If you're going to replace what is already there, I mean terraced levels rather than one wall. Typically when one makes a retaining wall, they level everything leading up to it. Putting down one wall would leave a hillside and then a small retaining wall at the bottom, or you would have a gigantic wall multiple feet high which must be back filled with dirt to match the level of the ground in front of your house. Neither option is appealing.
If you made two straight walls extending to the outer portion of the existing mulch bed you could get some extra soil and fill/level the space for each terrace. The end effect/desired result would be something like this:
Though yours would be on a smaller scale, and of course you could choose to landscape that any way you please.
The alternative with one wall would be a very high wall and back filling it with a whole lot of dirt which would likely not look all that nice, it would be difficult, more expensive, and create a greater safety hazard on your property. I'd go with terraces.
Just remember, if you do something like this you're going to want to build the lower wall first and then prepare well for the 2nd.
You will need:
- A hand tamper.
- A good shovel.
- A good level.
- A measuring tape.
- A chisel for prepping your lower level of blocks.
- A rubber mallet for knocking things into place well.
Measure out how long you want your wall to be, and make sure you have space enough to mark and dig out your base. Try to calculate your aggregate needs in advance- a 10 foot wall will need to sit on a base which (ideally) is about 10 feet 4 inches long, and depending on the width of your block (let's say it's half a foot wide) your base is going to be a foot and 2 inches wide. You may be up against a driveway which restricts the "4 inches on each side" rule, and certainly when you're up against the house you can't observe this rule. Anyway, you would have 10'4 by 1'2 base ideally at a depth of 6 inches. V= L*W*H so 10.33 times 1.16 times .5 = around 6 cubic feet unless my math skills have become atrophied from lack of having to calculate crap like this anymore.
The point is, in order to make it cost effective for yourself you should calculate in advance the volume which will be needed for the base of each wall. Also keep in mind the source of your gravel. If you have a work truck which can take a load of gravel being dumped in the bed, I'd buy in bulk and shovel it in. Sure, it's extra work, but buying it in bags or having bulk loads delivered can get pricey.
Level the ground all the way across, in small sections at a time. Be mindful of getting it perfect. Tamping the dirt prior to adding the gravel base and leveling the dirt will help make this a quicker process. Tamp, level, tamp again, level again. It's time consuming and tedious. I've also seen work that I did in year 1 landscaping which is in excellent condition today alongside work of a corner cutting crew which is lopsided and looking quite aged. You won't notice mistakes in months or a few years. After a decade however you'll know the difference between quality work and shoddy work.
When you complete your first wall, and after the glue is dry, I'd fill in, tamp, and level the dirt behind it so you have a flat surface. For the upper terrace, you will likely need to do a decent amount of digging to give yourself space to once again dig out a 6 inch base and repeat the process. Be mindful of keeping straight lines when putting together your wall and digging out your base. Tamp the **** out of everything- the dirt your base is sitting on, the dirt around your base, the base itself. You need solidly packed ground and a strong foundation. When you make the 2nd tier fill dirt in behind that, and level it off. Plant something nice in there for god's sake!