CPTu-Based State Characterization of Tailings Liquefaction Susceptibility Christina Winckler, URS Richard Davidson, URS
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 TAILINGS AND MINE WASTE 2014 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 TAILINGS AND MINE WASTE 2014 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) TAILINGS AND MINE WASTE 2014 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 TAILINGS AND MINE WASTE 2014 1. Static vs. Dynamic Pore Pressures TAILINGS AND MINE WASTE 2014 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 TAILINGS AND MINE WASTE 2014 ∆U 2. Normalized Pore Pressure Difference (P) • Contractive behavior: positive P-value • Dilative behavior: negative P-value TAILINGS AND MINE WASTE 2014 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 TAILINGS AND MINE WASTE 2014 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 TAILINGS AND MINE WASTE 2014 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 TAILINGS AND MINE WASTE 2014 Static Liquefaction Case History TAILINGS AND MINE WASTE 2014 Static Liquefaction Case History TAILINGS AND MINE WASTE 2014 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
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