The strength of an allan block retaining wall

Offering reliable landscaping services to homeowners in British Columbia since 2013

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Sam Maerz

Owner/Site Foreman

Retaining walls are shockingly much more complicated than I ever realized before learning how to build them. The Allan Block Engineering manual for instance is 140 pages long, most of it contains mathematical formulas I don’t have the time to understand, IF I ever could! So, for the purposes of this article, I will keep it simple, and talk about the most frequently occurring retaining wall strength issues and solutions.

Curves are Stronger than straight walls

 Curved retaining walls are stronger than similarly built straight walls. That’s not to say that straight walls aren’t strong enough, just know that a curved wall isn’t weaker, it’s stronger.

Knowing curved walls are stronger may not change the design, but it is a good back pocket bit of knowledge when you need to add some extra strength to a wall if your limited in access and can’t otherwise reinforce a wall.

An added bonus is when curved walls are not out of place, they look way better than a straight wall.

Unfortunately though there is significant extra work on cutting the caps and making them fit good.

A normal straight wall doesn’t require any cuts expect for the last cap, where as a curved wall may need two cuts per block. That’s about 5-10 min per cap in total to fit and place them well.



Plants can add strength

 Gardner’s rejoice! Plants can add strength to a retaining wall.

How to plant near an Allan Block retaining wall:

In most cases you will have 11” deep of soil to plant in (the rest is gravel)

Choose plants that can survive and thrive in 11” or less of soil depth

Be cognizant of excess watering on top of a retaining wall

Plant so that at full maturity the canopy of the plant is not encroaching on the wall

Choose plants that are recommended by a local horticulture who can see a photo of the retaining wall and measurements

Choose plants that do not have an aggressive/damaging root structure


Trees can cause damage to your Retaining wall

 Almost all trees are damaging to retaining walls if planted to close. Ensure at full maturity the canopy of the tree stays a safe distance away from the front or top of a retaining wall.

If you have an existing tree to work around it may not be possible to avoid it completely so using extra gravel in the area can help to “air prune” the roots, and cause the tree to avoid the wall.

Trees are very damaging and in the planning stage of the wall building suitable plans should be made to ensure the retaining wall is not damaged or effected in the future.


Blocks size matters

 Retaining wall block sizes makes a difference in terms of strength and staying power. Allan Block retaining walls are segmented block walls, meaning lots of individual blocks make up a greater structure.

The fewer joints, or independent blocks you use the stronger, and more uniform the wall is.

It is best practice to use the largest block available in a product range for the base course, then on consecutive courses, patterns, or smaller combinations of blocks can be used.


A classic size Allan Block is 18” wide. This is the size of the base course blocks, and usually the blocks that make up the rest of the wall.

There is a half width block available and these are used frequently by homeowners because they are much lighter.

Professionally though they can be used when you want to make a tighter radius curve. Being narrower than the full blocks they curve much tighter. A combination of the large and half blocks can be used, however strictly using the half blocks in most cases is not recommended by professionals.

During general construction it’s advised not to use tiny cuts. Instead cut down several surrounding blocks to ensure the smaller block is as big as possible.  

The bigger the cut block is, the more surface area it has to sit straight, level, and not visually stand out as a tiny cut.

Stairs should be planned out as best as possible to avoid small blocks. This is especially important, as stairs are actually individual walls (they don’t have a 2nd row to hold it in place).

Space For Working And Equipment

Without a doubt the most common issue we face when working on retaining walls is space issues.

This means if its awkward, or we can’t get our equipment in its not going to be as strong of a wall.

This is usually noticed on staircases where equipment wasn’t able to bring in lots of gravel, or compaction had to be done by hand.

Common wall issues are found in corners, stairs, remote areas where equipment couldn’t reach, or anywhere else the wall isn’t built to the same standards as the easily accessed areas.

It’s not impossible by any means to build strong walls with limited access, it just takes longer, and costs more.

Be aware of this, and discuss this in the planning stages, as the contractor will need you to provide the financial assistance so they can build you the wall you deserve and are expecting.