It's my belief that the fundamental reason that there are so many damaged and poorly repaired retaining walls in Seattle is because of serious underestimation of the forces involved in holding back earth. I suspect, but cannot prove, that many of Seattle's older concrete walls were built before concrete strengthening additives became commonly used, which partly accounts for why so many concrete slab walls are cracked.
Not only are many solid concrete walls cracked, but they are often pushed out of plumb. This is likely due to pouring the concrete into a trench that is too shallow, or not far enough "below grade."
Take a look at the wall in the banner photograph. Unfortunately the back corner of the wall in the alleyway is in shadow, so it's hard to see, but it starts out with stacked wood! In front you can see two different styles of small concrete block, which suggests that a major section was repaired, and that matching blocks couldn't be found. But I suspect the property owners reached the limit of their patience, and resorted to the massive Jersey Barriers you see at the corner!
Amazingly the sad wall seen above is not at all uncommon! The hydraulic force of just two feet or so of earth was enough tip this wall over. Because the soil is pushing evenly, the wall didn't crack in the middle, but the stresses accumulated at the corner. As we know, concrete is weak in tension and therefore the wall was literally pulled apart! An obvious example of why the inclusion of rebar, at the very least in the corner, might have prevented this.
Here we see a low stone wall exhibiting much the same behavior as the solid concrete wall.
I believe the operative word here is "plumb."
I hesitate to include the following image, as I'm not even sure it's supposed to be, or to have been, a retaining wall! But someone built it, and for some reason, so it's sad to see the effort come to naught.
C'mon Seattle, just say NO to crack!
