Puddle Wall vs. Puddle Trench.

There is a diversity of opinion among engineers in regard to the proper place for the puddle in dam construction. Theoretically, the inner face would be preferable to the center, for the purpose of preventing any water from penetrating the embankment. It is well known that all materials immersed in water lose weight in proportion to the volume of water they displace. If the upper half of the dam becomes saturated it must neccesarily lose both weight and stability. Its full cohesive strength can only be maintained by making it impervious in some way. The strength of an earth dam depends upon three factors:

These can be known only so long as no water penetrates the body of the dam. When once saturated the resultant line of pressure is no longer normal to the inner slope, for the reason that there is now a force tending to slide the dam horizontally and another due to the hydrostatic head tending to lift it vertically. When the water slope is impervious the horizontal thrust is sustained by the whole dam and not by the lower half alone. When once a passage is made into the body of the dam, the infiltration water will escape along the line of least resistance, and if there be a fissure it may become a cavity and the cavity a breach.

For practical reasons, mainly on account of the difficulty of maintaining a puddle face on the inner slope of a dam, which would require a very flat slope, puddle is generally placed at the center as a core wall.

It was thought possible at the Tabeaud dam to counteract the tendency of the face puddle to slough off into the reservoir by use of a broken stone facing of riprap. This covering will protect the puddle from the deteriorating effects of air and sun whenever the water is drawn low and also resists the pressure at the inner toe of the dam.