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Should You Fill Retaining Wall Blocks?

Should You Fill Retaining Wall Blocks?

There are over a dozen types of retaining walls and materials you can choose from — including things like railroad ties. However, homeowners prefer retaining wall blocks made of concrete that can either be solid or hollow. So, you might be wondering if you should fill the retaining wall blocks.

Retaining wall blocks should be filled, especially if they’re made of concrete and have hollow cores. You should fill the hollow cores of every retaining wall block with gravel or crushed rock before starting a new layer until you reach the top and install concrete caps.

Not only should you fill retaining wall blocks, but you must also backfill the structure, starting from the foundation or base in the trench. The rest of this article will explain why and how you should fill retaining wall blocks.

Why Should Retaining Wall Blocks Be Filled?

You should fill retaining wall blocks made of concrete with hollow cores for enhanced structural strength, stability, adhesion, and drainage. The integrity of retaining walls, thus longevity in the face of expected and unexpected challenges, partly depends on the infill.

Fill Concrete Retaining Wall Blocks for Enhanced Strength and Stability

Generally, retaining wall blocks made of concrete are solid or have a void. If the void accounts for more than 25% of the gross area or cross-section, the block is classified as hollow. This void may be in specific patterns, such as perforation, split, or partition, among others.  

Hollow concrete blocks are more affordable, lighter, and easier to work with. Also, they provide better insulation and soundproofing. However, you need to fill these voids to improve the structural strength, thus the stability of your entire retaining wall and water resistance. 

Just as steel is used for reinforced concrete, you must use an infill material to occupy the voids within the retaining wall blocks. You may use gravel to fill the hollow concrete blocks. Steel rods are unnecessary for reinforcing residential retaining walls that are only a few feet tall. However, some terrains and larger retaining walls merit stronger reinforcement for hollow concrete blocks.

Infill Improves the Adhesion of Hollow Concrete Blocks on Retaining Walls

You’re probably aware that concrete blocks must be laid in an interlocking pattern for retaining walls. This method is the same as the traditional bricklaying approach. Now, concrete blocks with solid cores have a lot of surface area for each layer to adhere to another. Retaining wall blocks with hollow cores have much less surface area, thus not sufficient contact for adhesion. 

As you fill the void in every hollow concrete block, the infill material forms its own interlock pattern, both vertical and horizontal. 

Plus, the infill material at the bottom and top of the void provides a surface for contact with the concrete blocks below and above. You’ll have much greater bonding between each layer of concrete block, all the way from the base to the top. 

This fundamental of stronger adhesion or bonding applies to both mortar and any concrete or masonry adhesive you may use. Also, the more roughness you have on the bonding surfaces, the better your adhesion will be. Concrete glue or masonry adhesive doesn’t work best on impeccably smooth surfaces. Hence, your filled retaining wall blocks will be more stable. 

Filled Hollow Cores of Concrete Retaining Wall Blocks Improve Drainage

Last but not least, the filled hollow cores of concrete retaining wall blocks enhance the drainage. You may live in a region where freeze and thaw cycles or sufficient rain and water pooling are routine. Filled concrete retaining wall blocks can resist both these natural extremes.

With the voids completely filled with gravel or crushed rock, there’s very little space for water to pool inside the concrete blocks of your retaining wall. Thus, the water can’t freeze and expand or thaw and contract. Also, the infill will compel water from runoff or rain out of the wall, down to the base where you have the perforated drain pipe to direct the flow away from your house. 

How To Fill Retaining Wall Blocks

First, you must choose a reliable material as the infill for your hollow concrete retaining wall blocks. Don’t choose sand, clay, or any material comprising them. Gravel or crushed rock works well. 

You may use concrete infill, too. Avoid materials that have dust or dirt as they can shift, settle, or undergo a structural transformation when exposed to water. 

Here’s a YouTube video to know how comprehensively you should fill every hollow concrete retaining wall block:

Second, you must work on the infill and backfill sequentially. You can’t increase the weight of your retaining wall by adding infill without accounting for it when you lay the backfill. The thumb rule is that a heavier retaining wall needs stronger foundational support, not just at the base or in the ground but also behind it alongside the soil or terrain that it’s supposed to stand against. 

Third, compact the infill for your retaining wall blocks, like you must do to settle and stabilize the backfill. If you have a shortage of infill during the construction, don’t add soil or other materials solely because they’re native to your area. Procure more gravel, crushed rock, or recycled concrete aggregate based on what you choose to fill the remaining hollow concrete blocks. 

Here’s a short but comprehensive YouTube video to help you build a retaining wall with filled blocks:


Retaining walls can last decades, if not centuries, provided you design a flawless plan, choose the right materials and don’t err at any stage of the construction. While this does add a bit of material cost, you can still stay within budget.

On a side note, we wrote an article about the cheapest ways to build retaining walls. Check it out — it may save you even more money.

Aim for perfection when filling the hollow concrete retaining wall blocks, as you’ll get only one shot at it until completion. 

Also, don’t forget to level every block of each layer or course of the retaining wall as you pour the infill material. Minor aberrations at different portions of your retaining wall due to uneven stacking or ineffective compacting of the infill material can lead to evident anomalies later.


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