****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.

  1. 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.

  1. 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.

[https://strukts.com/failure-mode-of-earthen-embankment-dam/sowers11]