As promised, this is the third in a five-part series on retaining walls and their construction. We've already talked about natural stone and pressure-treated timber walls, so on to one of my favorites, engineered walls -- oh yeah! Engineered walls are cast concrete blocks that are gravity-stacked and assembled kind of like a puzzle. I like this kind of wall because it is the strongest wall of the types I've enumerated. It is not as architecturally flexible as a stone wall, for example, but it will solve a bunch of grade change problems that the others may not. It should be noted that the majority of engineered wall work is for commercial applications, but small walls certainly have their place in a residential site.
Take a look at the gallery, so you see what some of the options are, and we'll continue.
First off, safety -- the wall blocks, sometimes called "cmu's" (concrete masonry units) are HEAVY -- some go to nearly 100 pounds each, so clearly this is not a "flipflop" job; additionally you should wear thick gloves, perhaps some sort of back brace, and hearing and eye protection during the times you are cutting the blocks. Don't forget to have your utilities marked, so as not to cut a power or gas line (clearly not a good thing) or a DSL cable (nothing says angry neighbor like an interrupted internet session.)
I am going to confine this feature to gravity-stacked wall construction by a homeowner, not exceeding 4' in height; if you plan to build taller than this, you should absolutely consider having an engineer do a layout for you. You may also need a building permit from your local authorities--check with them.
Materials: Hint -- I strongly recommend that you purchase all the heavy stuff noted below from a landscape or wall supplier and have them deliver it; even with our wall, the total weight of the materials is more than 3 tons. You don't want to haul that amount in your sports car and don't even think about using momma's van or you'll be curled up next to the blocks for a few nights.
- Wall units -- I generally use those measuring approximately 6" tall, 18" wide and 12" deep. This size yields a block about .75 face feet in area. To determine how many blocks you will need, multiply the length of the wall by the height and divide that number by .75. For example, a wall 10' long and 4 1/2' tall = 45 square feet in area (including the one buried level of blocks); divide 45 by .75 and you get about 60 blocks, or more than one pallet; I always get an extra 10% for mistakes (not that I would make any, of course!) and cutting. Keep in mind that your local vendor may require that you purchase blocks by the full pallet, so be prepared for this eventuality; if so, just adjust the size of your wall or use the blocks for other projects, like skateboard ramps or speed bumps for subdivision speeders (just kidding.) The blocks I use have a "lip" on the back, so I don't have to use pins to secure the courses of wall together.
Caps -- the caps I use are about 18" (or 1.5') long, so divide the length of the example wall (10') by 1.5' and you get approximately 7 caps; generally speaking, a vendor will sell you individual caps, but check to be certain.
-- a permeable fabric that will let water through, but not soil. You don't want the front of your wall to become stained; get one square foot of the stuff for each square foot of wall (above the ground) plus one square foot per lineal foot of wall; for us, it's about 50 square feet. The material generally comes in 100 square foot rolls, but you can always find a use for leftover material, as a weed block for example.
Gravel sand for the base (optional) -- this stuff looks like coarse, gray beach sand; you can put it in the bottom of your footing, but it's not required if you are accurate in your footing excavation as you may use the tamped excavated soil to level the bottom course. If you decide to use the fines, get enough to lay a base 2" thick for a footing that will be 18" from front to back and as long as the wall; for our 10' long wall that will be approximately 250 pounds. You can generally purchase the material by the bag from a home store, or get it in bulk from the folks who sold you the wall block.
Drainage gravel -- in theory, a wall less than 4' tall doesn't require drainage gravel; don't take a chance with that particular theory, however. Install drainage gravel the length and height (above the ground) of the wall and about 6" out from the wall; for us, that amounts to approximately 2000 pounds of #57 stone (the rock is about the size of your thumb.)
Cap glue -- for securing the caps to the topmost wall block; you'll need about one tube of concrete adhesive for each 5' of wall length; get it at the home store, plus a caulking gun if you don't have one.
-- you may hear this term bandied about when discussing your wall project; it's a high-strength polypropylene material that provides lateral strength like a "dead man
" or "tieback" for a wooden wall. If you keep your wall under 4' you don't need it; it's for the big boy walls.
Drain line -- use perforated (slits or holes; doesn't matter) 4" drain line covered with the landscape fabric to relieve any hydrostatic (water) pressure at the base of the wall. (You can get 100' rolls of sockpipe -- perforated pipe covered with a nylon sock -- but for a small wall just make your own.)
Time: for our 40 square foot wall, once you have all the materials on the site, it will take an easy weekend. Per unit of time expended, digging the footing and installing the first course is the hardest and most time-consuming; after that, it's not bad at all, slapping those wall blocks in place, especially if your pals lend a hand; I have it on good authority that the noted wall will cost you about a case of adult beverages.
4' or 6' level
and a small torpedo level
, for determining the back to front pitch of the wall.
3 pound hammer for use in splitting the blocks (and occasionally beating a block into position!)
Wide point stone chisel
for splitting the blocks. (if you have a big wall, you may want to consider a gas-powered saw
to make your cuts -- they are expensive to buy, best rent one.)
Round and flat shovels, for excavating the footing.
for use in compacting the soil in the footing.
After selecting your hillside site, begin your excavation at the lowest point of the wall, working both ways to the projected end points, digging a trench about 18" wide and 6" deep. Make the bottom as flat and level as you can; if the grade changes such that you need to "step the wall
", do so by changing the trench height in increments of one block. You may use the excavated soil to level the bottom of the trench.
Tamp, tamp, tamp the soil in the trench. If the soil is loose when you lay in the base course, that course will not stay level, the wall will not be level, and, in the extreme, will eventually fail and you will be subject to immense amounts of ridicule. (Did I mention you should tamp the soil?)
Lay in the first row (or lift) of wall blocks, ensuring that the top of the blocks is pretty well flush with the ground (i.e. bury the rascals); my rule of thumb is to bury the blocks 1 1/2" deep for each foot of height (4' wall = 6" trench.)
Level, level, level the base course. Use the 4 or 6' level to ensure the base is level from side to side and the torpedo level to do the same front to back. If the base is not level, the wall will not be level and, at a minimum, will look sloppy and, at a maximum, will fail and you will be subject to immense amounts of ridicule. (Did I mention level the wall?)
OK, the (technically) hard part is complete. Cut a block in half by scoring it on the top and bottom at the middle with your chisel and hammer, then giving it a good whack in the center. Lay this block at the end of the first (above-ground) lift, then add full blocks from side to side such that each block sits atop the joints of the blocks below it; if you place the blocks directly on top of each other, you will have a bunch of individual walls stacked next to each other; they will fail and you will be subject to ... you know the rest..
- At the level of the second lift, put your drain line behind the wall, with the lower end protruding from the end of the wall, cover it with the fabric, and begin backfilling with the drainage gravel, starting at the back of the wall and moving out about 6" from the wall, then start adding your excavated soil (or purchased soil, as necessary) between the gravel and the slope of the hill. Approximately every 6" high, tamp the soil. With this wall, you don't need to worry about setback or batter; the lip (or pins, if you used a pinned wall system) automatically sets it for you. Additionally, now is the time to put the fabric against the rear of the wall and hold it in place with the backfill gravel.
- Continue adding lifts to the wall, cutting blocks, staggering joints, and backfilling, until you reach your desired height, which should be about 2" above the soil's surface.
- At this point, lay out the caps, cut them as necessary, glue them with the construction adhesive, and you're finished -- what a deal!
That's the process; it's a lot easier than it reads and you'll be well satisfied to have taken on this project. Now you can sit back, finish off the leftover beverages, and bask in the admiring glow of family and friends.