CPTu-Based State Characterization of
Tailings Liquefaction Susceptibility
Christina Winckler, URS
Richard Davidson, URS
Lisa Yenne, URS
Joergen Pilz, Rio Tinto
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
Overview
• Tailings dam challenges
• Benefits of CPTu-based state characterization
• Material behavior: contractive vs. dilative
• Conventional triggering analysis
• Four methods of CPTu-based state characterization
• Static liquefaction case history
• Concluding remarks
TAILINGS AND MINE WASTE 2014
Tailings Dams Challenges
Recently placed, hydraulically deposited and highly interbedded, angular nonplastic sands and silts
TAILINGS AND MINE WASTE 2014
Cerro Negro Tailings Dams No. 3 and 2
Mw 7 La Ligua Chile Earthquake 1965
TAILINGS AND MINE WASTE 2014
Benefits of CPTu-Based State Characterization
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Contractive vs. Dilative Material Behavior
Dilative materials:
Contractive materials:
• Denser with higher tip resistance
• Looser with lower tip resistance
• Low dynamic pore pressure
• High dynamic pore pressure
• Less susceptible to liquefaction
• Susceptible to liquefaction
TAILINGS AND MINE WASTE 2014
State Parameter
Jefferies and Been (1985)
State parameter = difference between the in situ void ratio, e0 and the void ratio at
critical state, ecs at the same mean effective stress p’
contractive
dilative
TAILINGS AND MINE WASTE 2014
Contractive vs. Dilative Material Behavior
Robertson (2010)
• Liquefaction failures have
normalized CPTu data that plots
below the Qtn,cs = 70 line
• One mine tailings case history no.
35
• The Qtn,cs = 70 line is similar to the
state parameter Ψ = -0.05 line
• Zones 2 and 3 suggest clay-like
behavior, but may still be
vulnerable to significant strength
reduction
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Conventional Triggering Analysis
500
CPT-A
CPT-C
CPT-B
CPT-D
CPT-E
400
0.25
300
0.20
0.15
CPT-F
0.25
200
0.40
0.35
0.20 0.25 0.30
100
0.15
0.20
0
0
2400
200
2600
400
600
800
1000
1200
Distance (feet)
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1400
1600
1800
2000
2200
CPTu-Based State Characterization
• Evaluate the state of saturated non-plastic tailings
• Four methods:
1.Static vs. dynamic pore pressures
2.Normalized pore pressure difference
3.Normalized material properties
and state parameter
4.State parameter difference plots
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1. Static vs. Dynamic Pore Pressures
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2. Normalized Pore Pressure Difference (P)
• Calculate vertical effective stress, use
hydrostatic percentage at the time of CPT
• Calculate the static pore pressure using the
hydrostatic percentage at the time of CPT
• Calculate the change in pore pressure:
∆u = udyn – ustatic
• Calculate the normalized pore pressure
difference.
𝑢𝑑𝑑𝑑 − 𝑢𝑠𝑠𝑠𝑠𝑠𝑠
𝑃=
𝜎𝜎𝑣
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∆U
2. Normalized Pore Pressure Difference (P)
• Contractive behavior: positive P-value • Dilative behavior: negative P-value
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3. Normalized Material Properties and State
Parameter Plots
• Dilative behavior
TAILINGS AND MINE WASTE 2014
• Contractive behavior
4. State Parameter Difference Data Plots
• Perpendicular distance from
point to ψ = -0.05 line
• Plotted vs. normalized dynamic
pore pressure difference
Negative values Dilative
Positive values Contractive
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4. State Parameter Difference Data Plots
Quadrant 2
Quadrant 1
Quadrant 3
Quadrant 4
• Contractive behavior
Normalized Dynamic Pore Pressure
Difference
Normalized Dynamic Pore Pressure
Difference
• Dilative behavior
Quadrant 2
Quadrant 1
Quadrant 3
Quadrant 4
State Parameter Difference
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State Parameter Difference
Results
• Interior soundings (CPT-A and CPT-B): dilative behavior
• Near toe soundings (CPT-D, CPT-E, and CPT-F): contractive behavior
500
400
500
CPT-A
CPT-B
CPT-C
CPT-D
400
CPT-E
300
300
UNSATURATED WHOLE TAILINGS
200
100
SATURATED DILATIVE WHOLE
TAILINGS
1.
SATURATED CONTRACTIVE
0
CPT-F
200
WHOLE TAILINGS
100
0
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Static Liquefaction Case History
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Static Liquefaction Case History
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Limitations of CPTu-Based State
Characterization
• Must maintain full saturation of the CPTu pore pressure sensor
• Physical sampling of tailings to confirm material characterization through
microstratigraphy and index properties – identify meaning of CPT signatures
• Piezometer data to confirm PPD static pore pressure data
• Static and cyclic laboratory testing to assess quasi-steady state and triggering
potential
TAILINGS AND MINE WASTE 2014
Concluding Remarks
• Independent approach is proposed to evaluate whether material would behave in
a dilative or contractive manner during shear.
• With shallower tailings profiles, conventional triggering methods and the CPTubased state characterization method are in general agreement indicating tailings
liquefaction susceptibility.
• CPTu-based state characterization provides a continuous record throughout the
tailings profile and allows the state characterization and liquefaction vulnerability
to be extended to greater depth.
• Tailings are generally a highly interbedded material and properties can vary
significantly across a site and with depth; therefore, it is important to tie CPTu
characteristic signatures to the depositional regimes that formed them.
• It is essential to look at the preponderance of what the data is revealing. If all of
the independent methods point to the same interpretation, then confidence is
enhanced.
• If there are differences, then understanding the reasons for the differences can
be equally illuminating.
TAILINGS AND MINE WASTE 2014
Thank You
TAILINGS AND MINE WASTE 2014