FAILURE OF EARTHFILL DAMS
Some causes of failure of earthfill dams
The various causes of failure may be classified as:
a) Hydraulic failure
b) Seepage failure
c) Structural failure
a) Hydraulic failure:
Hydraulic accounts for over 40% of earth dam failure and may be due to one or more of the following:
i) By overtopping: When free board of dam or capacity of spillway is insufficient, the flood water will pass over the dam and wash it downstream.
ii). Erosion of downstream toe: The toe of the dam at the downstream side may be eroded due to i) heavy cross-current from spillway buckets, or ii) tail water. When the toe of downstream is eroded, it will lead to failure of dam. This can be prevented by providing a downstream slope pitching or a riprap up to a height above the tail water depth Also, the side wall of the spillway should have sufficient height and length to prevent possibility of cross flow towards the earth embankment.
iii) Erosion of upstream surface: During winds, the waves developed near the top water surface may cut into the soil of upstream dam face which may cause slip of the upstream surface leading to failure. For preventing against such failure, the upstream face should be protected with stone pitching or riprap
iv). Erosion of downstream face by gully formation: During heavy rains, the flowing rain water over the downstream face can erode the surface, creating gullies, which could lead to failure. To prevent such failures, the dam surface should be properly maintained; all cuts filled on time and surface well grassed. Berms could be provided at suitable heights and surface well drained.
Figure 3. Beginning of downstream failure
b). Seepage failure:
Seepage always occurs in the dams. If the magnitude is within design limits, it may not harm the stability of the dam. However, if seepage is concentrated or uncontrolled beyond limits, it will lead to failure of the dam. Following are some of the various types of seepage failure.
i) Piping through dam body. When seepage starts through poor soils in the body of the dam, small channels are formed which transport material downstream. As more materials are transported downstream, the channels glow bigger and bigger which could lead to wash out of dam
ii) Piping through foundation: When highly permeable cavities or fissures or strata of gravel or coarse sand are present in the dam foundation, it may lead to heavy seepage. The concentrated seepage at high rate will erode soil which will cause increase flow of water and soil. As a result, the dam will settle or sink leading to failure.
iii) Sloughing of downstream side of dam:
The process of failure due to sloughing starts when the downstream toe of the dam becomes saturated and starts getting eroded, causing small slump or slide of the dam. The small slide leaves a relative steep face, which also becomes saturated due to seepage and also slumps again and forms more unstable surface. The process of saturation and slumping continues, leading to failure of dam.
c) Structural Failure:
About 25% of failure is attributed to structural failure, which is mainly due to shear failure causing slide along the slopes. The failure may be due to:
i) Slide in embankment: When the slopes of the embankments are too steep, the embankment may slide causing failure. This might happen when there is a sudden drawdown, which is critical for the upstream side because of the development of extremely high pore pressures, which decreases the shearing strength of the soil. The downstream side can also slide especially when dam is full. Upstream embankment failure is not as serious as downstream failure.
ii) Foundation slide: When the foundation of an earthfill dam is composed of fine silt, clay, or similar soft soil, the whole dam may slide due to water thrust. If seams of fissured rocks, such as soft clay, or shale exist below the foundation, the side thrust of the water pressure may shear the whole dam and cause its failure. In such failure the top of the dam gets cracked and subsides, the lower slopes moves outward and forms large mud waves near the dam heel.
iii) Faulty construction and poor maintenance: When during construction, the compaction of the embankment is not properly done, it may lead to failure.
iv) Earthquake may cause the following types of failure to earthfill dams;
1 cracks may develop in the core wall, causing leakages and piping failure.
2. slow waves may set up due to shaking of reservoir bottom, and dam may fail due to overtopping
3. settlement of dam which may reduce freeboard causing failure byovertopping
4. slidding of natural hills causing damage to dam and its appurtenant structures
5. Fault movement in the dam site reducing reservoir capacity and causing overtopping.
6. Shear slide of dam
7. The sand below foundation may liquefy
8. Failure of slope pitching.