Building a retaining wall starts well before the first block is laid—it begins at the trench. The strength, longevity, and overall success of your retaining wall largely depend on how well the base is prepared. Skipping or rushing through the trenching process can lead to tilting, cracking, or complete failure down the line. Fortunately, understanding the correct base depth and preparation can help ensure a stable wall that stands the test of time.
“The bitterness of poor quality remains long after the sweetness of low price is forgotten.”
— Benjamin Franklin
Understanding Trench and Base Requirements
A common mistake in DIY wall construction is underestimating how deep to dig the retaining wall trench. The rule of thumb is to bury about 10% of the wall’s height plus an additional 4–6 inches for the gravel base. For instance, a 3-foot wall would typically require a trench around 10–12 inches deep. This includes 6 inches of compacted gravel and the partially buried first course of blocks.
The trench should also be twice as wide as the depth of your wall block to accommodate tools, gravel, and drainage systems. Clean edges and level depth across the entire run are essential to keeping your structure uniform and aligned.
Trenching Tools and Techniques for Success
Digging a straight, consistent trench across a slope or uneven yard is no small task. It's where efficiency meets precision. For tighter soil or clay-heavy regions, using the right tool can save time and ensure proper trench geometry. A trenching tool like a Tomahawk 3HP Honda Vibratory Rammer Tamper with GX100R Engine is ideal for compacting trench bases thoroughly—ensuring minimal settling after the wall is complete.
After digging, add a layer of crushed gravel. Compact this layer in increments using a tamper or plate compactor. The more thoroughly you compact, the less shifting or sinking you'll experience in the long run. Always check for level from front to back and side to side before placing your first row of blocks.
Final Considerations Before You Stack
Retaining wall performance hinges on more than height and width—it also depends on proper water drainage. Ensure your trench allows water to drain away from the wall. Use perforated drain pipe behind the base layer if your wall retains soil or is over two feet high.
Lastly, remember that landscape fabric, while not a substitute for good drainage, can help keep soil from migrating into the gravel base, maintaining long-term stability and cleanliness behind the wall.
If you're wondering how deep to dig, how to set up your base depth for the wall, or how trenching for retaining walls should be done—starting with solid ground preparation will prevent costly repairs and headaches later.
