Question

The three Primary failure modes for concrete dams are:

1-Stability under acting loads

2-Load-bearing capacity (due to e.g. degradation or cracks)

3-Water-tightness

Describe these-give the reason for when and how each of them Will occur?

Answer 1

The stability analysis of a dam section is carried out to check the safety with regard to
1. Rotation and overturning
2. Translation and sliding
3. Overstress and material failure

1.) Stability against overturning
Before a gravity dam can overturn physically, there may be other types of failures, such
as cracking of the upstream material due to tension, increase in uplift, crushing of the
toe material and sliding. However, the check against overturning is made to be sure that
the total stabilizing moments weigh out the de-stabilizing moments. The factor of safety
against overturning may be taken as 1.5. As such, a gravity dam is considered safe also
from the point of view of overturning if there is no tension on the upstream face.

2.) Stability against sliding
Many of the loads on the dam act horizontally, like water pressure, horizontal
earthquake forces, etc. These forces have to be resisted by frictional or shearing forces
along horizontal or nearly-horizontal seams in foundation. The stability of a dam against
sliding is evaluated by comparing the minimum total available resistance along the
critical path of sliding (that is, along that plane or combination of plans which mobilizes
the least resistance to sliding) to the total magnitude of the forces tending to induce
sliding.

NOTATION ΣΗ : NET HORIZONTAL FORCE NET VERTICAL FORCE o INCLINATION OF INTERFACE OF DAM FOUNDATION C COHESION OF MATERIAL AT THE PLANE A AREA OF CONTACT AT FOUNDATION NET DESTABILIZİNG FORCE ALONG A-A 2V Coso 2V / Sindi Cohesive force C.A Shear force (ZV Cosa-ΣΗ Sina)

3.) Failure against overstressing

A dam may fail if any of its part is overstressed and hence the stresses in any part of
the dam must not exceed the allowable working stress of concrete. In order to ensure
the safety of a concrete gravity dam against this sort of failure, the strength of concrete
shall be such that it is more than the stresses anticipated in the structure by a safe
margin. The maximum compressive stresses occur at heel (mostly during reservoir
empty condition) or at toe (at reservoir full condition) and on planes normal to the face
of the dam. The strength of concrete and masonary varies with age, the kind of cement
and other ingredients and their proportions in the work can be determined only by
experiment.

see following fig. for better understanding.

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