“That latter point means that, like all concrete structures, concrete dams will crack. These cracks will be tend to be vertical, since the concrete is free to grow and shrink in the vertical dimension, while the canyon it spans is of a fixed dimension. While an arch dam will have the pressure of the water pushing these vertical cracks closed, a gravity dam has no forces acting along its axis--well, except for the temperature changes. The easiest way to deal with this is to provide a known place for the concrete to crack. Therefore, the dam is divided into monoliths, where each monolith is a separately-placed block of concrete that is more or less structurally independent of the monoliths next to it. The dividing lines between monoliths are contraction joints, and will open and close with temperature changes, sometimes quite dramatically. This is the same reason sidewalks are cut into squares and concrete highways have the joints that go "ba-dump" as you drive over them--the concrete will crack, so provide a way to make it crack in a controlled, pretty manner.
[...]
This will hold true for all of the concrete, but once you get to the foundation, it gets a bit tricker. The soil is not a relatively well-controlled material like the concrete. It's typically full of joints and is permeable. At the base of the dam, it's typical to assume that the full pressure of the forebay is acting at the heel and the pressure of the tailwater is acting at the toe. The total force acting upwards is typically pretty large, which is a problem: this pressure will reduce the forces acting on the foundation, and therefore the force squeezing any joints in the rock. If you remember from high school physics, it's typically assumed that frictional forces are proportional to the force squeezing two surfaces together. Therefore, this uplift pressure can reduce these forces enough that a wedge of foundation rock will slide with the dam in a foundation failure. This is the typical cause of failure in a large gravity dam.
I've shown with a dashed line what that uplift pressure would look like under the monolith, if we let it reach its natural state. However, we have some tricks that we can use to dramatically reduce this uplift pressure. I've drawn a grout curtain and foundation drains extending from the bottom of the dam. The grout curtain is a "wall" of portland cement grout, formed by drilling through the bottom of the dam from the drainage gallery and injecting the grout under pressure. It's typically much deeper than I've shown here, as I ran out of room on the paper. This curtain will increase the path length for flow lines, which will primarily serve to reduce the volume of water flowing under the dam. ”
-- “Effort Post: Concrete Gravity Dam” https://www.datasecretslox.com/index.php/topic,215.msg3582.h...
(see also parts II and III at https://www.datasecretslox.com/index.php/topic,1012.msg19813... and https://www.datasecretslox.com/index.php/topic,1359.msg31228...)