Challenges in Seismic Stability Analysis
of Tailings Dams
David Zeng
Dept. of Civil Engineering
Case Western Reserve University
International Workshop on Seismic Stability of
Tailings Dams
November 10, 2003
Outlines
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Introduction
Seismic properties of coal refuse
Challenges
Objectives of the workshop
Three construction methods for tailings
dams
coarse refuse embankment
Fine refuse impoundment
Downstream construction
Fine refuse impoundment
coarse refuse embankment
Upstream construction
Fine refuse impoundment
coarse refuse embankment
Centerline construction
Failure of Tailings Dams
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1965 in Chile, El Cobre Dams failed, 2 million tons
of tailings traveled 12 km in minutes, killed more
than 200 people.
1985 in Chile, another tailings dam failed.
1978 in Japan, a tailings dam failure released
100,000 cubic yard of sodium cyanide.
In the US, failure of tailings dams occurred at
Buffalo Creek in West Virginia (118 deaths) and
Ages Creek in Kentucky
Tailings Dams in the US
There are 711 coal waste tailings dams in
the US. 241 of them are classified as
having high hazard potential by the FEMA
hazard rating system. MSHA and OSM
have the regulatory duties to oversee
the safety of tailings dams
Specific gravity of coal refuse
Dynamic tests on coal refuse
Dynamic tests on coal refuse
Shear modulus of coarse refuse
1.2
1
G/Gmax
0.8
0.6
0.4
0.2
0
0.001
Resonant Column Test(Confining Pressure=138 kPa)
Cyclic Triaxial Test 1(Confining Pressure=138 kPa)
Cyclic Triaxial Test 2(Confining Pressure=138 kPa)
Resonant Column Test(Confining Pressure=69 kPa)
Best Curve Fitting
Seed et al. for Sand (1986)
0.010
0.100
Shear Strain(%)
1.000
Shear modulus of fine refuse
1.2
Confining Pressure=407 kPa
Confining Pressure=380 kPa
Confining Pressure=228 kPa
Confining Pressure=186 kPa
Best Curve Fitting
Seed et al. for Sand (1986)
1
G/Gmax
0.8
0.6
0.4
0.2
0
0.0010
0.0100
0.1000
Shear Strain(%)
1.0000
10.0000
Damping Ratio of Coarse Refuse
30
Damping Ratio(%)
25
Confining Pressure=138 kPa
Confining Pressure=69 kPa
Best Curve Fitting
Seed at el. for Sand (1986)
20
15
10
5
0
0.001
0.01
Shear Strain(% )
0.1
1
Damping Ratio of Fine Refuse
30
Damping Ratio(%)
25
20
Confining Pressure=407 kPa
Confining Pressure=380 kPa
Confining Pressure=228 kPa
Confining Pressure=186 kPa
Best Curve Fitting
Seed et al. for Sand (1986)
15
10
5
0
0.001
0.01
0.1
Shear Strain(% )
1
10
Current State in Seismic Design of Tailings Dams
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!
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!
Design and construction standards of tailings
dams are not as high as that for earth dams.
Poor understanding about the seismic
properties of fine and coarse refuse.
There has been no seismic induced failure of
tailings dam in the US but the potential is there.
Seismic design is required by MSHA and OSM but
most people don’t know how to do it properly.
Challenge No.1: Can fine refuse liquefy
under earthquake loading?
0.6
Percent Fines =35 15 ≤
5
0.5
Cyclic
Stress
Ratio
(CSR)
0.4
0.3
SPT Clean Sand
Base Curve
0.2
0.1
0.0
0
10
20
30
Corrected Blow Count, (N1)60
40
50
SPT tests results in the field
1
Depth (m)
Soil Type
SPT Blow Count N
Modified N1 = CN1×N
9.1
coarse refuse
33
27
21.3
coarse refuse
29
16
33.5
coarse refuse
36
15
42.7
coarse refuse
27
10
45.7
fine refuse
11
4
51.8
fine refuse
11
4
57.9
fine refuse
8
3
62.2
fine refuse
13
4
CN is calculated using the relationship proposed by Liao and Whitman (1986) to normalize to an overburden pressure of 1 TSF
Particle size distribution curve
100
90
80
Percent Finer
70
60
50
40
30
20
10
0
100
Sample1
Sample2
Sample3
Sample4
10
1
0.1
Grain Size (mm)
0.01
0.001
Results of cyclic triaxial tests
Pore Water Pressure(kPa)
140
120
100
80
60
40
20
0
0
50
100
150
200
250
300
350
400
450
500
Time(s)
Excess pore pressure recorded in a cyclic triaxial test
(initial effective confining pressure σc’ = 186 kPa, cyclic deviator stress ∆σd = ± 64%
σc’)
Results of cyclic triaxial test
Deviator Stress(kPa)
150
100
50
0
-50
-100
-150
-7
-6
-5
-4
-3
Axial Strain(% )
-2
-1
0
1
Results of centrifuge tests
Results of centrifuge tests
27.9
14.4
9
LVDT1 1
10.5
2
PPT5
Model container
PPT6
1V:1.75H
3
fine refuse - stage 2 1V:1H
coarse refuse - stage 2 1
PPT3
PPT4
9.3
1V:1.75H
ACC3
3
1V:1H
fine refuse - stage 1
coarse refuse - stage 19.3
(in prototype scale, unit: m)
LVDT ACC1
Accelerometer (ACC)
Pore Pressure Transducer (PPT)
Results of centrifuge tests
Results of centrifuge tests
0
LVDT1
(mm)
-50
-100
8
0
-8
20
10
0
-10
2
0
-2
4
2
0
-2
-4
0.05
0
-0.05
0.08
PPT6
(kPa)
PPT5
(kPa)
PPT4
(kPa)
PPT3
(kPa)
ACC6
(g)
ACC4
(g)
0
-0.08
0.2
0
-0.2
0.2
0
-0.2
ACC3
(g)
ACC1
(g)
0
2
4
6
8
10
Time (second)
12
14
Challenge No.2: Can coarse refuse
liquefy under earthquake loading?
100
Coarse refuse after passing 19 mm (3/4 inch) sieve
Coarse refuse after passing 19 mm (3/4 inch) sieve
Coarse refuse-original
90
80
Percent Finer
70
60
50
40
30
20
10
0
100
10
1
0.1
Grain Size (mm)
0.01
0.001
Challenge No.3: Analysis of large deformation
Challenge No.4: In-situ measurement of soil
properties
! Consolidation condition of fine refuse
! Seismic properties of fine and coarse
refuse
! Liquefaction resistance
Challenge No.4: In-situ measurement of soil
properties
Sample
No.
Depth
(m)
Overburden
Pressure (kPa)
Pre-consolidation
Pressure (kPa)
Compression
Index
Rebound
Index
ST-1
48.8
638
91
0.133
0.031
ST-2
54.9
672
140
0.171
0.026
ST-3
60.4
702
110
0.208
0.044
ST-4
61.3
707
140
0.254
0.054
ST-5
61.9
710
130
0.239
0.061
Table 2 Results of consolidation tests on fine refuse
Challenge No.4: In-situ measurement of soil
properties
Objectives of the workshop
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To summarize the current state-of-the-art in
research and state-of-the-practice in design
To identify research needs and to recommend
strategies to meet these needs
To facilitate collaborations between
researchers, government agencies, and
practicing engineers