Test: Earthen Dams Failure - Civil Engineering (CE) MCQ

Over-topping of the dam generally occurs due to insufficient free-board, inadequate spillway capacity, and clogging of the spillway, etc. Frost causes heaving and cracking of the soil with seepage and consequent failure, an additional freeboard allowance of about 1.5 m should be provided as an additional safety measure.

The waves developed near the top surface due to the winds notch out the soil from the U/s face and the D/s toe of the dam may get eroded due to the cross currents and due to tailwater. Stone pitching or riprap should be provided to avoid such failures.

The cross-currents that may come from the spillway buckets is one of the reasons for the d/s toe erosion. To prevent the possibility of cross-flow towards the earthen embankment, sidewalls of the spillway also called diaphragm walls of sufficient height and length is provided.

The rainwater that acquires high downward velocities causes consequent erosion which can be reduced by the provision of berms.
The provision of berms serves the following purposes –

• It behaves like a good lining for reducing losses and leakage
• They provide protection against erosion and breaches due to wave action
• They help the channel to attain regime conditions as they help in providing a wider waterway
• It can be used as borrow pits for excavating soil to be used for filling.

The entire dam may slide or just one face slides over the foundation when the foundation of earth dams is made of soft soils. Then the foundation bulges out on the side of the movement. The top of the embankment gets cracked and lower slope moves outward forming large mud waves near the heel.

The progressive erosion and the subsequent removal of soil grains from within the dam or from the foundation is piping. The progressive removal of soil from the d/s face is sloughing.

The D/s slope is most likely to slide when the reservoir is full. For downstream face also, when the slope is steep and soil used is poor with inadequate compaction, saturation and softening of soil due to rainfall and seepage causes sliding of the downstream face.

When the upstream slope is steep and when the soil used in the construction of the dam is poor and compaction is not adequate, the sudden drawdown of water causes sliding of the upstream face. This failure seldom leads to catastrophic failures.

Failures of earthen embankment dams are grouped into three categories
a) Hydraulic Failure: They occur from the uncontrolled flow of water over and adjacent to the embankment and due to the erosive action of water on the embankment slopes. 4 out every 10 earthen dams have failed due to hydraulic failures.
Various hydraulic failures are:-
i) Overtopping:

(ii) Wave erosion: Notching of the upstream face by wave action that reduces the embankment cross section thickness and weakens embankment material.
(iii) Toe erosion: Erosion of downstream toe of the earth slope caused by misdirected spillway outlet discharge.
(iv) Gullying: Rainfall erosion of embankment slopes. Also caused by traffic from people and vehicles.
b) Seepage Failure: Seepage, if uncontrolled, can erode fine soil material from the downstream slope or foundation and continue moving towards the upstream slope to form a pipe or cavity to the pond or lake often leading to a complete failure of the embankment. This action is known as “piping.”1 out of every 3 failed dam cases has seepage failures.

c) Structural Failure: Sloughs, bulges, cracks or other irregularities in the embankment are examples of structural failure. About 1/4th of earthen dams have failed due to structural failures.
Hence, 40 % of earth dam failures are due to hydraulic failures, 33% of the earth dam failures are due to seepage failures, 25% of the earth dam failures are due to structural failures.

The saturation gradient line may cut the downstream end of the bank even after providing sufficient section for bank embankment. By the provision of counter-berms, the saturation line can be kept covered at least by 0.5 m.