Introduction: A Surprising Contractor Story
Let me start with a quick story from a contractor training I attended a while back. I was invited to this session where the presenter asked a room full of contractors a simple question: “What’s the best backfill to use for your segmental retaining wall project?”
To my surprise (and maybe a little bit of horror), I heard several voices confidently say “clay!” Now, if you know anything about retaining walls, you know that’s like saying you want to build a house on quicksand. It was a real eye-opener about how common some misconceptions are, and it’s exactly why we need to clear up why clay is a terrible choice.
The Slippery Slope: Understanding Clay’s Friction Angle
Let’s start with a simple analogy. Imagine you’re trying to brake your car. If you were braking on a slip-n-slide, you’d just keep sliding. That’s clay for you. It has a low friction angle because its tiny particles are smooth and slippery. When clay gets wet, it becomes even more like a slip-n-slide, and anything trying to anchor into it (like geogrid) won’t have much to grab onto.
Now, compare that to trying to brake on a solid asphalt or concrete road. That’s like using a well-draining, angular fill material. These materials have a high friction angle and interlock well, so your geogrid can “brake” and hold steady, keeping your wall stable.
The Drainage Dilemma: How Clay Holds Water
Clay isn’t just slippery; it’s also a water hoarder. Unlike sandy or crushed rock fills that let water pass through easily, clay holds onto moisture and prevents proper drainage. That means water buildup behind your wall, adding pressure and increasing the risk of failure. In other words, clay turns your retaining walls drainage into a swampy mess.
The Freeze-Thaw Fiasco: Clay’s Expansive Nature
Another big issue is how clay behaves in cold weather. When it gets wet and then freezes, it expands. When it dries out, it contracts. This constant cycle of expanding and contracting can cause the soil to heave and shift, putting additional stress on your wall. Over time, this “creep” can make your wall lean or even collapse.
The Particle Picture: Why Shape Matters
Finally, let’s zoom in and look at soil structure. Under a microscope, clay particles are like tiny, smooth marbles. They roll over each other with little resistance. In contrast, materials like sand, screenings, and crushed rock have angular, jagged edges that interlock and hold together. This interlocking property gives them a much higher friction angle and makes them the ideal choice for backfill.
What This Means for Your Retaining Wall: Real-Life Deformation and Failure
Now, let’s talk about what all this really means when you’re standing in front of a wall that’s been built with the wrong backfill. Picture a segmental retaining wall where the blocks are no longer aligned. You might see stair-stepped gaps forming as the wall deforms over time.
Or you might see tension cracks behind the wall and noticeable movement of the wall.
Both of these deformations are often a direct result of clay’s creep and the poor pull-out capacity of geogrid in clay. In the best-case scenario, you end up with a wall that just looks bad and doesn’t hold its shape well. But in the worst-case scenario, this kind of deformation can lead to a complete wall failure, especially when water builds up and adds even more pressure.
Don’t let the photo above be your retaining wall. It’s a visual reminder of why choosing the right backfill isn’t just a minor detail—it’s crucial to keeping your wall both stable and looking good for the long haul.
Conclusion
In summary, using clay behind a retaining wall is like trying to build on a slippery, waterlogged foundation that shifts with every freeze and thaw. Instead, opt for angular, well-draining materials that will keep your wall stable and strong for years to come.
If you need expert advice or help with your retaining wall project, don’t hesitate to reach out to us at DIY Retaining Wall Services.