Challenges in Seismic Stability Analysis of Tailings Dams

Transcription

1 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

2 Outlines! Introduction! Seismic properties of coal refuse! Challenges! Objectives of the workshop

3 Three construction methods for tailings dams Fine refuse impoundment coarse refuse embankment Downstream construction Fine refuse impoundment coarse refuse embankment Upstream construction Fine refuse impoundment coarse refuse embankment Centerline construction

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13 Failure of Tailings Dams! 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

14 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

15 Specific gravity of coal refuse

16 Dynamic tests on coal refuse

17 Dynamic tests on coal refuse

18 Shear modulus of coarse refuse G/Gmax 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) Shear Strain(%)

19 Shear modulus of fine refuse G/Gmax 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) Shear Strain(%)

20 Damping Ratio of Coarse Refuse Damping Ratio(%) Confining Pressure=138 kpa Confining Pressure=69 kpa Best Curve Fitting Seed at el. for Sand (1986) Shear Strain(%) 0.1 1

21 Damping Ratio of Fine Refuse Damping Ratio(%) 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) Shear Strain(%)

22 Current State in Seismic Design of Tailings Dams! 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.

23 Challenge No.1: Can fine refuse liquefy under earthquake loading? 0.6 Percent Fines = Cyclic Stress Ratio (CSR) SPT Clean Sand Base Curve Corrected Blow Count, (N 1 ) 60

24 SPT tests results in the field Depth (m) Soil Type SPT Blow Count N Modified N 1 = C N 1 N 9.1 coarse refuse coarse refuse coarse refuse coarse refuse fine refuse fine refuse fine refuse fine refuse C N is calculated using the relationship proposed by Liao and Whitman (1986) to normalize to an overburden pressure of 1 TSF

25 Particle size distribution curve Percent Finer Sample Sample2 Sample3 Sample Grain Size (mm)

26 Results of cyclic triaxial tests 140 Pore Water Pressure(kPa) Time(s) Excess pore pressure recorded in a cyclic triaxial test (initial effective confining pressure σ c = 186 kpa, cyclic deviator stress σ d = ± 64% σ c )

27 Results of cyclic triaxial test 150 Deviator Stress(kPa) Axial Strain(%)

28 Results of centrifuge tests

29 Results of centrifuge tests LVDT PPT6 PPT5 1V:1.75H Model container fine refuse - stage 2 1V:1H coarse refuse - stage 2 1 PPT4 PPT V:1H ACC3 1V:1.75H fine refuse - stage 1 coarse refuse - stage 19.3 (in prototype scale, unit: m) Accelerometer (ACC) Pore Pressure Transducer (PPT) LVDT ACC1

30 Results of centrifuge tests

31 Results of centrifuge tests Time (second) LVDT1 (mm) PPT6 (kpa) PPT5 (kpa) PPT4 (kpa) PPT3 (kpa) ACC6 (g) ACC4 (g) ACC3 (g) ACC1 (g)

32 Challenge No.2: Can coarse refuse liquefy under earthquake loading? Coarse refuse after passing 19 mm (3/4 inch) sieve Coarse refuse after passing 19 mm (3/4 inch) sieve Coarse refuse-original 70 Percent Finer Grain Size (mm)

33 Challenge No.3: Analysis of large deformation

34 Challenge No.4: In-situ measurement of soil properties! Consolidation condition of fine refuse! Seismic properties of fine and coarse refuse! Liquefaction resistance

35 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 ST ST ST ST Table 2 Results of consolidation tests on fine refuse

36 Challenge No.4: In-situ measurement of soil properties

37 Objectives of the workshop! 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