Like Hoover Dam, Washington's own Grand Coulee Dam illustrates a particular style of building a retaining wall to resist massive live loads. Because it has to span such a wide distance, and the canyon it's in is not narrow, a very thick wall was used. The mass of concrete in Grand Coulee is actually greater than that found in Hoover! But unlike almost all earthen retaining walls found in Seattle, you will notice that there are numerous triangular buttresses on the downriver side of the dam. Like the arch, triangular buttressing is a very old method of reinforecemt. It was taken to amazing architectural heights in some of the great cathedrals of Europe, when sections of the triangle were removed, creating "flying buttresses." Again, this photo of Grand Coulee Dam is not my own but from the Wikipedia entry on the dam.
Part of the stability of Grand Coulee Dam is due to its mass alone. You can see this used in some of Seattle's earthen retaining walls in the use of the massive stacked Jersey Barriers like the one seen below.
The only Seattle retaining wall that uses triangular buttresses that I've found is one on Beacon Hill in south Seattle. If you look at the photo closely, you can see that the wall cracked vertically, right behind the handrail mounting bracket. It was patched, but as you can see, the main wall along the sidewalk became pitched out. I'm sure that this was the point where the triangular buttresses were added, as a stopgap measure to prevent total collapse of the main wall.
