1Detection and Assessment of Internal Erosion atWAC Bennett Dam

Steve Garner, BC Hydro

182 m high2 km longZoned Earthfill Dam

2WAC Bennett DamLocation

Vancouver

WAC Bennett Dam

On the Peace River in North-eastern British Columbia

Williston Reservoir is 70 x 109 m3

Prince George

British

Columbia

3Working HypothesisGradations of Construction Materials

All materials come from moraine pits, deficient in medium sands Wide, blended till-like core, internally unstable Transition and Filter gap-graded in medium sand range

4Reservoir El 672

CoreSilty Transition

SandstoneShale

Shale

Scale: 0 30 60 90 m

Shell

Drain

TransitionFilter

Permeable Shales and Sandstones

Instrument Plane in Canyon Area

EP-06 EP-07 EP-08

1964

1965

1966

1967

Reservoir El 2205 ft (672 m)

Observation Well

Riser Pipe

5Observation Wells

8 leaky Observation Wells installed downsteam of dam axis

Includes an Observation Well near each Instrument Plane

Part of Instrument Island near Riser Pipes

6Performance Of Embankment Piezometers

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1965 1970 1975 1980 1985 1990 1995 2000 2005

G

r

a

d

i

e

n

t

EP08 TO TRANSITION

UPSTREAM SHELL TO EP06

BETWEEN EP'S

540

560

580

600

620

640

660

680

700

1965 1970 1975 1980 1985 1990 1995 2000 2005

E

l

e

v

a

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i

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n

H

e

a

d

(

m

)

RESERVOIR

PIEZOMETER TIPS

EP06

EP07

EP08

7Canyon Area (1974)

EP502032

EP511976

EP522160

EP532072

Res El 2205.6

2163 2065?

2145? 2115 2053?

2080 2055 1995

2165 2110?

( )

2163 2065?

2145? 2115 2053?

2080 2055 1995

2165 2110?

Instrument Plane 11974

Canyon Area1974

8Canyon Section (1978)

EP502019

EP511964?

EP522126

EP5320482160? 2065??

2150 2130 2050??

2085 2060 2010

2130? 2075?

Instrument Plane 11978

Canyon Section1978

9Canyon Section (1988)

2053

1991

1865

2057

2012

1946

EP501958

EP511888

EP522137

EP5320532140 2045

2140 2085 2020

1998 1905 1845

2145 2075

2140 2045

2140 2085 2020

1998 1905 1845

2145 2075

2140 2045

2140 2085 2020

1998 1905 1845

2145 2075

Instrument Plane 11988

Canyon Section1988

10

Canyon Section (1996)

( )

2049

1973

1842

2060

2002

1911

EP50 EP511843

EP522126

EP532030

( )

2132 2010

2120 2050 1975

1980 1880 1810

2130 2075

Instrument Plane 11996

Canyon Section1996

11

OW Flowmeter Measurements

0 0.2 0.4

2000

2050

2100

2150

2200

OW10

0 0.2 0.4

OW9

0 0.4 0.8 1.2

1700

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

OW2

0 0.4 0.8 1.2 1.6

1700

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

OW7

0 0.4 0.8 1.2 1.6

1900

1950

2000

2050

2100

2150

2200

October 1996December 2000September 2002September 2004September 2004 (verification)

OW5

0 0.4 0.8 1.2 1.6

1850

1900

1950

2000

2050

2100

2150

2200

OW4

End of 1966 Construction

End of 1965 Construction

End of 1964 Construction

12

OW Cross-hole Seismic Results

500 1000 1500 2000

2000

2050

2100

2150

2200

OW10 OW9

5001000150020002500

1700

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

OW2

5001000150020002500

1700

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

OW7

5001000150020002500

1900

1950

2000

2050

2100

2150

2200

OW5

5001000150020002500

1850

1900

1950

2000

2050

2100

2150

2200

OW4

ANALYSIS REPORT

DATE:

Fig. 3-42CDREPORT No.: E301

WAC Bennett Dam Oct. 2004SV

SHEAR WAVE VELOCITY PROFILESummary of OW Results

End of 1966 Construction

End of 1965 Construction

End of 1964 Construction

500 1000

1500

2000

13

2111 1994

2090.5 2010 1915

1966.5 1859.5 1789

2122 2066

InstrumentIsland

( )Res El 2192.6

1956.5

1919

Winter 1966/67

Winter 1965/66

Instrument Plane 12004 Seepage Model, Flowmeter and Seismic

OW

2

End of 1966 Construction

End of 1965 Construction

1000

1200

1400

1600

1800

2000

2200

2400

Shear Wave Velocity (feet/s)

1650

1700

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

OW2 OW7Flowmeter

14

WAC Bennett Dam - 1996

Sinkhole 1(BM 1)

Sinkhole 2(BM 2)

15

Sinkhole 1

2.5 m diameter at crest

7 m deep sinkhole

extremely loose zone to 80 m depth

variable zone to 125 m depth

16

Splitter Dikes

Splitter Dike 1

Splitter Dike 2

Splitter Dike 3

Splitter Dike 4

Sinkhole 1

Sinkhole 2

17

Splitter Dike Gradation

ShellTransition

Drain

Filter

Transition

Core

Crest of Dike

Splitter Dike

3" 1 1/2 " 3/4" 3/8" #4 #20#10 #40 #60 #100 #200

0

10

20

30

40

50

60

70

80

90

100

0.010.1110100

GRAIN SIZE (millimetres)

P

E

R

C

E

N

T

F

I

N

E

R

T

H

A

N

GRAVEL

COARSE FINE

SAND

COARSE MEDIUM FINESILT

U.S. STANDARD SIEVE SIZES

Drain

Transition and Splitter Dike

Sandstone/Shale Foundation

#20#4 #10

Flow Test

18

Slice 15 (590 mm)Local Collapse Near Rigid Cell

19

Slice 50 (30 mm)Loss of Fines Between Nested Particles

20

1850 1875 1900 1925 1950 1975 2000 2025

Working HypothesisSurmised Internal Erosion

Internal Erosion

Benchmark Tube

OW-6OW-5

21

1850 1875 1900 1925 1950 1975 2000 2025

Working HypothesisSurmised Present Condition

Remaining Damaged Area

Locally Remediated Area

Limit of Core at Crest

OW-6OW-5

22

500 1000 1500 2000 2500 3000 3500 4000 4500Shear Wave Velocity (feet/s)

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

E

l

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a

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i

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(

f

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)

Oct. 2000Aug. 2001Aug. 2002Sept. 2003Oct. 2003Sept. 2004Oct. 2005Core ecs-0.1

Core ecs-0.07

Transition ecs-0.05

500 1000 1500 2000 2500 3000 3500 4000 4500Shear Wave Velocity (feet/s)

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

E

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)

Oct. 2000Aug. 2001Aug. 2002Sept. 2003Oct. 2003Sept. 2004Oct. 2005Core ecs-0.1

Core ecs-0.07

Transition ecs-0.05

500 1000 1500 2000 2500 3000 3500 4000 4500Shear Wave Velocity (feet/s)

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

E

l

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a

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(

f

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t

)

Oct. 2000Aug. 2001Aug. 2002Sept. 2003Oct. 2003Sept. 2004Oct. 2005Core ecs-0.1

Core ecs-0.07

Transition ecs-0.05

500 1000 1500 2000 2500 3000 3500 4000 4500Shear Wave Velocity (feet/s)

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

E

l

e

v

a

t

i

o

n

(

f

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e

t

)

Oct. 2000Aug. 2001Aug. 2002Sept. 2003Oct. 2003Sept. 2004Oct. 2005Core ecs-0.1

Core ecs-0.07

Transition ecs-0.05

500 1000 1500 2000 2500 3000 3500 4000 4500Shear Wave Velocity (feet/s)

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

E

l

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v

a

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i

o

n

(

f

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e

t

)

Oct. 2000Aug. 2001Aug. 2002Sept. 2003Oct. 2003Sept. 2004Oct. 2005Core ecs-0.1

Core ecs-0.07

Transition ecs-0.05

500 1000 1500 2000 2500 3000 3500 4000 4500Shear Wave Velocity (feet/s)

1750

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

E

l

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a

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i

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(

f

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t

)

Oct. 2000Aug. 2001Aug. 2002Sept. 2003Oct. 2003Sept. 2004Oct. 2005Core ecs-0.1

Core ecs-0.07

Transition ecs-0.05

Transition PlaneChanges in Shear Wave Velocities

850 ft/s(259 m/s)

23

MP-Data and Shear Wave Velocities

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

E

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)

X-Hole Seismic MP Pressure Head

1800

1850

1900

1950

2000

2050

2100

2150

2200

2250

-10 -5 0 5 10 15 20 25Pressure Head (ft)

E

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(

f

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)

Oct-00

Aug-01

Aug-02

Sep-03

0

24

Tack(Thank You)

Conventional instrumentation can detect internal erosion

Conventional instrumentation can accelerate internal erosion

Cross-hole seismic has been used successfully for detecting and tracking internal erosion