Modular Block Retaining Walls
How Are They Built
Modular block walls have become very popular. They are probably the most common retaining wall constructed today. As you can see by the pictures, they are very attractive and the blocks come in many sizes, textures and colors. They can be used to build small walls to boarder gardens up to massive walls used to support entire building complexes.
Modular blocks are made of concrete but not all concrete blocks are made equal. Low-end inexpensive blocks are not made for heavy-duty use. The amount of load the wall must restrain and the environmental conditions the block must withstand will determine the grade/quality of the block that should be used. You should always consult the block supplier or a contractor when determining the correct type of block to use. For more pictures of retaining walls go to our modular block wall picture album.
There are many modular block manufactures and there are many styles and sizes of blocks from which to choose. Dealers, manufacturers or a contractor can help you select the block that best meets your needs and you can select the texture and color that appeals to you. Some of the common manufactures in the Minnesota region are Versa-Lok, Allan Block, Anchor, Keystone, Belgard and Rockwood, just to mention a few. Visit any of their web sites to see samples and learn much more about retaining wall blocks.
One of the advantages of modular block walls are that they are individual blocks so if something shifts a little, the wall won't break like a poured concrete wall. They are also porous so water will pass through the wall (the joints) which makes them less susceptible to hydrostatic pressure.
Now comes the more complex part of the story. Although it is a lot like stacking Lego's, it is a lot more complicated that that. Let's break a common wall down to all of its components and look at the purpose of each component. (See the illustration to follow the explanation)
The Block is the main structural component of the wall. The mass of the block helps provide the strength to restrain the material behind the wall. The bigger and stronger the mass, the more it can successfully restrain.
The Base is what the first row of blocks set on. Think of the base as the foundation of the wall. The first row or course of blocks is set below ground level. You have to dig a trench to install the base on which the wall sits. The base sits on the foundation soil or the existing soil that makes up the site. That existing soil must be solid and well packed. The base is made of compacted gravel commonly know as class V. The base is about 6 inches thick and twice as wide as the block. If you are setting a single row of blocks to be used as a garden boarder, the base is not too important. If you are building a 3-foot high wall, the base is extremely important. The base is used to make sure the wall is level and that the wall has a solid foundation to sit on. Before you build any wall, I suggest you read in depth about how to install the base.
Drainage behind the wall is also important. A small wall may not be much of an issue, but a 3-foot wall can have problems if drainage is not taken into consideration. You want water to drain from behind the wall rather than build up behind the wall. Drainage can be accomplished by using drainage rock and even adding drain tile with openings through the wall that allow the water from the drain tile to pass through. You generally want a layer of rock between the wall and the backfill soil to improve drainage.
Backfill or retained soil is the common term for the dirt used to fill the area behind the wall. In some cases, this is the soil that was excavated to make room for the wall. However, in many cases, additional material must be hauled on to the site to make up this back fill.
Wall Reinforcement or Geogrid/Geotextile is necessary any time there is going to be substantial load on the back of a wall. This type of materials is special high strength polymer "netting" designed for reinforcing retaining walls. Walls under 4 feet high that are not going to have a great deal of load on the backside of the wall, generally do not need reinforcement. If there is a large load to be restrained by the wall, such as a driveway with cars, a building, a large sloping hill or the wall is over 4 feet high, you need reinforcement. The reinforcement consists of a geotextile fabric or grid that goes between layers of the block, back into, and is held in place by reinforced soil. When this becomes necessary, it is time to hire a professional. Reinforced walls are generally beyond the do it yourself category. In many cases, these types of walls need city building permits and need to be built to an approved engineering drawing.
The above information is to give you an idea of what goes into a modular block retaining wall. Little short walls can be relatively easy to build. Large walls are complex and require careful planning and expertise to install correctly so that they last for many years.
If you don't think that installing a retaining wall is something that you want to take on yourself, contact Permagreen for a free consultation and estimate.
If you want more detailed information about installing modular block retaining walls, I suggest you visit the retaining wall block manufacturer's web site. Most of them have homeowner sites that will allow you to view or download manuals that will take you through a step-by-step process on how to install a retaining wall. One site I find good is by Versa-Lok retaining walls systems. They have good online do it yourself information on retaining walls. If you prefer a different company's block, visit their site or see if your local supplier has information.
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