What! Hoover Dam isn't a Seattle retaining wall! What's going on here... Well, Hoover Dam is an excellent example of a retaining wall that resists a massive force, in this case the weight of the water behind it, partly because of its shape rather than just its sheer mass or material strength. The curved shape tends to distribute the load evenly, and eventually terminate it at the ends of the arch, where it's set into solid rock. It should be noted that the photo above is from the Wikipedia entry for Hoover Dam and is not my own.
This principle has been known for hundreds, if not thousands of years, and is found in arches and domes that form the roofs of buildings. In those cases, the roofs are almost always supporting only their own weight, which is called a "dead load" in engineering terminlolgy. I chose to use Hoover Dam as an analogy for earthen retaining walls because both dams and earthen retaining walls are called upon to support "live loads" which are forces acting upon the structure in addition to the structure's own weight.
Unfortunately for earthen retaining walls, it's rare to find an example where the wall curves into the embankment. This is simply because property is more often than not laid out in square or rectangular grids. The only example of a retaining wall that I've found in Seattle that is curved into the embankment is the one seen below, found along Harbor Avenue in West Seattle.
