Retaining wall drainage is a system, or multiple systems, designed to remove water away from behind a retaining wall.
In most cases, a retaining wall drainage system will be made up of at least 1 or multiple methods of removing water safely away from a retaining wall.
It can be preventative, like a drainage swale where the ground is graded to move water away from the wall, clean gravel with void spaces so the water freely drains through it and away, or gravel with a drainage pipe.
Drainage is crucial to a long-lasting retaining wall for multiple reasons:
The first is to reduce the weight of the soil or ground behind the retaining wall. Wet soil is heavy and should be drained to reduce the soils weight as much as possible.
The higher the retaining wall the greater leverage is exerted against it, and the more crucial the drainage system becomes.
Water causes long term damage to any structure, whether that’s erosion, frost heave, or general deterioration to materials or stability of the surrounding soils.
An Allan Block retaining wall does not degrade in water like wood, but would be subject to frost heave, or expanding water if the water is left to freeze in the wintertime.
As water expands in and around the wall it can cause movement of the blocks, and stairs can become uneven, and likely to cause a hazard.
Allan Block retaining walls should be built with clear gravel (Clear gravel is defined as having uniform sizes only. ¾” clear only contains ¾” size rocks).
All gravel used under or behind a retaining wall needs to be compacted to prevent future settling, and to strengthen the wall by locking the block and gravel all together.
It’s important to note that the blocks by themselves are not what retains the soil. Even though they weigh 70lbs each, they are far too light to hold back the soil.
When you combine the weight of the blocks and gravel together you double the weight, and now can retain much more soil.
You combine the weight of the blocks together, by compacting the gravel and blocks together.
This is an often-overlooked step even by long time retaining wall builders.
The purpose of a drainage pipe behind the wall is to provide an outlet for the water to go, as well as providing a dedicated void space that can be cleaned or emptied should the drainage gravel ever get plugged.
Types of Retaining wall drainage pipes
The 2 most common drainage pipes are “Big-O”, and PVC. Both pipes are 4”, are perforated (slits or holes) to allow the water to enter them, and are approved by engineers and municipalities.
Big-O is a slang term for thin walled, corrugated, flexible drain pipe. It’s the lowest cost and the easiest to work with, but crushes easily. When burying Big o with gravel you cannot step on it, and must slowly bury it to prevent crushing. Over time the weight of the gravel, and soil may crush it.
PVC perforated pipe is more commonly used due to its strength, but because its rigid takes much longer to cut, prime and glue any fittings for direction changes.
There is an upgraded pipe called Baughman Tile High Octane. This specialty pipe contains all the very best pros of both other styles, and in B.C is an exclusive pipe of Back 40 Landscaping.
A retaining wall built without a drainage system will at some point show signs of failure. Failure is defined as either structural, or aesthetic.
For Allan Block retaining walls structural failure can be caused by excess water weight pushing a retaining wall out, or over.
This excess water can cause the foundation to erode, and the wall to sink or dip in places. Water movement causes erosion, and if left unchecked will cause varying levels of failure.
Allan Block walls are designed to handle the effects of rain and snow, and should be built with a drainage system to allow them to handle the weather around them and last for decades.
Every kind of retaining wall would benefit from a drainage system, and most certainly ones that are higher than 12” in height, are retaining poor draining clay soil, or have additional weight loads on them, such as a slope above the wall, driveway, or other additional weight loads.
