If you have an eroding slope on your property and you're thinking of building a retaining wall, keep in mind that there's a lot more involved than putting up the wall itself. Grading work will likely be required, and some sort of provision for drainage needs to be incorporated in the structure. It can all add up to making retaining walls an expensive proposition, but skipping steps like drainage features are a recipe for the wall to fail at some point in the future.
The Need for Drainage
When the soil behind a retaining wall is saturated from rain, it becomes many times heavier than in dry conditions. Under the extreme weight, there is a risk that the retaining wall could shift or collapse. Rather than build walls with immense foundations and specially engineered features to keep them from being pushed over by the weight of saturated soil, it is much easier to install features to drain away the water. This can take the form of holes in the wall to let the water pass through or drainage features behind the wall that safely carry away water during a rainstorm.
If you look at the large concrete retaining walls used in places like shopping center parking lots, you will notice a series of holes somewhere in the bottom half of the wall. These weep holes contain a small section of drain pipe to allow water to pass through the wall. The weep holes are usually 4 to 6 inches in diameter and placed every 3 to 4 feet, though these numbers can vary considerably depending on the scale of the wall. Sometimes the wall is stained below each hole where dirty water seeps through. Filter fabric should be used to cover the inlet of the drain pipe to filter the dirty water so it does not stain the exterior surface.
Another way to drain the soil behind a retaining wall -- that eliminates the need for weep holes -- is to install a perforated pipe in a bed of gravel behind the wall. The gravel allows the water to percolate easily towards the bottom of the wall where it then enters the holes in the perforated pipe and flows down to the end of the wall. At this point, the pipe can be left open so the water flows out onto the ground, or it can be connected to another drainage feature, if necessary.
Dry Stack Walls
Small walls under 3 feet in height may not require drain holes if the wall itself is not completely impermeable. For example, "dry-stacked" stone walls -- meaning those that are not held together with mortar -- have enough space between each stone to function as informal weep holes. Dry-stack concrete block walls under 3 feet in height should still have a drain behind them because the flat blocks fit together so tightly that water cannot drain through quickly enough during a heavy rain. Filter fabric is needed behind the dry-stack walls, too, to prevent soil from migrating through the cracks.