Building a Reliable Block Retaining Wall

Retaining walls serve a different purpose than outdoor freestanding walls—they're not built to fence in an area or to mark property but to aid in landscaping, holding back soil to fight erosion and let you reshape slopes. Whether it's for aesthetic purposes or to make sloped land usable for garden beds and outdoor spaces, a retaining wall is a valuable landscaping tool when used properly. With retaining wall blocks, it's also easy to make a DIY project of it—perfect for homeowners that want to take their landscape into their own hands. There's much to consider in designing and constructing a retaining wall, though, from aesthetics and material choice to managing water runoff and the weight of the retained soil.

Design Considerations

Fundamentally, retaining walls are built to hold back soil, and the weight of that soil puts tremendous pressure on the wall. Your wall won't be holding up an entire hillside, just the wedge of soil between itself and what is called the "failure plane"—the steepest slope that existing soil can naturally form before it collapses under its own weight. Driven by gravity, this wedge pushes against the retaining wall, with the pressure concentrated at the base.

Garden walls and short retaining walls made from stacked stone or concrete wall blocks can stand on their own when built correctly, using their own weight as well as friction between the wall and base and an interlocking block system to push back against soil. Large scale retaining walls—over 4 feet tall—need additional supports, and most municipalities require a building permit and input from a certified engineer before you can start constructing such a wall, so they're generally outside the scope of a DIY project. If you need to retain a greater slope, you can use multiple tiers of walls—not only does this create a distinctive terrace-like appearance, each individual wall has less pressure against it. As a rule of thumb, separate these tiers with distance twice that of the walls' height.

To make the wall as stable as possible, retaining wall blocks should have a well-compacted base for the lowest course (row of blocks) to rest on and be laid out with each course staggered from the one below. Also, two features are used to counteract the concentration of pressure at the base: the first course is buried partially by compacted soil to keep it from being pushed out and the blocks are stepped back so that the wall as a whole leans against the retained soil. Proper drainage is also vital, as groundwater getting trapped behind the wall adds extra weight against it. To aid in draining groundwater, be sure to install a drain pipe at the base and use gravel as backfilling behind the wall, since it drains more effectively.

Materials To Use

The most apparent material you'll need for your retaining wall is the blocks that make up the wall itself. A retaining block can come in a variety of shapes and compositions. Concrete wall blocks are common and reliable, not to mention they usually come ideally shaped for building retaining walls, but natural stone is a good choice for those who find concrete to look too artificial. If you choose natural stone, you best bet is to use quarried stone that has been cut at the quarry to be more easily stacked; "rubble" or "boulder" walls using irregular stacked stones are tricky to assemble in a stable structure. Additionally, retaining wall blocks usually interlock to improve stability. This is done either with lips and grooves in each block or with slots through which pins are inserted to anchor blocks to each other.

Along with the blocks, you'll need some other materials to help bring it all together. A compacted base material, either sand or gravel, is vital for the foundation. Additional gravel should be used for backfilling in place of native soil to keep it from collapsing and to improve drainage, and landscaping fabric laid between the backfilling and recompacted topsoil further aids in draining groundwater by keeping the soil from slipping into the gravel.

Key tools include a flat blade shovel, a hand tamper or vibrating plate compactor for compacting soil, stonecutting tools such as a diamond blade circular saw or a hammer and chisel, and a bubble or laser level and a set of stakes with string to ensure each course is level. Safety gloves and goggles are recommended, along with a dust mask while cutting, and a back brace can help with lifting heavy blocks.

Guidelines For Construction

The actual process of constructing a wall from retaining blocks can be done over the course of a weekend. To make sure everything goes as planned, here are some tips:

• Double check to ensure that you won't run into any utilities while digging the trench.
• To have level blocks, you need a level base. Flatten and level the underlying soil before laying sand or gravel, using the shovel or a level object like a 2-by-4 to scrape off extra soil. After that, about an inch of sand or gravel works for the base. Jiggling the first blocks a little when setting them in the base can help level them while also anchoring them in place.
• Spot-check regularly as you lay blocks to make sure they're level with each other. You may need to brush away loose dust, dirt or debris to make sure that further courses also lay level. Even a slight error in leveling at the base transfers upwards, destabilizing the entire wall.
• Check the level of an entire course by running string between two stakes or posts at either end.
• If you don't want to buy specially shaped corner or end blocks, you can use a chisel or diamond saw to cut blocks in half in order to maintain the staggered block system.
• Add small amounts of backfill at a time to let it settle and avoid creating gaps that can later collapse. As with setting the first bricks in the base material, jiggling the wall occasionally can help settle the backfill before compacting it.

With due diligence and the right equipment and materials, you can construct retaining walls on your own for DIY landscaping projects. Create terraces or gardens to liven up your backyard, protect your home's foundation from runoff and erosion or add a rustic touch to the scene, all without needing a professional engineer.

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Your Guide to Building a Strong Retaining Wall

Retaining walls can serve multiple functions. The most practical function of a garden wall is that it can offer extra support to prevent erosion. You want to battle the lateral force of slope through the design of the wall itself. In addition to a practical purpose, a retaining wall can add value and beauty to your property. You can create a number of different designs to accent your property. Retaining walls are more than simply stacking retaining wall block, however. Here are tips on an easy to install freestanding wall.

Choose Your Wall Blocks

There are different wall blocks that you can choose from. There are pros and cons to the different types of block. For instance, solid landscape blocks are heavier but you can cut them. Whereas hollow blocks are light but cannot be cut. When cut, it exposes the space within. In addition, hollow blocks generally require you to use backfilling.

In addition, some wall blocks are held together with pins, whereas others have a lip on the bottom. The pinned blocks are best for tight curves. The flat bottom is easier to stack, whereas the one with the small lip can wind up chipping. Chipping can weaken your wall.

Get a Building Plan

Not all retaining walls require a building plan or a permit, but some walls do. If your wall is over four feet tall, then you may need a building permit and an engineer to draw up the plans. The engineer can specify the depth and width of the base and how far you should dig for the base. The engineer can also tell you if you need soil reinforcement. Remember that the taller that your wall is, the more crucial it is for you to have a stable wall. Serious injuries could occur if your wall were to collapse. No matter the size of your wall, it does help to have contractors to assist with the plan and build.

Create Your Trench

Your trench should be big enough for the retaining wall block with at least eight inches of space behind it. Excavate enough to bury up to one layer of retaining blocks and up to eight inches of base material. In addition to depth and size, it should also be level. The base material’s first layer needs to be level. If it isn’t, the wall might tip after freeze and thaw cycles.

Make sure that you compact the soil at the bottom of the trench. You can use a hand tamper or a vibrating plate compactor. This is not an optional step! Some people choose to dismiss this step, but it can cause your wall to settle. Hand shovels and excavators can disturb the top inch or two of your soil.

It is also important that you have plenty of drainage between the wall blocks. Once you stack a few rows, you can backfill with rock and then lay down perforated drain tile. You also need to install drain tee fittings and a drain gate. You need to build your drainage system based on how much rainwater you expect. You can always cut one landscape block down for the drain gate. Remember to screw the drain tile parts together so that they don’t pull apart when you lay down more rock.

Use Crushed Stone

Before you build your garden wall, layer crushed stone as the base. This is better than gravel. It provides drainage and will need less compacting than gravel and other base materials. The best size for crushed stone is between half an inch and ¾ of an inch. Do not lay it down over half an inch beyond the final height that you want. This is about as much as it will go down when you use a hand tamper or plate compactor. When you lay the stone, it is basically already compacted.

Level Your Retaining Wall Blocks

When creating a free standing wall or stacked stone wall, your first course of retaining blocks is the most important. Your wall’s structural integrity relies heavily on this lawyer. Make sure that each block is level and even. You can use a rubber or plastic mallet to set the blocks. Use a torpedo level to make sure that it is completely level. This first course should also be as close to the middle of the trench as possible.

To ensure that your wall remains level, make sure to sweep the blocks before you stack them.  If there any pebbles on the surface, it can completely throw the wall out of alignment. It leads to a chain reaction with every block above it. Eventually, your top course will have a hump that you could have avoided by sweeping the blocks.

Space Out Your Tiers

If you want a tiered block system, then you are going to want to make sure that each tier is set back enough that the weight and pressure will not destroy the lower levels. You should separate wall tiers by a distance that isn’t less than the twice the height of the wall below it. For instance, if the bottom wall is two feet tall, then the above needs to be at least four feet tall behind it.

Tighten Your joints

The joint between retaining wall blocks have to be as tight as possible. if you have large gaps, then water might be able to seep into the wall. Retaining wall blocks can be prone to chipping if water is allowed to stand within the wall.

Backfill Your Retaining Wall

Once you are finished, you can backfill. You need to compact angular stone as you backfill. You will want to backfill about eight to 10 inches below the top of your capstones. Then, you can fill the rest with topsoil or turf.

It takes design and work to create the perfect retaining wall. You need the right retaining wall blocks, a plan and the best quarried stone. Once you have all of the materials and tools to make it happen, you can create a garden wall that adds value to your property and structural integrity to your landscape.