Allan Block Retaining Wall Base

Building a strong, long-lasting retaining wall starts from the ground up—literally! Learn how a well-prepared base sets the foundation for durability and success.

There Are Two Parts To A Retaining Walls Base:

First the material the blocks will be sitting on, and second the way the retaining wall blocks are placed. (Basing the wall).

The foundation of an Allan Block retaining wall is, as you can imagine, critical. The base gravel is what supports the retaining wall, and is the foundation to a long-lasting wall that doesn’t sink or settle in time.

Look around and you will notice most walls settling in places.

Knowing and using the best gravel for your retaining wall is more important than choosing a gravel that is easy to work with.

Sand and fine gravel are tempting to use as they level really easily. The problem with this is that over time this material moves and washes away. Using a coarse gravel is harder to use, and level, but it will last a lifetime, and never erode.

For a typical retaining wall, we excavate 10”-12” of soil. This provides enough room for a minimum of 6” of gravel, 4” of buried block.

When excavating you must dig down to undisturbed soil. If over excavating has to be done for site services or other reasons, soil cannot be added back to raise the grade. Instead, you need to add compactable gravel, until the desired elevation is reached.
Once excavated the ground will be flat, and clean of all organic matter. Any big rocks should be removed so a compactor can rest completely on the soil. The area is then compacted as required.

In some cases, the soil can be flat, while other times we may want a slope to aid in the drainage of the “trench”. Any pipe that is installed is placed directly on the soil, and at the back of the trench, making sure it won’t be under the block wall.

The Gravel

Once the soil is clean, compacted, and the drainage pipe (if required) is in, we begin to import the angular ¾” clean gravel. If a small compactor is available, the gravel is imported in small layers and compacted.

The bigger the compactor, the deeper the gravel can be before its compacted. This is called compacting in lifts. 6-8” lifts are normal for small compactor under 200lbs. and a minimum of 2 passes is required.Water is used to keep the dust down, and to lubricate the rocks as they rotate to lock together.

With a perfectly prepared base we are able to base each block in about 5 min, while a bigger rock sub base will take longer.

Size and Depth

The size and depth will vary a lot depending on site conditions. If building on poor soil you will need a much deeper, and wider base. Typically, this is about 6” minimum of gravel, and about 30” wide. This gives room for 12” of drainage gravel behind the wall, 12” of room for the Allan Block itself, and 6” of working room in front of the wall.

The size of this Base trench will affect the quality, lifespan, and cost of the retaining wall. The bigger the base, the stronger your wall will be (within reason of course).

Basing the Wall

Once the trench is dug, the base rock can be placed and compacted. We use a laser level to ensure the exact grades are met based on the property elevations.

The most important step of ensuring that the retaining wall is level and straight is this first row of blocks. When a wall is based, each block must be perfectly level front to back, side to side, and block to block.

In addition to being level, the blocks must also perfectly follow either a stringline or curved fiberglass rod depending on the design of the retaining wall.

The first row of most retaining walls takes more time than all the other rows combined.

At Back 40 Landscaping we are trained and certified in building a proper retaining wall base, ensuring your wall will not just look great but last.