Poured concrete VS. allan block

Offering reliable landscaping services to homeowners in British Columbia since 2013

Sam Maerz

Sam Maerz

Owner/Site Foreman

About twice a month we are asked what the differences are between a poured concrete and an Allan Block retaining wall. So, in this article I will highlight some key points to consider when comparing these two options.

 

FACTORS

POURED CONCRETE RETAINING WALL

ALLAN BLOCK CONCRETE RETAINING WALL

PRICE

$$$

$$$

COLORS

Grey is standard, custom available

Charcoal, Grey, Silverado, Rocky Mountain, Sandstone

DESIGN

Majority are smooth/flat face, custom available

Vertical, or stepped back, Rough natural looking face

EXCAVATION

Minimum 16” behind and 10” underneath. Requires significant soil disposal.

Minimum 16” behind and 10” underneath. Requires significant soil disposal.

DRAINAGE

Drainage rock behind the wall to reduce the soil weight surcharge on the wall. Drainage underneath the wall to prevent frost heave/settling

Drainage rock behind the wall to reduce the soil weight surcharge on the wall. Drainage underneath the wall to prevent frost heave/settling

MATERIAL

Concrete

Concrete

DURABILITY

Concrete is a lifetime, material, but design and installation are what determines if it will last a lifetime.

Concrete is a lifetime, material, but design and installation are what determines if it will last a lifetime.

STRENGTH

Rigid and Strong

Flexible and Strong

CRACKING

Will most likely crack and then fail at that point.

Independent blocks that will move as needed. Does not crack, and like a tree in the wind its flexibility is its strength

ab-retaining-wall

It’s a common misconception that a poured concrete retaining wall will be faster to pour, and therefore cheaper to build compared to an Allan Block retaining wall.

Price

The cost of these 2 walls is pretty close when compared apples to apples. However, what often tends to happen is a simple poured wall 6-8” thick is compared to an Allan Block wall that has been installed based on Allan Block engineering and contractor training.

The major differences are a poured wall is straight up and down and has no mechanical advantage (it doesn’t lean into the bank its retaining). Poured walls typically don’t have a lot of drainage behind them, so the soil is wet and heavy behind the wall (this puts added pressure on the wall). And from a design stand point most concrete poured walls are standard grey in color and have a smooth flat face.

Allan Block has a setback to it so it leans into the soil its retaining, contains a drainage zone of 16” behind it so the soil its retaining is as light as possible, comes in multiple colors, and has a natural rough face.

If a Poured concrete wall, and an Allan Block wall are built the same they will be very similar in price. If they are not built the same, the poured concrete wall will fail much sooner.

 

Colors

A poured concrete wall will typically be grey in color, but for an additional cost, custom colors are available.

Allan Blocks are available in 5 main colors. Charcoal, Grey, Silverado, Rocky Mountain, and Sandstone. Other colors are available, but vary by region.

 

Design

The holy grail in landscape design incorporates curves, and elevation changes. Squares and straight lines are boring, and generally not desired.

Ultimately a poured retaining wall has the most customizable design in terms of color, shape, etc. There really isn’t anything that you can’t do with a poured concrete wall as you can build a mold in any shape, and texture. HOWEVER, this is at a cost, and up to the contractor’s skill level.

In reality what most poured walls look like is they have a smooth face, and are much less likely to incorporate gentle flowing curves as the molds are made out of wood. Forming curved molds takes a lot of extra time, and stamping a texture onto the face of a poured retaining wall is very rare.

Allan Blocks, while they are limited in colors, can handle curves very easily, and take very little extra time to build. They have a natural “split” face, and come in difference sizes so even pattern walls can be built. The added depth of a rough split face blends into most landscapes more naturally when compared to a poured concrete wall.

 

 

 

 

Excavation

Both walls should require the same size of excavation.

A poured wall will need an area excavated behind it to form up the heel and Tow footing for the foundation (think upside down T), as well as for the drainage zone of the wall.

 

An Allan Block wall doesn’t have the same size foundation footing requirements as it instead leans into the bank for its mechanical advantage. It does however require the same amount of drainage behind the wall.

 

Drainage

Drainage is easily one, if not the most important component to EVERY style of retaining wall. Retaining walls are installed to hold back soil. In the Fraser Valley this soil is wet, and heavy the vast majority of the year. So, if this soil is not drained it will put an incredible amount of weight surcharge on the wall. This weight year after year will eventually cause a wall to fail. Every wall is susceptible to this, whether it’s a poured concrete wall, an Allan Block wall, or even a Timber retaining wall.

As well as wanting to reduce the weight of the soil a retaining wall has to retain, you also want to keep water as far away as possible from the wall to eliminate freezing water expansion during the winter.

Poured concrete walls are especially suspectable to cracking, and any additional water that freezes in and around the wall may be too much for them to handle.

 Frost heave is a phenomenon that occurs when water freezes, and expands to push something out of the ground, in this case a concrete retaining wall.

Allan Block can be affected by this, but inherently they handle it much better. They are installed on a base of gravel that allows water to freely flow away. They are also individual blocks so they do not crack, instead they have the ability to move as the seasons change.

 

 

Material

Both a poured concrete wall and an Allan Block wall are made of concrete. While these mixes can be different, in all reality there are not a lot of noticeable differences in the actual hardness, and water handling capabilities of either. Both will physically last for well over a lifetime outside in the elements.

 

Durability

Because Concrete is a lifetime product the factors that influence how long a poured concrete wall or Allan Block wall will last basically come down to design and installation.

Most retaining walls over 4’ in height will require a permit and engineering. In these cases, the engineer will design and guarantee the installation. This ensures both styles of concrete walls are going to last a lifetime.

Walls under 4’, or illegally constructed walls without an engineer are where you want to ensure that the physical/mechanical design of a poured wall is sufficient.

The biggest issue we see with poured concrete retaining walls is the homeowner, or the builder, are not expert retaining wall builders. Hard concrete that looks thick does not equal a structurally sounds design.

If you’re having an Allan Block wall built its easy-to-follow Allan Block installation guides, and/or hire an Allan Block certified contractor. Every Allan Block wall that is built following “Best practises” is engineered, because the “Best practises” are based on engineering!

 

 

Strength

The strength of a poured wall is described as “Rigid and Strong”. On the surface this may sounds great, but one of their weaknesses is in their rigid design. EVERY poured concrete slab will crack. This is why patios, driveways, and sidewalks have joints in them. Concrete will crack, it’s only a matter of time. For a poured concrete retaining wall at some point, it will crack.

Most likely this will be 5-25 years later, unless significant reinforcement is done. Rebar, and special concrete mixes can make a huge difference, so consider hiring an engineer if you’re unsure what reinforcement should be included.

Allan Blocks greatest strength is in its “Flexible and Strong” design. Instead of cracking the individual blocks can move with any ground settling, or frost heave. The blocks are small enough, and hollow so they do not crack themselves. Just like a willow tree that bends in the wind vs cracking, an Allan Block retaining wall can live with nature and adjust.

Cracking

In general, every poured concrete retaining wall will crack and fail. Now, to be really clear if its reinforced well enough, and built strong enough it won’t crack or fail. In reality though most residential poured walls are built without engineering, and will crack. When they crack, they will fail at that point. If you want to prove this for yourself drive around an old neighbourhood that used poured concrete retaining walls. You will rarely find one that has not cracked.

Allan Blocks themselves do not crack. Instead, the blocks shift and move with the ground, and change of season. If the wall is constructed to Allan Block best practises it will last a lifetime, however, if it is not built correctly, it will ultimately suffer the same failure as a poured wall.

 

 

 

Retaining walls: Poured Concrete vs. Allan Block

Because both retaining walls are built using concrete the winner between a poured concrete wall vs. Allan Block comes down to design and installation.

A properly engineered poured concrete wall is better than a poorly built Allan Block wall,

and

a Properly built Allan Block wall is better than a poorly designed Poured Concrete wall.

 

When in doubt hire a qualified company and or engineer!