We have raised beds out back, partly due to the poor red rock "soil" that our house is built on, so we can have decent dirt for growing, and partly to screen the bottoms to keep out the gophers.
We made them 6 or so years ago out of "landscape timbers" from Home Depot. They were inexpensive at less than $2 each for an 8 footer on sale. However, they are starting to decay.
I looked at replacing them with other things, especially self stacking concrete block systems. They all seem to run $3.50 to $5.00 or more per face square foot. A little too rich for us right now for the size beds we need to replace.
The other day I was out in the Think Tank, AKA spa, having a glass of wine, and I got to thinking. (duh) What about making soil cement out of the abundant red rock? Actually, it would be more like soil concrete, rather than the classic soil cement. Just Portland cement and the red rock.
So, I did a little semi-scientific testing. First, the classic jar test. Soil and water in a Mason, shake and let settle for a day.
Because of the soil and the grading that was done for our house, there isn't really any topsoil or organic matter. All I did was take out any weed root balls.
Hey, not too bad! About 16% fines, and then gravel and stones.
Then I decided to do a little screening. I shoveled a sample into the wheelbarrow, and then screened it through 1/2" and 1/4" hardware cloth. and window screen.
Again, not too bad! About 30% sand and clays < window screen size, 35% window screen to 1/4", 15% "pea gravel" 1/4" to 1/2", and 20% > 1/2".
So, the next test was to make a block. I made a damp mix if 1 part Portland cement and 6 parts red rock. (A classic concrete is 1:5, but I don't need a lot of compressive strength for raised bed walls).
The mix was packable, not pourable. I tamped it into the mold pretty well. This picture is after one day to cure a little, unmolding, and then distressing with a wire brush right away. The right side is distressed, the left as it came out of the mold.
That looks pretty good! I'll let it cure for another week or 2 and then see how strong it is.
Not including form lumber or rebar, I can make raised bed walls for well under a dollar a square foot.
