10 problems with Allan Block concrete retaining walls you can’t afford not to know

Offering reliable landscaping services to homeowners in British Columbia since 2013

Sam Maerz

Sam Maerz

Owner/Site Foreman

Retaining walls are a great focal point for any outdoor space, and can be built using a variety of methods and materials. The thing that homeowners need to keep in mind, is that the materials and methods used for a retaining wall build are extremely important. Retaining walls that are not built correctly, or with the proper materials, will not last. Retaining walls do the important job of keeping soil retained, and when they fail can damage your surrounding outdoor areas, walkways, flower beds, and whatever else your landscaping consists of. 

What are the biggest problems with Allan Block concrete retaining walls?

#1 Allan Block walls are usually not built correctly

This is the FIRST problem with retaining walls. THEY ARE NOT BUILT CORRECTLY. How can we be sure that retaining walls are not built correctly? Just take a look at any neighbourhood and you will see dozens of walls on the verge of collapsing, or that have already collapsed. 

Almost every contractor builds their retaining wall differently. The methods that they use to build their retaining walls are based on speed, ease, cost, and general practices.  

Let’s say that the retaining wall that is being built for your outdoor space is made out of Allan Block. Allan Block concrete retaining wall blocks have one method, with specific site variations, that need to be used when building a retaining wall. 

If the Commercial Installation Manual is calling for reinforcement it needs to be done regardless of what the site looks like. 

 

If a retaining wall is not built correctly what happens?

Retaining walls that are not built correctly  have varying levels of failure, from minor separation between the blocks, to catastrophic failure where large portions of the wall fall over.

Allan block retaining  walls are a mortarless system, that interlock in a dry stacking pattern. They are flexible in nature, and unlike a solid concrete structure will not form cracks.

However poor retaining wall design, installation method, or any other cost saving method will result in failure on some level.

 Anything outside of Allan Blocks allowable movement for a flexible structure is a failure.

When a homeowner is spending 10s of thousands of dollars there should be a strict standard for quality.

Retaining walls are a big investment when built to best practices. Not only should they do their job of holding back hundreds of thousands of pounds of force ( retained soil), they should be straight, true, and an aesthetically pleasing center piece in your outdoor space and landscaping backdrop.

#2 Wrong design for your site – Lack of education

The second problem with retaining walls is they are not designed correctly for the application and site they are installed on. This is certainly not as much of an issue when a professnal engineer is involved, however you still want an experienced installer.

The majority of poor designs come from a lack of education, and cost cutting. ( Such as not installing proper drainage, or using enough gravel behind the retaining wall) The technical installation manual found HERE is a lengthy document that does a very good job of explaining the best practices, and Alan Blocks Contractor courses do an excellent job bringing that document and the field work together in plain English.

An example of this would be a site with limited room behind the wall for excavation.

It could be for many reasons, but to keep it simple we will say that you need a wall on the edge of your property between your neighbors. Taller walls require Geogrid going back 60-75% of the wall height. A 6’ high wall would require an excavation site of 3.6’ behind the wall for the lower grids, and 4.5’ for the last top grid.

In order to do this an installer / homeowner would have 3 commonly known choices.

1: Ask their neighbor to allow excavation of their property, which likely isn’t going to happen. Excavating 4 dump truck loads of soil for a 75’ long wall is going to create a scenario they probably won’t have any interest in dealing with. And more importantly structural components of YOUR wall shouldn’t be on THEIR property.

2: Move the wall inside the property line 4.5’, which with the cost of land also likely isn’t going to happen.

3: Cheat the required excavation, and fallback on previous jobs where this was done, and so far, may have gotten away with.

Let’s focus on option 1 & 2, because cheating the wall should never be an option. Both  options 1 & 2  are acceptable building practices. They allow us to build a strong wall capable of retaining the soil that in this example can be as much as 1,350,000 lbs. (3000lbs/sq ft).

1: Excavating into your neighbors’ yard may be suitable if they are onboard

Perhaps you both want the retaining wall, there is no finished landscaping to worry about in the area, there is either no fence, or an old one that should be replaced anyway, or generally speaking you have a great relationship with them, and they want to allow it.

 

2: Retaining walls are usually used to create more flat usable space

In nature elevation changes happen with slopes, and distance.

Giving up 4.5’ of yard space is usually not an ideal solution either. Sure, you can make a flower bed or  garden bed on the top, but at 6’ high that’s really impractical to maintain.

This solution may be a good fit for someone who has a large property, however not for the majority of homes in subdivisions.

This really only leaves homeowners with one option approved by Alan Block. It’s called No-Fines Concrete Backfill.

The basic principles are:

Recognize you need a solution for tight access areas where large open excavation behind the wall is not practical or the best option.

Confirm No-Fines Concrete Backfill is an appropriate solution.

Install the wall in accordance to the installation manual instructions. BASIC instructions below.

  • Mix it yourself as concrete companies don’t make it (this wall will need 20 cubic yards – 2 concrete trucks)
  • Cut at least 1 wing of the wall block off to help secure the block face to the concrete backfill. 450 cuts
  • Mix and pour every 2 rows of block to ensure all voids are filled. Fill the hollow core block and a minimum 12” behind.

Now this all sounds like a fantastic solution, and for structural integrity of the wall it is.

But it comes at a cost:

Cutting every single block adds a minimum of 450 cuts to the project.

Because the concrete is added after every 2 rows this means that even if a concrete truck could deliver it you wouldn’t be able to build the wall at the rate of which the concrete was pumped into the backfill area.

This leaves you to mix 20 cubic yards in small batches. A process that adds considerable labor costs, and time.

The two positives however are you need less space and excavation to build the wall, and more importantly in space restricted areas this is the only solution.

#3 Price of a concrete Allan Block retaining wall

A retaining wall that is 75’ long by 6’ high – 450 sq ft will cost in almost every case less than $50,000. Some variables to the cost include: access issues to the site, time of year, if the excavated soil has to be hauled and disposed of, method of construction, permits, design engineering, and possibly geogrid soil engineering.

So why is price a problem? Building a proper retaining wall takes time, the right materials, and the right method. Most homeowners shop around and choose a contractor based on the price of a wall because they know that they need a wall for their outdoor space, but are not comfortable allotting their money towards it. While this is understandable, a retaining wall that is not build correctly WILL fail, and will cause more work and money to fix in the long run. 

For a more in- depth look at what retaining walls costs click here. 

#4 Cheating the excavation area – Limiting the MASS of your wall

Our 4th problem on this list is Hardscape Retaining Walls require a tremendous amount of excavation and in most cases soil disposal.

Geogrid is used to combine the weight of the blocks, gravel and soil behind the wall.

This large amount of mass is what is actually resisting the load on the wall. It should almost NEVER be just the retaining wall blocks holding the weight load.

The weight load could be as simple as soil, a parking spot, or a road. Each scenario requires a different amount of load bearing capacity.

In general, 60% of the wall height is used to determine how far back needs to be excavated to install the Geogrid.

The picture shows Geogrid locking all of the mass of the wall, BLOCKS, GRAVEL and SOIL together.

geogrid-retaining-wall

#5 Unsuitable backfill material – What to backfill a retaining wall with?

Problem number 5 has a direct correlation to #4 (excavating and soil disposal) the majority of the soil that is hauled offsite needs to be replaced with Gravel.

This is why so many contractors cheat the excavation zone, and use UNSUITABLE BACKFILL MATERIAL. (Like using soil). 

If 2 loads of soil are hauled out, and 2 loads of gravel are hauled in, this quadruples the backfill material costs when compared to not removing soil at all which is by far the most common INCORRECT method.

When your comparing proposals this will be very apparent in the pricing.

We tend to take retaining walls for granted, and expect them to stand forever because they are made out of concrete.

Concrete is a great building product, but if we expect them to last, we need to do what we can to ensure they don’t fail. ALMOST EVERY failed retaining wall is because of the lack of gravel behind the wall.

Allan Blocks requires a minimum of 16” of clear draining rock, OR 12” of No Fines Concrete directly behind the wall, AND prefers the infill soil area shown in this picture to be a compactable material, such as ¾” Minus or road-base.

You will almost never find these 2 types of gravel behind your wall without hiring a qualified contractor.

If your wall is not built using proper backfill material you will have a problem with your retaining wall.

#6 No drainage system – Does my retaining wall need a drain?

We all know how much water can affect the weight of something. Multiply that over the length of your wall, for several feet behind your wall, for its entire life, it’s no wonder why you should want a proper drainage system behind your wall.

If you have a small wall, and Geogrid is NOT required than you really only need 1 pipe. This is called the Toe drain in the photo.

If you have a larger wall and Geogrid is required than you will need two drains, the Toe drain, and the Heel drain.

retaining-wall-drain

Common problems with retaining wall drainage are that there either no drains at all, pipes are not protected from migration of fine materials (think clay soil plugging up the pipe), or the drains don’t drain to a good location.

The best method is to connect directly to your existing drainage system or storm connection.

Having this water pool in your yard somewhere will cause you long term problems.

Redirect the water away from your wall, and get it off your property or at least to an acceptable designed area.

Remember water freezes in the wintertime, and will cause movement in your wall if it’s not drained away

#7 Filter fabric – Do retaining walls need filter fabric?

Without a doubt the most misinformed topic with retaining walls is filter fabric.

Filter fabric does just like its name suggests, it filters something.

There are 2 main types:

The first is a fabric that claims to allow water to pass through, and the 2nd is a tested fabric that actually does.

It is not uncommon for a filter fabric to claim to have a flow rate of 100 gal/min/ft. when they may not even allow 5 gal/min/ft.

In a typical scenario filter fabric will only be used at the top section of the wall to filter the 11” of soil placed on top of the wall.

This keeps soil from migrating down through the clear drain rock directly behind the wall, and equally as important it allows any water to pass through into the drainage zone.

In the diagram you will notice the fabric on the 2nd row from the top extending back past the midway point of the backfill point.

reinforced-retaining-wall

Using an untested fabric will create a load surcharge on top of the wall where it has the greatest leverage to overturn the wall.

There are exceptions to when you would use filter fabric in other places other than the top of the wall:

Lakeshore scenarios in which you need to control erosion of your base material, or to encompass your drainage system if not using a clear material to prevent fines from entering and clogging the pipe.

#8 Quality of work – How hard is it to build a retaining wall?

Most walls are not built correctly whether from contractors cutting corners by not using enough material or not following proper specifications . To build a structural retaining wall takes considerable effort, and skill.

To build a reinforced retaining wall that will last, it takes the proper hand tools, gas powered concrete saw, laser level for perfect accuracy, string lines for straight and true walls, and the combined construction experience to put all this together and to address potential issues before they become a problem.

There is also a fitness requirement, ability to handle stress, and working in the elements. As well as a skill to run heavy equipment: Excavators, skid steers, dump trucks etc.

#9 Upkeep – What are maintenance requirements of a retaining wall?

Allan Block walls have relatively little yearly maintenance, however as with any excavated area some settling is likely to take place over the first year or two.

It will be minor if properly compacted when the area is backfilled.

Should there be any unusual or excessive settling these areas should be corrected. Low areas will allow water to pool, and could make the problem worse over time.

Correcting grade issues early will ensure you get a lifetime of performance from your Allan Block wall.

After construction It is important to finish the landscaping around your wall.

Vegetation, whether that’s grass, or plants is a great way to control erosion, direct water where you want it, and generally sturdy up the soil surrounding your wall.

Weeds or vegetation growing in your wall are possibilities, and if caught early are easy to pull before they establish. Otherwise spraying them with a roundup or selective herbicide just like your lawn will be effective if done annually.

Annual spring inspection of the wall and surrounding area should alert you to anything out of the ordinary.

If a wall is not properly installed, or an adequate drainage system was not installed you may have some movement in the wall.

Early recognition or hiring a qualified contractor can correct or prevent this from happening in the first place.

#10 DIY – Should I build my own retaining wall?

Retaining walls are a large investment built to Allan Blocks specifications, and at least double the cost when they are not.

Oftentimes homeowners have given it their all, used up a summer of holidays, stressed themselves, and each other, spent valuable money they couldn’t afford to waste, and then had to call for help.

It’s always the base material and first course of wall block that has to be fixed.

This means every ounce of effort they gave has to be taken out, and redone.

 

So, in these cases, NO they should not have tried to build their own wall.

If you think about it you really only get the advantages of a concrete retaining wall if it’s built well:

  • Lifetime durability
  • Increased amount of flat usable land
  • Focal point in any landscaping backdrop
  • Higher resale value if your Realtor & Home Inspector are in the know
  • And of course peace of mind

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