FAILURE OF EARTHEN DAMS (SEEPAGE THEORY)

1. FAILURE OF EARTHEN DAMS
NAME- SABYASACHI SWAIN
SUBJECT- THEORY OF SEEPAGE
M.TECH 1ST YEAR
BRANCH- WRD & IE
ROLL NO- 13244006

2. FAILURE OF EARTHEN DAMS
• Earth dams are less rigid and hence more
susceptible to failure.
• An earthen dam may fail due to improper
designs, faulty constructions, or lack of
maintenance etc.

3. FAILURE OF EARTHEN DAMS
• The various causes leading to the failure
of earth dam can be grouped into
following three categories1. Hydraulic failures
2. Seepage failures
3. Structural failures

4. FAILURE OF EARTHEN DAMS
1. HYDRAULIC FAILURES-
• About 40% of earth dam failures have
been attributed to these causes.
• The failure under this category may occur
due to the following reasons:

5. FAILURE OF EARTHEN DAMS
a) Overtopping failure:
The water may overtop the dam , if
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•
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
The design flood is under-estimated.
The spillway is of insufficient capacity.
The spillway gates are not properly
operated.
Sufficient freeboard should, therefore be
provided.

6. FAILURE OF EARTHEN DAMS
b) Erosion of upstream face:
• The wave action due to the wind and tides will
cause the erosion of the dam material at the
upstream face, if the material could not
sustain wave velocity.
• It will lead to washing out of the dam material
or overturning of the protection slab or slip of
the upstream face.

7. FAILURE OF EARTHEN DAMS
Safety measures:
Upstream stone pitching .
Provision of riprap.
Proper design.

8. FAILURE OF EARTHEN DAMS
c) Cracking due to frost action:
• Frost in the upper portion of the dam may
cause heaving and cracking of the soil with
dangerous seepage and consequent failure.
An additional freeboard allowance should be
provided for dams in areas of low
temperatures.

9. FAILURE OF EARTHEN DAMS
d) Erosion of the downstream slope by
gully formation:
• Heavy rains falling over the downstream face
• The erosive action of moving water
• The formation of gullies on the downstream
face , leading to dam failure.

10. FAILURE OF EARTHEN DAMS
Proper maintenance.
Filling the cuts from time to time , especially
during rainy season.
Grassing the slopes.
Providing proper berms at suitable heights.

11. FAILURE OF EARTHEN DAMS
Provision of berms in an earthen dam

12. FAILURE OF EARTHEN DAMS
e) Toe erosion:
• Similar to the wave action at the upstream of
the dam, downstream dam slope is also
susceptible for erosion by the downward
water.
• Erosion due to the cross-currents that may
come from the spillway buckets.
• Erosion due to tailwater.

13. FAILURE OF EARTHEN DAMS
Provision of a downstream slope pitching or a
riprap up to a height slightly above the normal
tail water depth.
By providing the rock toe drainage.
Sidewalls of the spillway must be of sufficient
height and length.

14. FAILURE OF EARTHEN DAMS
2. SEEPAGE FAILURES• Uncontrolled or concentrated seepage
through dam body or foundation may lead to
failure of the dam.
• Piping (through foundation and dam body)
• Sloughing

15. FAILURE OF EARTHEN DAMS
• Piping is the progressive erosion and
subsequent removal of soil grains from within
the body or foundation of the dam.
• Sloughing is the progressive removal of soil
from the wet downstream face.
• More than one-third of the earth dams have
failed due to these reasons.

16. FAILURE OF EARTHEN DAMS
(a)Piping through foundations:
• When highly permeable cavities or strata of
coarse sand or gravel are present in the
foundation, water may seep at a huge rate
through them.
• This concentrated flow at a high gradient
may erode the soil.
• Increased flow of water and soil results in
creation of hollows below the
foundation, into which the dam may sink
down.

17. FAILURE OF EARTHEN DAMS
Piping through the dam foundation

18. FAILURE OF EARTHEN DAMS
(b) Piping through dam body:
• When concentrated flow channels get
developed in the body of the dam, soil may be
removed as in case of foundation piping.
• Formation of hollows in the dam body and
subsequent subsidence of the dam body.
• Flow channels develop due to faulty
construction, insufficient compaction, cracks
in embankment due to settlement, shrinkage
cracks etc.

19. FAILURE OF EARTHEN DAMS
Piping through the dam body

20. FAILURE OF EARTHEN DAMS
All these can be removed by better
construction and better maintenance of dam
embankment.
Thorough compaction of soils near the outlet
conduits and preventing the leakage through
them to avoid contact seepage.
For pervious seams in foundation, relief drain
with filter can be provided.

21. FAILURE OF EARTHEN DAMS
(c) Sloughing:
• If the seepage line exists at the downstream
face of the dam, the portion of the toe of the
dam below the exit point will always remain in
the wet condition.
• This will cause the reduction of the stability of
the slope and small size sliding may occur.
• The repetition of wetting and sliding will
continue further to cause dam failure, termed
as sloughing.

22. FAILURE OF EARTHEN DAMS
3. STRUCTURAL FAILURES• About 25% of overall dam failures.
• Generally, these are caused by shear failures
leading to sliding.

23. FAILURE OF EARTHEN DAMS
(a)Failure due to the pore water pressure:
• Excessive pore water pressure in confined seams
of sand and silt, artesian pressure in
abutments, etc. may reduce shear strength of
soil.
• Loose sand foundation may fail by
liquefaction or flow slides.
Adequate compaction of soil.

24. FAILURE OF EARTHEN DAMS
(b) Upstream slope failure:
• Steep slope, weak embankment soil, sudden
drawdown of reservoir.
• Cause the seepage force acting along the
sliding direction causing increase of the
driving force for slope direction.
• Seldom lead to catastrophic failures.
Can be avoided by flatten slopes, provision of
rock berms and better operating rules.

25. FAILURE OF EARTHEN DAMS
Upstream slope slide due to sudden drawdown

26. FAILURE OF EARTHEN DAMS
(c) Downstream slope failure:
• When the reservoir is at maximum level and if
the steady stage seepage is at maximum
rate, the downstream slope is more
vulnerable to slide due to seepage force acting
in the direction of driving force for the sliding.
Flatten slope, better soil, increased
compaction, internal drainage with protective
filters and surface drainage facilities.

27. FAILURE OF EARTHEN DAMS
Downstream slope slide during full reservoir condition

28. FAILURE OF EARTHEN DAMS
(d) Foundation slide:
• The dam body as a whole may slide, if the
foundation is from the silt or soft soil.
• The slow consolidation process and expansion
of clay soils due to the saturation will decrease
the shear strength of the foundation soil.
Flatten slopes, broad berms, stabilization of
soil, deep drain trenches with protective filters
for drainage, relief wells.

29. FAILURE OF EARTHEN DAMS
Sliding due to soft or weak foundation

30. FAILURE OF EARTHEN DAMS
(e) Other modes:
• Failure by spreading.
• Failure by leaching.
• Failure due to earthquake – most devastating
of all types of failures.
Proper design, regular inspection and better
operating rules.

31. REFERENCES
Garg, S.K., (2012), “Irrigation Engineering and
Hydraulic Structures.” 24th Edition, Khanna
Publishers.
Wallingford H.R.,(2008), “Dam Breach Floods –
An Introduction”, www.hrwallingford.com.
Dam Failure- Wikipedia, the free encyclopedia.
en.wikipedia.org/wiki/Dam_failure.
Creager, W.P., Justin, J.D. and Hinds, J., (1955),
“Engineering for Dams.” Vol. 3, 1st
Edition, John Wiley & Sons,INC., Chapman &
Hall, LTD.

32. THANK YOU