What Is Retaining Wall Drainage?
Retaining wall drainage is a system, or multiple systems, designed to remove water away from behind a retaining wall.
It is can be preventative, like a drainage swale where the ground is graded to move water away from the wall, it can be clean gravel with void spaces so the water freely drains through it and away, or it can be gravel with a drainage pipe.
In most cases, a retaining wall drainage system will be made up at least 1 or multiple methods of removing the water safely away from the retaining wall.
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 much heavier, and therefore should be drained to reduce the soil 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.
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.
In cases where a wood retaining wall is built, water should be removed from behind and underneath the wall, so that the wood does not prematurely rot out, or fall over from an unnecessary weight surcharge.
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, stairs can become uneven, and even become a tripping hazard.
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.
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.
A high rate of compaction is achieved simply by placing the rocks on the ground (as long as they are contained).
If you were to use different sizes of gravel, the rocks would have to rotate and move around to find the tightest fit (compacted).
This takes longer to achieve full compaction, is harder to measure as you can’t visually see if its compacted, and puts the wall at risk of settling as it further compacts over the years.
The specifications on the type of approved gravel are very clear to mention that you do not want to use a gravel with fines, or “minus” material.
This type of gravel requires a lot of compaction, and when compacted is very slow to drain water through it.
The fines can plug up any drainage pipes underground, and in general will not drain water away from the wall which is what you want.
Clear gravel is angular in nature, and locks in place. Round rock does not lock together, and is not a suitable gravel to use under or behind your retaining wall.
Clear gravel can be any size between .5” – 1.25”. The most common size used in the Fraser Valley is ¾”. Because ¾” clear gravel is used so much its readily available, and competitively priced.
Drainage zones underneath and behind an Allan Block wall will vary on application. However, a typical or minimum number is 6” of ¾” clear gravel below the wall, and 12” behind the wall.
The typical or minimum amount of gravel to use is a starting point, and can quickly change by specific site conditions. Poor draining onsite soils should prompt you to increase the drainage zone, or any other weight surcharge like a shed, slope above the wall, parked vehicle etc.A pro tip is to consider using gravel much further behind the retaining wall.
On engineered walls you will have to excavate much further behind the wall to place the geogrid.
While you can use soil as backfill on top of the geogrid, you may want to use gravel.
The backfill material has to be compacted, and soil is virtually impossible to adequately compact in a timely manner, or if its wet.
Make sure to plan accordingly prior to the project begins, as this is a huge change to the scope of work in terms of excavation, soil disposal, gravel purchasing, and importing.
Placing drainage gravel under or behind a retaining wall might seem not worth talking about, but considering how much gravel is needed, it quickly becomes a logical challenge to do right.
This will be covered more in greater detail later, but importing enough gravel requires special equipment due to the weight and volume of the gravel required. You can’t simply use a wheelbarrow.
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 weight 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. Yes, you run the compactor on top of the blocks, just be sure to use a narrow smaller compactor.
This is an often-overlooked step even by long time retaining wall builders.
Make a minimum of 2 passes on top of the blocks, and extend all the way towards the back of the cut.
All compactors have specific depths of gravel they can compact. Smaller 100-200lb compactors can do about 6-8” of gravel, while larger 1000lb compactors can do up to 24” thick. Within 3’ of a retaining wall you should use a small 100-200lb compactor. Beyond 3’ you can and should use a larger heavier compactor.
Placing drainage gravel under or behind a retaining wall might seem not worth talking about, but considering how much gravel is needed it quickly becomes a logical challenge to do right.
We will cover this more in greater detail later, but importing enough gravel requires special equipment due to the weight and volume of the gravel required. You can’t simply use a wheelbarrow.
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 weight 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.
Yes, you run the compactor on top of the blocks, just be sure to use a narrow smaller compactor.
This is an often-overlooked step even by long time retaining wall builders. Make a minimum of 2 passes on top of the blocks, and extend all the way towards the back of the cut.
All compactors have specific depths of gravel they can compact. Smaller 100-200lb compactors can do about 6-8” of gravel, while larger 1000lb compactors can do up to 24” thick.
Within 3’ of a retaining wall you should use a small 100-200lb compactor. Beyond 3’ you can and should use a larger heavier compactor.
The purpose of a drainage pipe behind the wall is to provide an outlet for the water to go, as well as a dedicated void space that can be cleaned or emptied should the drainage gravel ever get plugged. Just be sure to include clean outs every 50’, and discharge the pipe to daylight.
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 approved by engineers and municipalities.
Big-O is a slang term for thin walled, corrugated, flexible drain pipe. It’s the lowest cost, easiest to work with, but crushes easily.
When burying 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.
It’s much more expensive than Big-O, and during the Covid Pandemic was 4x the normal cost, IF you could find it. It’s more readily available as of January 2023, but has doubled in its normal cost.
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.
Installing a drainage system, whether that’s just gravel, or gravel and a pipe, is by far the hardest part of building a retaining wall.
Excavation of the “extra” soil, the 12” minimum creates a lot of digging, moving it to the front yard in most cases, trucking, and paying to dispose of it. Then you must buy, truck, and move the same amount of gravel back in its place.
Because of the cost, labor, and knowledge of why it’s important, its very frequently overlooked, skipped, or missed.
I’ll give a quick example to illustrate the labor and cost involved with installing a drainage zone.
Retaining wall 3’ high, 60’ long.
Without a drainage zone:
10.11 yds3 of soil removal
No gravel importing
Total: 10.11 yds3 (61 wheelbarrows, 1 full size dump truck, or 10.5 pick-up trucks)
With a drainage zone:
Cost of pipe, and labor to instal
20.23 yds3 of soil removal
10.11 yds of gravel imported
Total: 30.34 yds3 (182 wheelbarrows, 2.5 full size dump truck, or 30.5 pick-up trucks)
It’s completely understandable that if your using wheelbarrows at some point you’re going to want to cut back or eliminate the drainage zone, especially considering retaining walls are always needed for elevation changes (you’re not wheelbarrowing uphill for long).
We can say with all certainty that a retaining wall built without a drainage system will at some point show signs of failure. Failure is defined as either structural, or aesthetically.
For Allan Block retaining walls structural failure can be caused by excess water weight pushing a retaining wall out, or over.
It 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.
For a wood retaining wall excess water will present the same weight issues, but as well will cause it to rot out much faster than it normally would if there was adequate drainage.
Allan Block walls can aesthetically fail when water has caused blocks to open up, or separate. This can happen during winter when water freezes and expands. Blocks may go back to normal, or they may permanently stay out of place.
Often time these gaps will then start leaking out dirt and be a constant eye sore. While not structurally deficient, a retaining wall that is moving will not perform as a focal point that it did, or should be.
Straight, true retaining walls are huge focal points. They are the holy grail in landscape design because they have height, depth, and mass.
Maximize your investment by getting all the benefits you deserve. A strong, beautiful lifetime lasting retaining wall.