SOME CONSIDERATIONS IN THE ASSESSMENT OF THE

PERFORMANCE OF TAILINGS DAMSDURING EARTHQUAKES

by

I. M. IdrissDepartment of Civil & Environmental Engineering

University of California, Davis, CA 95616-5294e-mail: imidriss@aol.com

Presented at the

International Workshop on SeismicStability of Tailings Dams

Case Western Reserve University Cleveland, Ohio

November 9-10, 2003

FOR PAST 40± YEARS, THERE HAVE BEEN SOME 80CASES OF TAILINGS DAM FAILURES

MOST OF THESE FAILURES HAVE BEEN ASSOCIATEDWITH EXCESSIVE WATER DUE TO RAIN, BURSTPIPES � ETC

SOME 15 CASES ARE EARTHQUAKE-RELATED �LIQUEFACTION OF TAILINGS

AMONG THE MORE PROMINENT CASES ARE THEFAILURE OF THE EL COBRE

TAILINGS DAMS IN THE 1965 CHILE EARTHQUAKE.

1,900,000 m3CopperChileEl CobreOld Dam

28 march 1965

Tailings traveled 12 km down-stream, destroyed the town of El Cobre and killed more than 200 people

350,000 m3Liquefaction of Tailings during earthquake �leading to dam failure & flow slides

CopperChileEl CobreNew Dam

28 march 1965

ImpactReleaseType of IncidentOre Type

LocationDate

1.1. GEOLOGIC & SEISMOLOGIC CONSIDERATIONSGEOLOGIC & SEISMOLOGIC CONSIDERATIONS

2.2. SITE CHARACTERIZATIONSITE CHARACTERIZATION

3.3. ANALYSESANALYSES

4.4. ASSESSMENT OF PERFORMANCEASSESSMENT OF PERFORMANCE

ELEMENTS OF A SEISMIC SAFETYELEMENTS OF A SEISMIC SAFETYEVALUATION FOR TAILING DAMSEVALUATION FOR TAILING DAMS

5.5. REMEDIATION (or DESIGN MODIFICATIONS),REMEDIATION (or DESIGN MODIFICATIONS),AS NEEDEDAS NEEDED

!!EXISTING DAMSEXISTING DAMS""OLD DAMSOLD DAMS

""RECENTLY CONSTRUCTED DAMSRECENTLY CONSTRUCTED DAMS

!!NEW DAMSNEW DAMS

These elements must be considered withinThese elements must be considered withinThe context of :The context of :

From Vick (1983)

From Vick (1983)

From Vick (1983)

EFFECTS OF FINES CONTENT ONSUSCEPTIBILITY OF COHESIONLESS SOILS

TO LIQUEFACTION

PROBABLY, THE BEST MEANS FOR CHARACTERIZINGTAILINGS IS THE USE OF CPT WITH JUDICIOUSLY

SPACED SAMPLED BORINGS

UNDISTURBED SAMPLING IN "COHESIONLESS"TAILINGS IS COMPARABLE TO SAMPLING IN

CLEAN SANDS � MAY EVEN BE MORE DIFFICULT

RECONSTITUTED SAMPLES ARE ADEQUATE FOR"NEW TAILINGS", BUT COULD PROVIDE MISLEADING

RESULTS BECAUSE THEY WOULD NOT REFLECTTHE EFFECTS OF AGING

THEREFORE,IN MOST INSTANCES WE WILL HAVE TO RELY

ON FIELD-BASED PROCEDURES FORASSESSING THE CYCLIC RESISTANCE OF TAILINGS

FromRobertson & Wride (1997)

FromCarraro, Bandini & Salgado (2003)

FromCarraro, Bandini & Salgado (2003)

Normalized Corrected CPT Tip Resistance, qc1N

0 50 100 150 200 250

Cyc

lic S

tres

s R

atio

, CSR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Earthquake Magnitude, M = 7½; σσσσ'vo = 1 atm

RecommendedRelationship

Suzuki et al (1997)

Liquefaction - Adjusted PointsLiquefaction - Revised Points

No Liquefaction - Revised Points

CPT-Based Published Case Histories of Clean Sands(Fines Content, FC ≤ 5%)

Magnitude = 7½ and σσσσ'vo = 1 tsf (≈≈≈≈ 1 atm)

Normalized Corrected CPT Tip Resistance, qc1N

0 50 100 150 200 250

Cyc

lic S

tres

s R

atio

, CSR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Earthquake Magnitude, M = 7½; σσσσ'vo = 1 atm

RecommendedRelationship

Suzuki et al (1997)

Liquefaction - Adjusted PointsLiquefaction - Revised Points

No Liquefaction - Revised Points

Robertson & Wride (1997)

Normalized Corrected CPT Tip Resistance, qc1N

0 50 100 150 200 250

Cyc

lic S

tres

s R

atio

, CSR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Earthquake Magnitude, M = 7½; σσσσ'vo = 1 atm

RecommendedRelationship

Suzuki et al (1997)

Liquefaction - Adjusted PointsLiquefaction - Revised Points

No Liquefaction - Revised Points

Robertson & Wride (1997)

Carraro et al (2003)

SPT-Based Published Case Histories of Cohesionless Soilswith Fines Content Greater than or Equal to 35%

Modified Standard Penetration - (N1)60 - Blows/ft0 10 20 30 40

Cyc

lic S

tres

s R

atio

(CSR

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

50

50

55

80

75

40

60

92

75

38

67

484035

43

37

50

Fines Content(FC)

LiquefactionNo

Liquefaction

Revised Values forCases Published in 2000

Revised Values forCases Published in 1984

Derived Curve(FC ≤≤≤≤ 5%)

Derived Curve(FC = 35%)

NCEER (1997)(FC ≥ ≥ ≥ ≥ 35%)

CPT-Based Published Case Histories of Clean Sandsand for Cohesionless Soils with Fines Content, FC ≥ 35%

Normalized Corrected CPT Tip Resistance, qc1N

0 50 100 150 200 250

Cyc

lic S

tres

s R

atio

, CSR

0.0

0.1

0.2

0.3

0.4

0.5

Earthquake Magnitude, M = 7½; σσσσ'vo = 1 atm

RecommendedRelationship for

Clean Sands

92 75

75

35

35

40

Clean Sands

Liquefaction - Adjusted PointsLiquefaction - Revised Points

No Liquefaction - Revised Points

50

66Fines Content, FC

WHAT ABOUT RESIDUAL STRENGTHFOR TAILINGS?

Sr Independent of σσσσ'vo vs Sr / σσσσ'vo

Adjustments for Fines Content

Extrapolation to Larger Values of Blow Count

ISSUES TO BE CONCERNED WITH

Equivalent Clean S and S PT Corrected B low count, (N1)6 0 cs

0 4 8 12 16 20

Residual S

hear Strength, S

r - psf

0

20 0

40 0

6 0 0

8 0 0

10 0 0

120 0

1

2

4

3

5

1. Lower San Fernando — fines content 25%2. Fort Peck Dam — fines content unknown3. Calaveras Dam — fines content unknown4. M ochi-Kochi Tailings — fines content 8 0 % 5. Lake M erced — fines content 3%

Range of residual strengthscalculated by S eed (19 8 7)

Equivalent Clean S and S PT Corrected B low count, (N1)6 0

0 4 8 12 16 20

Residual S

hear Strength, S

r - psf

0

20 0

4 0 0

6 0 0

8 0 0

10 0 0

120 0

3

45

6

7

89

1

2

Legend for Case Histories

Earthquake-Induced -- S PT & other param eters w ere estim ated

Earthquake-Induced -- S PT & other param eters w ere m easured

Construction-Induced

1. Upper S an Fernando — fines content 25 %2. Low er S an Fernando — fines content 25 %3. La M arquesa - D /S — fines content 20 %4 . W hiskey S prings — fines content 4 0 %

5 . Juvenile H all — fines content 6 5 %6 . La M arquesa - U/S — fines content 30 %7. Lake M erced — fines content 3%8 . M ochi-Kochi Tailings — fines content 8 0 %9 . La Palm a — fines content 15 %

S trengths calculatedby S eed (19 8 7)

S trengths calculatedby O lson & S tark (20 0 2)

Relationship Recom m endedfor Estim ating ResidualStrength Using M edian

M easured SPT

Equivalent Clean S and S PT Corrected B low count, (N1)6 0 cs

0 5 10 15 20

Sr/

σ σσσ'vo

0 .0 0

0 .0 4

0 .0 8

0 .12

0 .16

0 .20

(N1)6 0 values from

O lson & S tark (20 0 2)

A ll of Values -- as published byO lson & S tark (20 0 2)

CC S F S F

FP FP

(N 1)6 0 values from

S eed & Harder (19 9 0 )

Q uestionableParam eters

r

vo

S

'σσσσ

Equivalent Clean S and S PT Corrected B low count, (N1)6 0 cs

0 5 10 15 20

Sr/

σ σσσ'vo

0 .0 0

0 .0 4

0 .0 8

0 .12

0 .16

0 .20

All of Values -- as published byO lson & S tark (20 0 2)

C S F

FP

r

vo

S

'σσσσ

Equivalent Clean S and S PT Corrected B low count, (N1)6 0 cs

0 5 10 15 20

Sr/

σ σσσ'vo

0 .0 0

0 .0 4

0 .0 8

0 .12

0 .16

0 .20

All of Values -- as published byO lson & S tark (20 0 2)

r

vo

S

'σσσσ

Equivalent Clean S and S PT Corrected B low count, (N1)6 0 cs

0 5 10 15 20

Sr/

σ σσσ'vo

0 .0 0

0 .0 4

0 .0 8

0 .12

0 .16

0 .20

All of Values -- as published byO lson & S tark (20 0 2)

r

vo

S

'σσσσ