Introduction to Retaining Walls
First, a retaining wall is a structure that holds back, or “retains,” lateral faces of dirt, water, or other materials. Retaining walls are used to prevent erosion or collapse of higher level ground onto a certain building, structure, or general area. The earth that is retained is often referred to as backfill. The word backfill is also used as a verb to describe placing more dirt behind a retaining wall to make the ground more level.
Retaining walls can be the answer if you are blessed (or cursed) to a contoured lot. They can turn a steep slope into a terraced yard, creating a stair step effect instead of a slope.
Retaining walls can be constructed from many different materials using many different techniques. There are several factors that influence which choice will be the best choice for each situation, including cost, height of the wall, soil characteristics, and ground water conditions.
Generally, most municipalities require that any retaining wall over four feet in height be engineered or approved by a licensed engineer. It is important to check local building codes before starting any retaining wall projects, even if it is below four feet tall. After all, retaining walls should be structurally sound before there is any thought of aesthetic appeal. The retaining wall must be able to stand up to the pressure exerted by the backfill, also called lateral earth pressure. This lateral earth pressure is a majority of the force that the retaining wall will need to bear. Lateral earth pressure is determined by studying the weight of the soil unit, slope of the soil, soil type (for plasticity and adhesion purposes with clay soil types), and ground water levels. Engineers do many calculations with these factors to determine what design to implement with the wall. To homeowners and contractors that do not want to go through engineering steps, it is generally a good rule to overdo it. Many structural methods to build retaining walls with withhold earth, at least temporarily, with a height up to four feet, are adequate, which is why most local building codes require engineering if the wall will be over four feet tall.
However, even methods that may seem adequate for retaining earth can fail if they are not built properly. Many times, it is elements below grade that can make (or break) a retaining wall. Problems including bowing, slanting, or cracking are evidence of retaining wall failure because it was not successful in retaining the dirt. Cutting costs with a bad contractor, insufficient materials, or neglecting the below grade elements can turn out to be more expensive over time if replacement or repair is necessary often or if collapse of earth causes damage to nearby structures. For example, erosion can cause foundation issues if dirt erodes from under the foundation. This is just one example how a retaining wall failure can cause expensive problems aside from the cost to repair or replace the retaining wall.
Another aspect that is often neglected or underestimated that can cause failure of a retaining wall is drainage. Inadequate drainage of retaining walls can mean that water is trapped behind the wall, adding to the weight that the wall must hold back. Usually weep holes and/or drainage pipes are used to flush rain water and ground water from behind the wall. It is important that the drainage system, whatever it be, have a filter barrier to keep dirt from clogging the drain or weep hole.
Options for Retaining Wall Materials
· Railroad Crossties – Railroad crossties, also shortened to railroad ties, can be used as a retaining wall material; however, as with most every retaining wall material, it must be installed correctly in order to be a solution. For example, most railroad tie retaining walls will need a substantial footer to tie the crossties to in order to keep the bottom from sliding out. Also, there needs to be perpendicular members placed into the ground behind the wall to keep the wall from overturning. These members, called deadmen, should be at least 1.5 times as long as the wall is high and should be placed at minimum every 6 feet or so. Because railroad ties are wood (even though they are treated most of the time), there is a risk that they may rot over time or be subject to termite infestation, which can lead to a weak wall likely to bulge or break. And, even though there are areas for water to slowly seep though, it is still a good idea to have a filtered drain or weep holes designed into the wall.
· Treated Wood – Treated wood retaining walls offer many of the same risks as retaining walls because over time it is susceptible to rot and termite infestation. It is a labor intensive construction, but it is usually lightweight and decently strong if built correctly. It is recommended to build a concrete footer to attach the wall to in order to keep it from shifting. Construction usually requires a clear area to be backfilled after the wall is built, so it is not always the best material to use if you are replacing an existing retaining wall. This wall still needs weep holes or drains designed into the wall. Standing water behind the wood will only increase risk of rot.
· Concrete – Concrete is a great material to use for retaining walls. Of course, there is a right and a wrong way to build a concrete retaining wall, which makes choosing your contractor a big job. There are many different designs that can lead to a successful concrete retaining wall, but one thing is for sure: there needs to be a footer beneath the wall to ensure success. Concrete is one of the most durable man-made products known to man. Most concrete poured today is engineered to last for over 50 years. Plus, concrete offers many decorative options as well. Aside from a plain, white concrete retaining wall, you can also pour a colored concrete retaining wall, stain a retaining wall after it has cured, pour into a patterned form, apply a vertical stamp material and pattern after poured, etc. Sealing the concrete wall can lead to an easy-to-maintain surface that rinses free of most dirt and grime.
· Blocks or Paver Stones – Engineered blocks that stack together to make a wall must be as simple as building with Legos, right? Nope. These retaining walls should also have a significant footer in order to support not only the weight of the dirt but also the weight of the wall. These blocks can be a unique look, but it may even be necessary to place a concrete retaining wall behind the blocks if there immense lateral earth pressure. In such a case, the concrete would be the structural element, using the blocks as a visual feature.
· Rock or Brick – One of the favorite looks for a retaining wall is natural stone or brick. These materials usually look more permanent and traditionally classy. However, just as we have seen with the other materials, it is necessary to have a concrete footer below grade to carry the weight and ensure success of the retaining wall. Like architectural blocks or paver systems, it may be necessary to back a masonry retaining wall with a concrete retaining wall for structural support.
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