Failure mode of earthen embankment dam

1. Hydraulic failure

a) Overtopping failure:

If the flow takes place above the crest level of the embankment dam due to the various causes such as inadequate flood discharge, incorrect spillway design , insufficient free board consideration etc then the dam body is susceptible of erosion by the over flowed water.

b) Wave erosion:

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.

c) 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. Ti could be checked either by providing the riprap protection of by providing the rock toe drainage.

d) Surface erosion of the D/S slope:

Heavy intensity of the rainfall may cause the erosion of the surface of the dam and may form the several gullies along its slope direction. These gullies may further erode to larger size, if rainfall commence for the long duration causing serious damage to the dam. Turfing or providing contour drainage for arresting the high velocity flow at berms may lessen the severity of the erosion.

2. Seepage failure:

a) Piping failure:

If the exit gradient of the seepage flow is more than the critical gradient .i.e. if the seepage force is much than the resisting force (submerged weight of the soil) of the soil, then soil particles are susceptible for the dislocation. The soil particle at the surface is much vulnerable for the dislocation as there is no overburden support from the above. As the surface particles are dislocated, the process will be even accelerated towards the upstream direction because seepage gradient will further increase continuously. This will cause the pipe like flow inside the dam body or in the foundation such that it will lead to the unwanted settlement of the dam body.

b) 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 be continued further and ultimately dam may lead to failure. This phenomenon is termed as sloughing.

3) Structural failure:

a) Failure due to the pore water pressure:

If the pore water presenting in the embankment body is draining slowly (rapid rise of layer during construction, no drainage provision for the pore water escape) then much of the stress will be beard by the water itself and effective overburden stress on the soil will be less. These pore water pressures will cause the decrease of the shear strength of soil and may cause the failure of embankment slope in case of even small magnitude of shearing load. In experience, it is found that pore water pressure at the central part of the dam is nearly equal to the overburden pressure due to the weight of the soil above it. It is particularly dangerous in case of the earthquake condition if occurred during the dam constructing period.

b) sudden drawdown on the upstream face which cause the seepage force acting along the sliding direction causing increase of the driving force for slope direction.

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.

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.

e) Failure by spreading:

When the earthen dam is located above the stratified deposit that contains layer of silt clay, the fail of dam with the spreading of the fill materials may happen.

f) Failure by leaching:

Water may leach the soluble slats e.g. iron oxide, calcium carbide, present in the foundation or at the abutments, causing formation of large unwanted cavities leading to excessive settlement of the dam.

g) Failure due to earthquake; most devastating of all.

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