DEAR MIKE: It's time to landscape my back yard, which is currently dirt. I have a 2-foot-high bluff where I'd like to put a retaining wall, but I'm uncomfortable with masonry work. I've seen stackable blocks in stores, and wonder if they would be suitable because I plan only to go about 1 1/2 feet high with the wall. Besides, I think it would add visual excitement to my yard. -- Frank K.
DEAR FRANK: Visual what? Most of my buddies get their visual excitement from certain Internet sites. But hey, if modular blocks do it for you, then go for it.
The blocks you describe are perfect for the job. The good news is they come in different sizes and cost from about $1.50 to $4 each. The bad news is that your back will probably ache afterward, because the blocks weigh between 25 and 45 pounds each, depending upon the type you use.
You may think that your retaining wall has to support all of the dirt behind it, but it doesn't. It only needs to hold back the amount of dirt between the wall itself and the bluff you mentioned.
The bluff is undisturbed soil and is naturally compacted. This soil has a slope, called a failure plane, beyond which it won't support weight. You are going to fill in this area between the retaining wall and the failure plane with porous gravel. If you use regular soil, it will get saturated and may slide down the slope, taking out the new wall.
The pressure on the base of the retaining wall can be staggering. With soil weighing in excess of 100 pounds per cubic foot and a nice slope to slide down, particularly when it becomes wet (and heavier), there are some rules that should be observed.
The base of the wall must be very solid. This is done by digging a trench into which will sit the bottom course of the wall. The rule is that the bottom course must be buried at least one-tenth the height of the wall. This will help the wall hold up against the forces behind it.
Also, once you dig the trench for the bottom course, you must compact the base material with a tamper. The base material should be a mixture of gravel and/or finely crushed rock. This will compact well and is also porous enough to allow water to drain through. Make sure that the trench is compacted and level.
As you place the bottom course into the trench, level each block front to back and side to side. Also level each block against its neighbor. Use a mason's string along the back edge to keep the blocks in line.
Lay landscaping fabric in the trench and run it up the failure slope. This will prevent soil and silt from mixing with the gravel, which would prevent good drainage.
The villain in this picture is water. If water gets behind the wall and has no way to escape, it will cause big problems. That's why you use porous gravel behind the wall, so that the water can quickly drain down to the subsurface drain. Water can also seep through the blocks (since they are not mortared) to reduce pressure against the wall.
Use 3-inch or 4-inch perforated PVC and run it behind the first course of the wall. Any water that gets behind the wall will have an easy way out and not cause excessive pressure at the wall's base.
You'll notice that modular retaining blocks have a lip or a lug built into each block. This is engineering brilliance. These make it so that as each course of block is laid over the other, the wall steps back toward the failure plane. In effect, the wall pushes back against the soil. The back yard bully is born.
As you lay each successive course, fill in the area behind it with porous gravel and compact it with a tamper. Then sweep the dirt off of the existing course so that full contact is made by the next course. As you lay the next course, stagger the blocks, that is, each block should overlap the one beneath it by half its width. Then add a little gravel (3-4 inches) and tamp it down. If you have a large area between the wall and the failure plane, or if your wall is somewhat taller, you would also use reinforcement grid. This material, along with good compaction, will help direct the pressure downward instead of outward.
The height of the retaining wall should be the same height as the soil surface behind it. If the wall is too tall, then you risk water puddling behind it.
If you need to cut some of the blocks, you can rent a concrete or block saw, or you can chuck a masonry blade (about $20) into a circular saw. Use shallow passes, and when you can go no deeper, split the block with a masonry chisel.
Some block manufacturers offer slick options, such as capstones for a finished look. You can also buy false blocks into which you can install lights or stereo speakers. Maybe this will give you the visual excitement you seek.