How to Build an Allan Block Retaining Wall: A Step-by-Step Guide

A retaining wall can serve both functional and aesthetic purposes, whether you’re looking to manage sloped areas or add dimension to your backyard. Among the best options for DIYers and professionals alike is the Allan Block retaining wall system. Known for its durability, flexibility, and clean design, Allan Block offers a simple yet effective way to build a retaining wall that stands the test of time.

In this guide, we’ll walk you through the essential steps to successfully build an Allan Block retaining wall. Whether you’re an experienced DIY landscaper or tackling your first major project, these practical tips will help you create a sturdy, visually appealing wall that complements your outdoor space.

Why Choose an Allan Block Retaining Wall?

Before we dive into the construction process, it’s important to understand why an Allan Block retaining wall is a great choice for your landscape. Allan Block products are designed with versatility and durability in mind. They’re easy to install and offer a professional finish without the need for heavy machinery or specialized skills.

Benefits of Allan Block Retaining Walls:

• Durability: Made from high-strength concrete, Allan Block walls are built to last, even in extreme climates.
• Versatility: Whether you’re constructing a small garden wall or a large retaining structure, Allan Block offers a variety of block sizes and shapes to fit your needs.
• Ease of Installation: The interlocking block system simplifies the installation process, making it manageable for DIY projects.

For homeowners in Edmonton, Canada, Allan Block walls are especially useful due to their resilience against the region’s freeze-thaw cycles, ensuring your wall stays intact for years to come.

Step 1: Plan Your Allan Block Retaining Wall

The first step in building an Allan Block retaining wall is planning. Determine the height of your wall and what you’re retaining, as this will dictate the materials and preparation needed.

• Wall Height: For walls taller than 3-4 feet, geogrid reinforcement is required. Walls shorter than this can often be built without additional reinforcement.
• Excavation Requirements: For taller walls, you’ll need to excavate several feet behind the wall to accommodate geogrid and drainage systems. For smaller walls, excavation can be more limited.

Pro Tip:

For every 8 inches of wall height, you’ll need to bury 1 inch of the first row of blocks. This ensures stability, especially during Edmonton’s freeze-thaw cycles.

Step 2: Excavate and Clear the Area

Start by clearing the area of any organic material, such as grass, roots, and debris. Next, excavate a trench for the foundation of your retaining wall. The depth of the trench should be approximately 6-8 inches plus 1 inch for every 8 inches of wall height.

• Width: The width of your trench should be at least twice the width of the Allan Blocks, ensuring a stable foundation.
• Excavation Depth: For walls over 3 feet tall, extend the excavation area behind the wall by 60% of the wall’s total height.

Step 3: Prepare the Base

A strong foundation is essential for building a lasting Allan Block retaining wall. After excavation, it’s time to prepare the base.

1. Add Geotextile Fabric: To reduce frost heaving, lay a geotextile or commercial-grade landscape fabric at the bottom of the trench.
2. Compact Road Crush: Add at least 6 inches of compacted road crush to create a solid base. For taller walls, increase the thickness of the base. Use a compactor to compact the material in 1-inch lifts.
3. Ensure Levelness: Use screed poles to ensure that your base is perfectly level. This step is crucial for the stability of your wall.

Pro Tip:

Overbuild your base slightly to allow for adjustments during installation. This will make it easier to correct any slight deviations.

Step 4: Install the First Course of Allan Blocks

With your base prepared, it’s time to lay the first course of Allan Blocks. Start by placing the blocks in the trench, ensuring that each one is level. For straight walls, use a string line as a guide to maintain alignment.

• Pressure Placement: Apply even pressure when setting each block into place. Avoid dropping the blocks, as this can disturb the level foundation.
• For Curved Walls: Remove the tabs from the back of the blocks to allow for tighter curves.

After laying the first course, fill the blocks with drain rock (14-20mm construction rock). Avoid placing any backfill behind the wall at this stage.

Step 5: Install Geogrid (If Required)

For walls taller than 3-4 feet, geogrid reinforcement is necessary to maintain stability. Geogrid is a synthetic mesh that strengthens the wall by providing extra support against lateral earth pressure.

1. Placement: After placing the first row of blocks, install the geogrid across the wall, extending it behind the wall by 60% of its height.
2. Ensure Flatness: Ensure the geogrid lies flat and is free from any folds or wrinkles.
3. Backfill: Place and compact gravel up to 1 foot behind the wall after the geogrid is installed. Compact each layer as you build to prevent settling.

Pro Tip:

For taller walls, geogrid should be installed on every second layer (e.g., rows 1, 3, 5, etc.) to ensure stability.

Step 6: Build Up the Wall and Backfill

As you build each additional course, stagger the blocks so that the seams between them do not align. This improves the structural integrity of the wall.

1. Offset Each Course: Offset the blocks by half a block from the previous row for a more secure structure.
2. Backfill and Compact: After placing each row, backfill the area behind the wall with washed rock and compact the material to avoid settling.
3. Weeping Tile Installation: Install a socked and perforated weeping tile along the slope behind the wall to ensure proper drainage. Proper drainage helps prevent water buildup, which can cause the wall to shift or crack, especially during Edmonton’s freeze-thaw cycles.

Step 7: Install the Capstones

Once you’ve built the wall to your desired height, it’s time to install the capstones for a finished look.

• Cutting Curved Caps: For curved walls, you may need to cut capstones to fit the shape of the wall. Use a saw to make precise cuts, and ensure the capstones are level.
• Adhesive: Secure the capstones with a strong adhesive, such as Xtreme Bond, to keep them in place over time.

Pro Tip:

Avoid over-reliance on shims when leveling the capstones. If minor adjustments are needed, pennies can serve as an inexpensive alternative.

Maintenance Tips for Allan Block Retaining Walls

Maintaining your Allan Block retaining wall will ensure it stays in top condition for years to come. Here are a few maintenance tips to keep in mind:

• Inspect for Shifting: Regularly check for signs of movement or settling, especially after heavy rainfall or snowmelt in Edmonton.
• Clean the Blocks: Remove dirt, moss, and debris from the wall periodically to maintain its appearance.
• Check Drainage: Make sure the drainage system is functioning properly to prevent water buildup behind the wall.

Conclusion: Ready to Build Your Allan Block Retaining Wall?

Building an Allan Block retaining wall is a rewarding project that can enhance both the aesthetics and functionality of your landscape. With the right preparation, tools, and techniques, you can construct a durable wall that withstands the test of time. Whether you’re retaining soil or creating an elegant garden feature, an Allan Block wall is a versatile solution for any outdoor space.

At Edmonton Landscape Center, we offer all the materials and advice you need to get started on your retaining wall project. Visit us today or contact our team for expert guidance and high-quality products to help you achieve professional results!

Frequently Asked Questions (FAQs)

How tall can I build an Allan Block retaining wall without reinforcement?

Walls shorter than 3-4 feet typically don’t require geogrid reinforcement, but taller walls should include geogrid to ensure stability.

What type of drainage is needed for an Allan Block retaining wall?

A weeping tile drainage system is recommended to prevent water buildup behind the wall, which can lead to shifting or cracking.

How much geogrid is needed for a retaining wall?

Geogrid should extend 60% of the height of the wall behind it. For taller walls, geogrid should be installed on every second row of blocks.

Can I build an Allan Block retaining wall myself?

Yes, Allan Block walls are designed for DIY projects. With proper planning and attention to detail, you can build a durable and attractive wall.

What tools do I need to build an Allan Block retaining wall?

You’ll need a shovel, level, compactor, string line, saw (for cutting blocks), and a strong adhesive for the capstones.