Failure of gravity dam occurs due to overturning, sliding, tension and compression. A gravity dam is designed in such a way that it resists all external forces acting on the dam like water pressure, wind pressure, wave pressure, ice pressure, uplift pressure by its own self-weight.
Gravity dams are constructed from masonry or concrete. However, concrete gravity dams are preferred these days and mostly constructed.
The advantage of gravity dam is that its structure is most durable and solid and requires very less maintenance.
Causes of failure of a Gravity Dam:
A gravity dam may fail in following modes:
- Overturning of dam about the toe
- Sliding – shear failure of gravity dam
- Compression – by crushing of the gravity dam
- Tension – by development of tensile forces which results in the crack in gravity dam.
Overturning Failure of Gravity Dam:
The horizontal forces such as water pressure, wave pressure, silt pressure which act against the gravity dam causes overturning moments. To resist this, resisting moments are generated by the self-weight of the dam.
If the resultant of all the forces acting on a dam at any of its sections, passes through toe, the dam will rotate and overturn about the toe. This is called overturning failure of gravity dam. But, practically, such a condition does not arise and dam will fail much earlier by compression.
The ratio of the resisting moments about toe to the overturning moments about toe is called the factor of safety against overturning. Its value generally varies between 2 and 3.
Factor of safety against overturning is given by
FOS = sum of overturning moments/ sum of resisting moments
Fig 1: sum of external horizontal forces greater than vertical self-weight of dam (overacting, sliding occurs)
Sliding Failure of Gravity Dam:
When the net horizontal forces acting on gravity dam at the base exceeds the frictional resistance (produced between body of the dam and foundation), The failure occurs is known as sliding failure of gravity dam.
In low dams, the safety against sliding should be checked only for friction, but in high dams, for economical precise design, the shear strength of the joint is also considered.
Factor of safety against sliding can be given based on
- Frictional resistance
- Frictional resistance and shear strength of the dam
Factor of safety based on frictional resistance:
Gravity Dam Failure due to Tension Cracks
Masonry and concrete are weak in tension. Thus masonry and concrete gravity dams are usually designed in such a way that no tension is developed anywhere. If these dams are subjected to tensile stresses, materials may develop tension cracks.
Thus the dam loses contact with the bottom foundation due to this crack and becomes ineffective and fails. Hence, the effective width B of the dam base will be reduced. This will increase pmax at the toe.
Hence, a tension crack by itself does not fail the structure, but it leads to the failure of the structure by producing excessive compressive stresses.
For high gravity dams, certain amount of tension is permitted under severest loading conditions in order to achieve economy in design. This is permitted because the worst condition of loads may occur only momentarily and may not occur frequently.
Gravity Dam Failure due to Compression
A gravity dam may fail by the failure of its material, i.e. the compressive stresses produced may exceed the allowable stresses, and the dam material may get crushed.