### Zoning of High Non-Overflow Dams

A high gravity dam may be divided into seven zones according to design and stability requirements.The characteristics and limits of these zones are described below:

1. Zone: is a rectangular section from the top of the dam to the water surface. The resultant force passes through the mid-point of the base.

2. Zone: is also a rectangular section and extends to a depth where the resultant in the reservoir full condition reaches the outer middle third point of the base.

3. Zone: upstream face of the dam is vertical but the downstream face is gradually inclined so that the resultant in the reservoir full condition has exactly at the outer middle third point of the base. This zone extends to a depth where the resultant in the reservoir empty condition reaches the inner middle third point of the base.

4. Zone: in this zone both the upstream and downstream faces are inclined so that the resultant both in the reservoir full and empty conditions lie at the middle third point. The zone extends to a point where maximum permissible compressive stress is reached at the toe of the dam.

5. Zone: the slope of the downstream face is further increased to keep the principal stresses within permissible limits. Resultant in the reservoir full condition is kept well within the middle third section. The resultant reservoir empty condition follows the upper middle third section. This zone extends to a depth where the stress at the heel of the section reaches the permissible limits in the reservoir empty case.

6. Zone: the slope of the upstream face is rapidly increased so as to keep the principal stress at the heel within the permissible limits in the reservoir empty condition. The inclination of the downstream face should also be adjusted so that the principal stress at the toe does not exceed the maximum allowable stress. The resultants in both reservoir empty and full conditions lie within the middle third section. This zone extends to a point where the slope of the downstream face reaches 1:1. This normally happens when the dam is 80 to 90 meters high.

7. Zone: in this zone the inclination of both upstream and downstream faces increase with the height of the dam. Consequently, at some plane the value of (1 + tan2) may become so great that the principal stress at the downstream face may exceed the allowable limit. If one reaches this zone during design, it is better to avoid it and start again with a fresh design with increased crest width and/or better quality concrete.

Zoning of overflow Dams (Spillways)

1. Zone: the resultant in the reservoir full condition is outside the middle third point both horizontal and vertical forces are existing. End of zone I is at a depth where the resultant intersects downstream middle third point. Upstream face needs reinforcement to take tension.

2. Zone a: this is the zone below zone1. The end of zone Ia is established by the plane where only friction is sufficient to resist sliding.

3. Zone: similar to zone II of non overflow dam with the only difference that the downstream face is inclined in overflow dams. The rest of the zones are similar to those of non-overflow dams.