CAUSES OF FAILURE OF GRAVITY DAMS
Following are the modes of failure of a gravity dam
I. Overturning
II. Sliding
III. Crushing of masonry due to compression
IV. Rupture from tension
V. Foundation failures.
1. Overturning:
The overturning of the dam section takes place when the resultant force at any section cuts
the base of the dam downstream of the toe, on the other hand, if the resultant cuts the base
within the body of the dam, there will be on overturning.
For stability requirements, the dam must be safe against overturning. The factor of safety
against overturning is the ratio of the stabilizing moments (+ve) to the overturning
moments,
i.e., F.S = Σ stabilizing moments = Σ Μs
Σ Overturning moments Σ Μo
The F.S against overturning should not be less than 1.5
2. Sliding:
A dam will fail in sliding at its base, or at any other level. The sliding takes place when the
total horizontal forces are greater than the combined shearing resistance of the joint and
the static induced by the total vertical forces. To avoid sliding the F.S against sliding should
be greater than 1 i.e.,
F.S = µ (V-U) > 1
ΣH
Where V = Total vertical forces
U = Upward force,
ΣH = sum of the horizontal forces
µ = Co-efficient of friction. It varies from 0.65 to 0.75
3. Crushing of masonry due to Compression:
Failure due to crushing of masonry is likely to occur when compressive stress developed in
the body of the dam section is greater than the maximum permissible compressive stress of
the material used in dam section. To avoid this failure the stresses developed in the material
must be kept within the permissible limits.
4. Failure due to Tension:
The dam may fail when tension is produced in the concrete or masonry. When the cracks are
produced due to tension water seeps in and uplift is increased. It reduces self-weight and
cracking is further extended due to more tension. For no tension to develop, the eccentricity
should be less than. b/6 in other words, the resultant should always lie within the middle
third.
Following are the modes of failure of a gravity dam
I. Overturning
II. Sliding
III. Crushing of masonry due to compression
IV. Rupture from tension
V. Foundation failures.
1. Overturning:
The overturning of the dam section takes place when the resultant force at any section cuts
the base of the dam downstream of the toe, on the other hand, if the resultant cuts the base
within the body of the dam, there will be on overturning.
For stability requirements, the dam must be safe against overturning. The factor of safety
against overturning is the ratio of the stabilizing moments (+ve) to the overturning
moments,
i.e., F.S = Σ stabilizing moments = Σ Μs
Σ Overturning moments Σ Μo
The F.S against overturning should not be less than 1.5
2. Sliding:
A dam will fail in sliding at its base, or at any other level. The sliding takes place when the
total horizontal forces are greater than the combined shearing resistance of the joint and
the static induced by the total vertical forces. To avoid sliding the F.S against sliding should
be greater than 1 i.e.,
F.S = µ (V-U) > 1
ΣH
Where V = Total vertical forces
U = Upward force,
ΣH = sum of the horizontal forces
µ = Co-efficient of friction. It varies from 0.65 to 0.75
3. Crushing of masonry due to Compression:
Failure due to crushing of masonry is likely to occur when compressive stress developed in
the body of the dam section is greater than the maximum permissible compressive stress of
the material used in dam section. To avoid this failure the stresses developed in the material
must be kept within the permissible limits.
4. Failure due to Tension:
The dam may fail when tension is produced in the concrete or masonry. When the cracks are
produced due to tension water seeps in and uplift is increased. It reduces self-weight and
cracking is further extended due to more tension. For no tension to develop, the eccentricity
should be less than. b/6 in other words, the resultant should always lie within the middle
third.
