The various modes of failures of earth dams may be grouped under three categories: 1. Hydraulic failures 2. Seepage failures, and 3. Structural failures

1. Hydraulic Failures: This type of failure occurs by the surface erosion of the dam by water. This may happen due to the following reasons:

(a) Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway. Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through thespillway. Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided. Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the dam’s downstream face, either complete or partial failure is inevitable (Figure 73).

(b) 2. Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 74).

(c) 3. Erosion of downstream slope by rain wash. Though the downstream face of an embankment is not affected by the reservoir water, it may get eroded by heavy rain water flowing down the face, which may lead to the formation of gullies and finally collapse of the whole dam (Figure 75).

(d) 4. Erosion of downstream toe of dam by tail water. This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base. (Figure 76).

1. Seepage Failure:

The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side. Unless a proper design is made to prevent excessive seepage, it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse. Seepage failures may be caused in the following ways:

(a) Piping through dam and its foundation: This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 77)

(b) Conduit leakage: This is caused due to seepage taking place by the surface of a conduit enclosed within an embankment dam (Figure 78). The seepage of water may be from the reservoir to the downstream or due to the water leaking out of the conduit through cracks that might have developed due to unequal settlement of dam or by overloading from the dam. Further, the cracking of a conduit may also be caused when the soil mass lying below it settles and the conduit is not sufficiently strong to support the soil mass lying above.

(c) Sloughing of downstream face: This phenomenon takes place due to the dam becoming saturated either due to the presence of highly previous layer in the body of the dam. This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 79)

1. Structural Failure These failures are related to the instability of the dam and its foundation, caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage-failures). These failures can be grouped in the following categories:

(a) Sliding due to weak foundation: Due to the presence of faults and seams of weathered rocks, shales, soft clay strata, the foundation may not be able to withstand the pressure of the embankment dam. The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 80) causing large mud waves to form beyond the toe.

(b)Sliding of upstream face due to sudden draw down: An embankment dam, under filled up condition develops pore water pressure within the body of the dam. If the reservoir water is suddenly depleted, say due to the need of emptying the reservoir in expectation of an incoming flood, then the pore pressure cannot get released, which causes the upstream face of the dam to slump (Figure 81).

(c)Sliding of the downstream face due to slopes being too steep: Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and / or too steep in relation to the shear strength of the shoulder material. This causes a sliding failure of the downstream face of the dam (Figure 82).

(d) Flow slides due to liquefaction: Triggered by a shock or a movement, as during an earthquake, some portion of the dam or foundation may destabilize due to the phenomena called liquefaction. Here, even cohesion less soil cannot drain quickly enough as the movements are so sudden that the rate of extra loading on the soil becomes greater than the rate of drainage of the seepage water out of the soil. This causes excess pore water pressure to develop, where both the effective stress and the strength decrease. Under circumstances when the effective stress drops to zero, which means the soil loses all its shear strength, it behaves like a dense liquid and slides down, and the dam slumps.

(e) Damage caused by burrowing animals or water-soluble materials: some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly, carrying fine material along with. This phenomenon consequently leads to piping failure within the body of the dam, finally leading to a complete collapse. Similarly, water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure.

(f) Embankment and foundation settlement: Excess settlement of the embankment and/or the foundation causes loss of free board (Figure 83). The settlement may be more in the deeper portion of the valley, where the embankment height is more