Seismic Stability of Tailings Dams, an Overview

GEI Consultants, Inc.

BY

Gonzalo Castro, Ph.D., P.E. Principal

International Workshop on Seismic Stability of Tailings DamsCase Western Reserve University, November 2003

Seismic Stability of Tailings Dams, an Overview

GEI Consultants, Inc.

Small cyclone to separate sands from fines (slimes)

GEI Consultants, Inc.

Large cyclone

GEI Consultants, Inc.

Methods of tailings dams construction

GEI Consultants, Inc.

Typical tailings dam under construction,steep terrain

GEI Consultants, Inc.

Upstream construction, flat terrain

GEI Consultants, Inc.

Perimeter hydraulic fill dam

GEI Consultants, Inc.

Tailings dam failure, Cerro Negro 1985 Chile Earthquake

GEI Consultants, Inc.

Tailings dam failure, Veta de Agua1985 Chile Earthquake

GEI Consultants, Inc.

Typical Seepage Lines

GEI Consultants, Inc.

Drained vs. undrained soil behavior

Drained Behavior

Rate of shear is slow enough to allow dissipation of excess pore water pressures with a corresponding volume change.

Undrained Behavior

Rate of shear is rapid enough not to allow dissipation of excess pore water pressures. Volume remains constant.

GEI Consultants, Inc.

Soil type and rate of loading

GEI Consultants, Inc.

Dilative vs. contractive behavior

Dilative StateDilative StateDilative StateDilative State

Tendency for volume increase due to application of shear.

Contractive StateContractive StateContractive StateContractive State

Tendency for volume decrease due to application of shear.

GEI Consultants, Inc.

Drained vs. undrained soil behavior

GEI Consultants, Inc.

Definition of strength terms

GEI Consultants, Inc.

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.

GEI Consultants, Inc.

The state diagram

GEI Consultants, Inc.

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.

GEI Consultants, Inc.

Type of stress strain response

GEI Consultants, Inc.

Stress strain diagram for the unstable case

GEI Consultants, Inc.

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.

GEI Consultants, Inc.

Stress strain diagram for the stable case

GEI Consultants, Inc.

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.

GEI Consultants, Inc.

Deformation accumulation for stable case

GEI Consultants, Inc.

Seismic analysis of tailings dams

Selection of Shear Strength for Stability Analyses

! Sands" Loose saturated Sus" Dry or dense saturated Sds

! Saturated clays" Large strain at peak Sup

! Silts" Moderate strain at peak Sup or Sus ?

GEI Consultants, Inc.

Typical undrained stress strain behavior of fine tailings

GEI Consultants, Inc.

Peak undrained strength ratios for tailings and natural soils

GEI Consultants, Inc.

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

GEI Consultants, Inc.

Effect of cyclic straining compared with common assumption