Seismic Stability of Tailings Dams, an Overview

Transcription

1 Seismic Stability of Tailings Dams, an Overview BY Gonzalo Castro, Ph.D., P.E. Principal International Workshop on Seismic Stability of Tailings Dams Case Western Reserve University, November 2003

2 Small cyclone to separate sands from fines (slimes)

3 Large cyclone

4 Methods of tailings dams construction

5 Typical tailings dam under construction, steep terrain

6 Upstream construction, flat terrain

7 Perimeter hydraulic fill dam

8 Tailings dam failure, Cerro Negro 1985 Chile Earthquake

9 Tailings dam failure, Veta de Agua 1985 Chile Earthquake

10 Typical Seepage Lines

11 Drained vs. undrained soil behavior Drained Behavior Undrained Behavior Rate of shear is slow enough to allow dissipation of excess pore water pressures with a corresponding volume change. Rate of shear is rapid enough not to allow dissipation of excess pore water pressures. Volume remains constant.

12 Soil type and rate of loading

13 Dilative vs. contractive behavior Contractive State Dilative State Tendency for volume decrease due to application of shear. Tendency for volume increase due to application of shear.

14 Drained vs. undrained soil behavior

15 Definition of strength terms

16 Steady state of deformation! The state in which a mass of particles is continuously deforming at constant: " Volume " Effective Normal Stress " Shear Stress " Strain Rate! Steady state exists only during deformation and after particles are oriented as well as they can be.! At steady state the specimen has lost all memory of initial structure.

17 The state diagram

18 Main types of tailings dam response to seismic shaking! Seismic shaking causes sufficient strength reduction so the dam becomes unstable.! Embankment and foundation soils retain sufficient strength so the dam remains stable, limited (and possibly damaging) deformations are possible.

19 Type of stress strain response

20 Stress strain diagram for the unstable case

21 Main features of unstable case! Triggering strain is low for loose saturated sands (0.2 to 1%), high for clays (10% or higher).! Earthquake triggers the failure if accumulated strain reaches value of triggering strain.! When failure is triggered in sands, seismically induced pore pressure may be less than 50%.! Failure is typically a major slide, e.g., lower San Fernando dam.

22 Stress strain diagram for the stable case

23 Main features of stable case limited deformations! Pre and post earthquake shear stresses are about the same.! Movements occur mainly during shaking.! Significant movements usually develop when pore pressures become high.! Displacements are less than 3 m, often substantially lower.! Dam is stable with the soils at steady state strength.

24 Deformation accumulation for stable case

25 Seismic analysis of tailings dams Selection of Shear Strength for Stability Analyses! Sands " Loose saturated S us " Dry or dense saturated S ds! Saturated clays " Large strain at peak S up! Silts " Moderate strain at peak S up or S us?

26 Typical undrained stress strain behavior of fine tailings

27 Peak undrained strength ratios for tailings and natural soils

28 Effect of cyclic straining on peak undrained strength of low plasticity clayey silt

29 Effect of cyclic straining compared with common assumption