failure of earhen dam

NAME- ANIKET GUPTAM.TECH -1ST YEARBRANCH- GEOTECHROLL NO- 2K15/GTE/03

FAILURE OF EARTHEN DAMS

FAILURE F EARTHEN DAM

• 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.


• The various causes leading to the failure of earth dam can be grouped into following three categories-

1. Hydraulic failures2. Seepage failures3. Structural failures


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:


a) Overtopping failure:The water may overtop the dam , if

• 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.


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.


Safety measures:

Upstream stone pitching .

Provision of riprap.

Proper design.


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.


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.


Proper maintenance.

Filling the cuts from time to time , especially during rainy season.

Grassing the slopes.

Providing proper berms at suitable heights.


Provision of berms in an earthen dam


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.


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.


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


• 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.


(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.


Piping through the dam foundation


(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.


Piping through the dam body


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.


(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.


3. STRUCTURAL FAILURES-

• About 25% of overall dam failures.

• Generally, these are caused by shear failures leading to sliding.


(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.


(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.


Upstream slope slide due to sudden drawdown


(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.


Downstream slope slide during full reservoir condition


(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.


Sliding due to soft or weak foundation


(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.

THANK YOU

failure of earhen dam

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