Post on 15-Jan-2016
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
Homogenous clay dam
Homogenous clay dam
Homogenous clay dam
Homogenous clay dam
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
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
Rockfill dam with moraine core
Breaching mechanisms in a rockfill dam with moraine core
2 2003-09-15 – 2003-09-19
Rockfill dam with moraine core
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
Rockfill dam with moraine core – breaching by overtopping.
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
Rockfill dam with moraine core – breaching by overtopping.
Rockfill dam with moraine core – breaching by overtopping.
Rockfill dam with moraine core – breaching by overtopping.
Rockfill dam with moraine core – breaching by overtopping.
Rockfill dam with moraine core – breaching by overtopping.
Rockfill dam with moraine core – breaching by 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 overtopping.
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
Rockfill dam with moraine core – breaching by piping/internal erosion
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)
Rockfill dam with moraine core – breaching by piping/internal erosion
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
Homogenous moraine
Homogenous moraine
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.
Summary / conclusions
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.
Summary / conclusions
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.
Summary / conclusions
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.
Summary / conclusions
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.
Summary / conclusions
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.
Summary / conclusions