Why So Many Failing Retaining Walls?
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:
- 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.
- 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.
- 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 help anchor the wall to the soil it is supporting, increasing stability.
- Engineering. Certain conditions call for a retaining wall to be professionally engineered before it can be built. Retaining walls over one metre high must be designed by a qualified structural engineer. The engineer designs the retaining wall so that it is able to resist soil pressures and is a stable structure.
All types of retaining walls require a system to minimise the build up of water pressure exerted upon the retaining wall by the water contained within the soil. This pressure is called 'hydrostatic pressure'.
Minimising hydrostatic pressure can be achieved by carefully controlling the backfill that is placed behind the retaining wall. It must be a coarse, well draining material such as scoria An agricultural drain placed behind the wall within the free draining backfill will help drain the water away to the stormwater drainage system. A series of weep holes in the base of the wall will also minimise the effects of hydrostatic pressure exerted upon the wall 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.