Retaining walls are so much more than beautiful landscape elements. They are workhorses built to restrain the earth behind them. They need to hold up under constant pressure as the force of gravity pushes against them, or else they will fail. Failed walls will sag, crack, bulge, lean, produce large gaps, or collapse. Often times, the only way to fix a failed retaining wall is to tear it out and rebuild it.
The good news is that most retaining wall failures can be avoided. Let’s take a look at 4 preventive measures you can take to help ensure your retaining wall will hold up.
1. Drainage. The lack of proper drainage is the most common reason for retaining wall failure. When water is absorbed into the soil behind a retaining wall and it has no place to go, the pressure behind the wall is increased. If the water continues to build up, it will eventually push the wall out, causing it to bulge or collapse. Installing drain pipes and using a clean, granular rock for both the backfill and the base of the wall will allow water to drain out. The pressure will be released and wall failure will be prevented.
2. Compaction. Poor compaction will eventually cause a retaining wall to shift, producing large gaps within the wall. The soil needs to be compacted once at the point of excavation, again after the gravel base has been added, and each time backfill is added. (Backfill should be added after each course of the wall has been installed). The most effective way to do this is to use a piece of machinery called (you guessed it!) a compactor.
3. Reinforcements. The type of reinforcements needed depends on a number of factors – the soil conditions, the type of material being used to build the wall, and the height of the wall. The most common type of reinforcement for modular block retaining walls and rock retaining walls is geogrid. Geogrid is a mesh-like fabric that helps stabilize the soil and secure the wall. It is normally added every 2 or 3 courses.
A wall built in clay-like soil will need more geogrid than a wall built with sandy soil. Likewise, a tall wall will need geogrid for extra reinforcement, whereas a short wall normally will not. Timber retaining walls require deadmen, which are timber secured perpendicular to the wall. Deadmen helpanchor the wall to the soil it is supporting, increasing stability.
4. Engineering. Certain conditions call for a retaining wall to be professionally engineered before it can be built. Walls exceeding a certain height (usually 3 – 4 feet, depending on soil conditions) should always be engineered. Retaining walls that will need to hold up against additional pressure, such as a wall restraining a parking lot, should be engineered. Local city or county ordinances may also require engineering for retaining walls.
Before You Get Started
Before you get started, do your homework. If you are hiring a retaining wall builder, talk to them about retaining wall failure. Review your contract carefully to make sure all the costs needed to build the wall properly are included. If you are going to do it yourself, be sure to follow the installation instructions. Check into local ordinances for retaining wall requirements. Don’t hesitate to consult with your supplier or a professional retaining wall builder if you need assistance.
When building properly, retaining walls have the potential to last hundreds of years. Investing in the proper materials and taking the time to build it right will mean all the difference between a retaining wall that lasts and a retaining wall that fails.