Segmental Retaining Walls
Segmental Retaining Walls
There are many parts of the nation that are blessed with level or slightly sloping ground. Other parts are hilly and mountainous. If you want to make a hilly area flat, you need retaining walls. This simple building practice has been around for thousands of years. Farmers in southeast Asia have incorporated retaining walls for years to grow rice and other staples on hillsides. The retaining walls allow them to create steps or flat areas as they climb a hillside.
We tend to use retaining walls here in the USA to create level play areas, parking areas or terraces in a yard or garden. They can be as simple as a two foot high stacked rock wall or a highly engineered poured concrete wall 40 or 50 feet tall.
You can build a retaining wall using any solid material. Earth is used as a "retaining wall" or dam to hold back lake water; wooden timbers are used in garden landscaping. Concrete, concrete block and stone are used effectively as common retaining wall materials. The newest and most interesting materials, however, are segmental retaining wall systems. These building materials are individual concrete block masonry units that are made to stack and interlock with one another. They require no mortar to hold them together.
These segmental systems can be used to create a two or three foot wall, or a wall in excess of 25 to 30 feet. They can be installed either by a homeowner or by a professional. However, walls in excess of four feet high should be designed or engineered before installation. Why, you ask? Well, it has something to do with wedges and weight.
The Sliding Wedge
The fill material behind a retaining wall is anything but stable. If it were stable, the wall would be unnecessary! Each backfill material (soil type) has its own angle of declination. This is the angle at which it would normally develop if left to weather. In other words rain and gravity would pull a certain amount of this material down. Retaining wall engineers call this material the sliding wedge. The mass of material gets wider at the top as the height of the wall increases. This means that the higher a retaining wall is, the stronger it must be.
This relationship, however, is not on a one to one basis. In other words, a retaining wall eight feet tall is not just twice as strong as a wall four feet tall. The eight foot wall needs to be four times stronger!
Batter but no Cake.....
Traditional retaining walls and the new segmental walls can be battered. This term refers to a backwards tilt. If you decide to build a retaining wall plumb, you are maximizing the sliding wedge of material behind the wall. If, instead, you tilt the wall back towards the hillside, you reduce the amount of material in the sliding wedge by an amount equal to the degree of tilt.
Segmental wall systems take advantage of this opportunity. In fact, many manufacturers of these products offer you different degrees of batter.
Year Round Material
Segmental wall systems allow year round construction possibilities. We already discussed that mortar is not required between the masonry units. And rarely are thick concrete footers required. The block wall usually rests on a level bed of gravel. These requirements mean that these walls can often be built in the dead of winter. As long as you make sure the ground below the wall is not frozen, you can usually proceed. Traditional materials require special protection so that freezing weather does not harm the poured concrete or mortar between stones or block.
Curves, stepped terraces, serpentine walls, steps, waterfront walls and plain old straight walls are a breeze with the new segmental retaining walls. Each manufacturer makes a wide variety of special interlocking blocks that allow you to make just about any shape you desire. Blocks of different colors are sometimes available. This allows you to incorporate a design or colored bands within a wall system.
Weight - BIG Differences
Not all of the segmental blocks weigh the same. This may impact you if you decide to build a wall yourself. Some wall blocks are heavy because they are solid or nearly solid. Other manufacturers use a hollow block that you fill with gravel as you construct each course.
These hollow block are as strong as solid block. The gravel fill, especially angular gravel, actually helps to interlock the blocks with one another. In addition, the gravel inside the block provide a quick way for water to exit the wall system.
Segmental walls are here to stay. These new mortarless systems can work for you. I suggest you give them serious consideration.