PHYSICAL MODELING OF BREACH FORMATION Large scale field tests Kjetil Arne Vaskinn, Sweco Gröner...

PHYSICAL MODELING OF BREACH FORMATION

Large scale field tests

Kjetil Arne Vaskinn, Sweco Gröner Norway

Test site

Test area at the reservoir lake Røssvatnet

TustervassdammenRøssvatnet

Test area at the reservoir lake Røssvatnet

Instrumentation

Water levels (stage-discharge measurements):Upstream of the damDownstream of the dam

Discharge through the gates at Røssvassdammen

Photo (several points)Video (3 different cameras)

Pressure sensors in the damBreach sensors

Large scale field-tests 2002

Test Dato Type of dam Objectives of the tests

1-02 2002-09-11 – 2002-09-12

Homogenous clay Breaching mechanisms in a dam made by cohesive material

2A-02 2002-10-01 - 2002-10-02

Homogenous gravel dam. Rockfill on the downstream slope

Stability with flow through and over the dam

2B-02 2002-10-10 – 2002-10-12

Homogenous gravel dam.

Stability with flow through and over the dam

2C-02 2002-10-15 – 2002-10-16

Homogenous gravel dam.

Breaching mechanisms

3-02 2002-10-24 – 2002-10-25

Homogenous rockfill 300-400 mm

Stability with flow through the dam

2 m

2.0

16 m 1

Rock

1 Clay, moisture content 30%, placed in 0.15 m layers compacted with dozer

Homogenous clay dam

R

EL. V

EKTM

ENG

DE

AV K

OR

N <

d %

0

10

20

30

40

50

60

70

80

90

100

Sample level 4,25. 9. Sept. 02 Sample level 4,75. 9. Sept. 02

2 60 26 10 100 200 600µm 1 6 10 20

Fine Medium Coarse

60mm

0,075 0,125 0,25 0,5 1 2 4 8 19 31,5 63

1 20

SUM

TIL

BAKE

HO

LDT

MAT

ERIA

LE %

CLAY SILT SAND GRAVEL

Fine FineMedium Medium CoarseCoarse

100

90

80

70

60

50

40

30

20

10

0

Homogenous clay dam

• A 0.5 m deep and 3 m wide channel at the top of the dam for initiation of the breach

• Due to high water content in the clay deposit (w = 28-33%) construction of the dam became difficult. To improve construction the layer thickness was increased to 0.4 m and the compaction pressure was reduced.

Homogenous clay dam

Homogenous clay dam

Water elevation and discharge Homogenous clay dam

Outflow from dam, test 1-02

0

50

100

150

200

250

300

350

400

450

500

13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:30

Time (hr)

Q (

m3/s

)

367

367,5

368

368,5

369

369,5

370

370,5

371

Flow VM5

Level VM2

2 m

5m

Concretesill

Clay

Rock

Homogenous (minimum cohesive) dam. Gravel 0-60 mm, fines (0,074mm)<5%, 4 mm<d50<10 mm, dmax<60 mm

0.5 meter layer, compaction by 4 tons roller compacter, 2 layer with pore pressure sensors.

Gravel dam

Sieve curves at every layer dam 2-02 (sandy gravel) 0.074 0.149 0.297 0.59 1.19 2.38 4.76 9.52 19.05 38.1 76.2

0 10 20 30 40 50 60 70 80 90

100

0.01 0.1 1 10 100 d [mm]

% finer than d

U.S. Standard Sieves (mm) 1 1.5 2 2.5 3 3.5 4

Gravel dam

Gravel dam

Left hand side Right hand side

Gravel dam

Gravel dam

Large scale field-tests 2003

Test Dato Type of dam Objectives of the tests

1A-03 2003-07-28 – 2003-08-15

Rockfill dam with moraine core

Resistivity and SP investigation for initial dam breach studies and internal erosion detection

1B-03 2003-08-19 - 2003-08-22


Breaching mechanisms in a rockfill dam with moraine core

2 2003-09-15 – 2003-09-19


Breaching by internal erosion/piping

3 2003-10-07 – 2003-10-08

Homogenous moraine dam

Breaching by internal erosion/piping

Rockfill dam with moraine core – breaching by overtopping.

5.9m1

1.5

3m

1

4

0.65m

1.5m

Concrete sill and V-notch weir

Clay

Rock

0.24m deep and 8 m wide notch in dam crest during breach test

2 Rock from tunnel spoil 0-500mm3 Rockfill 300-400mm

21

3

Defects built into dam for test of leakage detection

Pressure tranceducers

Moraine1

Grain distribution moraine and rockfill dam 1-03

0,074 0,149 0,297 0,59 1,19 2,38 4,76 9,52 19,05 38,1 76,2 152,4 406,4

0

10

20

30

40

50

60

70

80

90

100

0,001 0,01 0,1 1 10 100 1000

d (mm)

Re

lati

ve

we

igh

t o

f g

rain

s <

d in

%

U.S. Standard Sieves (mm)

Moraine

Moraine < 19 mm

Rockfill


VM2 and overtopping discharge

370,0

370,1

370,2

370,3

370,4

370,5

370,6

370,7

370,8

370,9

371,0

21.08 08:30 21.08 09:30 21.08 10:30 21.08 11:30 21.08 12:30 21.08 13:30 21.08 14:30

Time

Sta

ge (m

asl

)

0

1

2

3

4

5

6

7

8

9

10

Dis

char

ge (m

3/s)

level Top core

Dam crest overtopping


VM5 discharge

0

50

100

150

200

250

21.08 11 21.08 12 21.08 13 21.08 14 21.08 15

Time (dd.mm hh)

Dis

ch

arg

e (

m3

/s)


Rockfill dam with moraine core – breaching by piping/internal erosion

6m

1

1.51

4

Concrete sill and V-notch weir

6m

Small defect Large defect

3m

Clay

Rock

Defects built into dam for initiation of piping, two 200 mm half-pipes embedded in uniform sand

Moraine, vibratory compaction, 0.5 m layer thickness1

2 Rock from tunnel spoil 0-500mm, vibratory compaction, 1 m layer thickness

3 Rockfill 3-400mm, vibratory compaction, 1 m layer thickness

1

23

VM5, Stage and discharge

0,0

20,0

40,0

60,0

80,0

100,0

120,0

140,0

160,0

180,0

200,0

19.09 11:00:00 19.09 12:12:00 19.09 13:24:00 19.09 14:36:00

Time

Dis

char

ge

(m3/

s)


Homogenous moraine

4.5m

1

1.3 Concrete sill and V-notch weir

4.5m

3m

Clay

Rock

Defect built into dam for initiation of piping. 200 mm half-pipe with slots embedded in uniform sand

Moraine, vibratory compaction, 0.5 m layer thickness1

1

12

Remaining portions of previous test dam2

2

Homogenous moraine

0

20

40

60

80

100

120

140

160

180

200

08.1013:00:03

08.1013:14:27

08.1013:28:51

08.1013:43:15

08.1013:57:39

08.1014:12:03

08.1014:26:27

08.1014:40:51

08.1014:55:15

Time

Dis

ch

arg

e (

m3

/s)

Homogenous moraine

Analysis of the data has started and is likely to continue for some years. The data will assist in the development of understanding and validation of predictive models.

Prior to this analysis, some initial, broad observations may be made based upon field observations and data analysis to date.

These include the following:

Summary / conclusions

1. The failure processes of the different embankments have been observed.

2. Features such as cracking, arching (pipe formation), headcut formation and progression were all observed.

3. Existing breach models does not accurately simulate many of these features.


4. The first phase in the external erosion of the downstream slope due to overtopping is slow and very gradual.

5. When the scour and unraveling finally reaches the upstream edge of the dam crest, the breaching is rapid and dramatic.

6. The same general observations were made for the rockfill, gravel and clay dams.

7. The opening of the breach first progresses down to base of the dam, before it expands laterally. The sides of the breach were very steep, almost vertical, in all three materials.


8. The rate of breach growth for the homogeneous clay and gravel dams was not as expected.

9. The clay dam failure more quickly, whilst the gravel dam more slowly than expected.

It is likely that this was due to the condition of material and nature of construction / compaction.

This demonstrates the significant impact that material condition and construction method may have on breach formation and hence the need to consider these aspects within predictive models.


10.The internal erosion process, initiated at the defects built into the moraine core of the rockfill dam (Test 2-2003), took a very long time to develop, even in this dam with no filters between the moraine core and the downstream rockfill.

11.Breaching of the dam did not take place until the erosion had proceeded up to the dam crest, and then the dam failed by overtopping as in Test 1-2003, but the breach opening was not so wide.


12.The difference in rate of embankment failure for the homogeneous moraine embankment and the composite moraine / rockfill embankment was significant.

This demonstrates the importance of the interaction between layers of material within a composite structure.

This has implications for overall dam stability and in the development of predictive breach models.


13.Many of the field test scenarios simulated typical rockfill embankment dams. As such, there was surprise that the rate and mechanisms of failure observed were typically more resistant than existing analyses and guidelines prescribe.


PHYSICAL MODELING OF BREACH FORMATION Large scale field tests Kjetil Arne Vaskinn, Sweco Gröner...

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