The design which uses least concrete of all is the overhung or doubly curved arch dam, which uses a three-dimensional arch like part of a dome. The Coolidge Darn built in Arizona in 1929 is an example of a `multiple dome’ — three part-domes with buttresses in between.
The whole dam is looks like shell type so, it is also called as shell arch dam.
Because of double curvature, the thickness of arch wall is reduced. But this non-vertical type dams are difficult to construct compared to other three types. They also require stronger foundations. Shell arch dams are more suitable for very narrow valleys.
Picture Credit : Google
