Causes of Failure of Earthen Dams

Earth dams are less rigid and hence moresusceptible to failure. Every past failure of sucha dam has contributed to an increase in theknowledge of the earth dam designers. Earthendams may fail, like other engineering structures,due to improper designs, faulty constructions,lack of maintenance, etc.

Classification of failure of earth dams :-

(i) Hydraulic failures – 40 %

(ii) Seepage failures – 30 %

(iii) Structural failures – 30 %

Hydraulic failures

About 40% of earth dam failures have been attributedto these causes. The failure under this category, mayoccur due to the following reasons:

(a) By over topping

The water may overtop the dam, if the design flood isunderestimated or if the spillway is of insufficientcapacity or if the spillway gates are not properlyoperated. Sufficient freeboard should, therefore, beprovided as an additional safety measure.

(b) Erosion of upstream faceThe waves developed near the top water surface due tothe Winds, try to notch-out the soil from the upstreamface and may even, sometimes, cause the slip of theupstream slope. Upstream stone pitching or riprapshould, therefore, be provided to avoid such failures.

(c) Cracking due to frost action

Frost in the upper portion of the dam may cause,heaving and cracking of the soil with dangerousseepage and consequent , failure. An additionalfreeboard allowance up to a maximum of say 1.5 mshould, therefore, be provided for dams in areas of lowtemperatures.

Figure – 5 Berm in d/s side at suitable interval

(d) Erosion of downstream face by gully formation

•Heavy rains falling directly over the downstream face andthe erosive action of the moving water, may lead to theformation of gullies on the downstream face, ultimatelyleading to the dam failure.

• This can be avoided by proper maintenance, filling thecuts from time to time especially during rainy season, bygrassing the slopes and by providing proper berms atsuitable heights (Fig. 1), so that the water has not to flowfor considerable distances.

•The proper drainage arrangements are made for theremoval of the rain water collected on thehorizontal berms. Since the provision of bermsensures the collection and removal of water before itacquires high downward velocities, the consequenterosion caused by the moving water (run off) isconsiderably reduced.

(e) Erosion of the down stream toe

The d/s toe of the earth dam may get eroded dueto two reasons, i.e.

(i) The erosion .due to cross currents that maycome from the spillway buckets; and

(ii) The erosion due to tail water.

• This erosion of the toe can be avoided byproviding a downstream slope pitching or a riprapup to a height slightly above the normal tail waterdepth.

• Side walls of the spillway (called diaphragmwalls) must be of sufficient height and length, asso to prevent the possibility of the cross flowtowards the earthen embankment.

Seepage Failures

• Controlled. seepage or limited uniform seepage is in-evitable in all earth dams, and ordinarily it does notproduce any harm. However, uncontrolled or concentratedseepage through the dam body or through its foundationmay lead to piping or sloughing and the subsequent failureof the dam.

• Piping is the progressive erosion and subsequent removalof the soil grains from within the body of the dam or thefoundation of the dam.

• Sloughing, is the progressive removal of soil from the wetdownstream face. More than 1/3rd of the earth dams havefailed because of these reasons.

(a) Piping through foundations

•Sometimes, when highly permeable cavities or fissuresor strata of coarse sand or gravel are present in thefoundation of the dam, water may start seeping at ahuge rate through them (Fig. 2).

•This concentrated flow at a high gradient, may erodethe soil. This leads to increased flow of water andsoil, ultimately resulting in a rush of water and soil,thereby creating hollows below the foundation. Thedam may sink down into the hollow so formed,causing its failure.

Figure – 2 Piping through the dam foundation

(b) Piping through the Dam body

•When the concentrated flow channels get developed inthe body of the dam, (Fig. 3) soil may be removed in thesame manner as in foundation piping, leading to theformation of hollows in the dam body, and subsequentsubsidence of the dam.

• These flow channels may develop due to faultyconstruction, insufficient compaction, cracks developed inembankment due to foundation settlement, shrinkagecracks, animal burrows, etc. All these causes can beremoved by better construction and better maintenance ofthe dam embankments.

Figure – 3. Piping through dam body

•Piping through the dam body, generally get developednear the pipe conduits passing through the dam body.Contact seepage along the outer side of conduits mayeither develop into piping, or seepage throughleaks in the conduits may develop into piping.

•This can be avoided by thoroughly and properlycompacting the soils near the outlet conduits and bypreventing the possibilities of leakage throughconduits, but preventing the formation of cracks in theconduits.

•These cracks in the conduits are caused by differentialsettlement and by overloading from the embankment.When these factors are controlled, automatically, thepossibility of piping due to leakage through theconduits is reduced.

(c) Sloughing of D/S Toe

• The process behind the sloughing of the toe issomewhat similar to that of piping.

• The process of failure due to sloughing starts whenthe downstream toe becomes saturated and geteroded, producing a small slump or a miniatureslide.

• The miniature slide leaves a relatively steep facewhich becomes saturated by the seepage from thereservoir and slumps again, forming a more unstablesurface.

•The process continues till the remaining portion of thedam is too thin to withstand the horizontal waterpressure, leading to the sudden failure of the dam.

Structural failures

About 30 % of the dam failures have been attributed tostructural failures. Structural failures are generallycaused by shear failures, causing slides.

(a) Foundation slide (i.e. overall stability of the dam)

• When the foundation of the earth dams are made ofsoft soils, such as fine silt, soft clay, etc., the entiredam may slide over the foundation.

• Sometimes, seams of fissured rocks, shales or softclay, etc. may exits under the foundation, and the dammay slide over some of them, causing its failure.

• In this, type of failure, the top of embankment getscracked gets cracked and sub-sides, the lower slopemoves outward forming large mud waves near theheel, as shown in Fig. 4.

Figure – 4. Sliding due to soft or weak foundation

• Excessive pore water pressure in confined seams ofsand and silt, artesian pressure in abutments, orhydrostatic excess developed due to consideration ofclay seams embedded between sands or silts, etc. mayreduce the shear strength of the soil, until it becomesincapable of resisting the induced shear stresses,leading to the failure of the dam foundation withoutwarning.

• Loose sand foundations may fail by the liquefactionor flow slides.

(b) Slide in Embankments

•When the embankment slopes are too steep for thestrength of the soil, they may slide causing damfailure.

•The most critical condition of the slide of the u/s

slope is the sudden draw sudden drawdown of the

reservoir (Fig. 5) and the d/s slope is most likely to

slide, when the reservoir is full (Fig. 6).

Figure – 5. u/s slope slide due to sudden draw-down

M.W..L.

Figure - 6. d/s slope slid during full reservoir condition

•The u/s slope failures seldom lead to catastrophic

failures, but the d/s slope failures are very serious.

• These failures, generally occur due to development

of excessive unaccounted pore pressures which may

reduce the shearing strength of the soils as explained in

the previous article. Many embankments may fail

during the process of consolidation, at the time

construction or after the construction.

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