evaporation in drying box
Preliminary Results
5 Days 8 Days 11 Days
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