A gravity dam may fail in following ways:

(i) By overturning

(ii) By crushing or compression

(iii) By development of tension

(iv) By shear failure or sliding

(i) By overturning

• If resultant of all forces acting on a dam at any one of its sections, passes outside its toe, the dam shall rotate and overturn about its toe.
• Practically, such a condition shall not arise.
• The ratio of writing moments about toe (anticlockwise) to the overturning moments about toe (clockwise) is called factor of safety. Its value ranges both 3 and 4.

(ii) By crushing or compression

• A dam may fail due to failure of materials, i.e., the compression stress produced may exceed allowable stress and dam material may get crushed.

  $P=\text{Direct stress + Bendng stress}=\cfrac{\sum Y}{B}+\cfrac{M}{I}Y=\cfrac{\sum Y}{B}\pm \cfrac{M \cdot e}{B^{2}/6}Y$

  $\cfrac{P_{max}}{P_{min}}= \cfrac{\sum Y}{B}\left [ 1 \pm \cfrac{6e}{B}\right]$

• If $P_{min}$ comes out to be negative, it means tension shall be produced at the appropriate end.

• If $P_{max}$ exceeds allowable compressible stress of dam material, the dam may crush and fail by compression.