Why Retaining Walls Fail And Common Mistakes You Can Avoid

If you’re staring at a sloped backyard and dreaming of a level patio, you’re likely thinking about a retaining wall. But a retaining wall isn’t just a garden feature. It’s a dam built to hold back thousands of pounds of moving earth. When these walls fail, they don’t just lean; they collapse, taking your yard and causing drainage blowouts that can drown your perennials.

At Matthew’s Evergreen Farm, I’ve spent years fixing DIY projects that turned into nightmares. In Wisconsin, a poorly built retaining wall is a ticking time bomb. Between our heavy clay soil and brutal freeze-thaw cycles, your wall must be smarter than the physics trying to push it over. If you have a steep landscape, a good retaining wall is the only thing keeping the soil anchoring your Wisconsin trees from sliding into your neighbor’s yard.

What Is A Retaining Wall’s Job?

Simply put, a retaining wall turns a hill into a staircase. It’s a structural barrier designed to fight “lateral pressure.” The natural urge of soil is to slump downward. This pressure triples the moment it rains, and that soil becomes a heavy, waterlogged mess.

Here are 3 important things a good retaining wall must do:

• Hold Back the “Slumping” Soil: The wedge of soil behind your wall always wants to slide downward. Your wall’s first job is to act as a sturdy dam that keeps that massive weight exactly where it belongs.
• Manage Water: Water is incredibly heavy. Your wall needs a built-in way to let moisture flow through or around it; otherwise, trapped water will create “hydrostatic pressure” and push your blocks right over.
• Stay Level Through the Seasons: In Wisconsin, the ground expands and shrinks as it freezes and thaws. Your wall must be built on a rock-solid, level gravel base so it stays straight, even when the earth shifts during a spring melt.

Common Retaining Wall Mistakes And How To Avoid Them

1. Setting Blocks Right on Top of the Grass

   Many beginners try to save time by placing their first row of blocks directly on the lawn or loose dirt. Without a solid foundation, there’s nothing to prevent it from sliding forward under the weight of the earth.

   To prevent this, professionals dig a shallow trench and bury the entire first row of blocks underground on a tightly packed gravel base (usually 1/10th of the wall’s height). This is called a “toe” and creates a lock that holds the rest of the wall in place.

2. Trapping Water Behind the Wall

   Water is the most common cause of wall failure. When it has nowhere to go, that immense pressure often results in a bulging wall or a total blowout. The best way to solve this is by placing a plastic drain pipe at the bottom of the wall and surrounding it with a thick layer of crushed stone, giving water a clear path to exit safely.

3. Filling the Gap with Backyard Dirt

   It’s tempting to use the dirt you just dug up to fill the space behind your new wall, but native soil (especially clay) acts like a sponge. When that wet dirt freezes in winter, it expands and pushes against your stones. Instead of dirt, fill the wall with clean crushed stone. This stone doesn’t hold water or expand, keeping the pressure off your wall.

4. Failing to Compact the Sub-base

   If the gravel under your wall is loose, the heavy blocks will eventually settle unevenly. This leads to ugly gaps. Most professionals use a heavy vibrating machine called a plate compactor to smash the gravel base until it is rock-hard. They do this in thin layers to make sure the foundation is solid from the bottom up.

5. Building Too High Without Reinforcement

   Most standard landscape blocks are only designed to hold back a hill that is 3 feet tall or less. If you go higher without extra support, the wall will eventually tip over.

   For anything taller, pros use geogrid—a strong mesh fabric that pins the wall back into the hillside. My best advice would be to call in a retaining wall contractor to ensure the physics are sound.

6. Building a Perfectly Vertical Wall

   It sounds counterintuitive, but a wall that stands straight up and down is actually quite weak. It has no means to fight back against the weight of the earth. Over time, even a tiny shift forward makes the wall look like it’s falling.

   In this case, like most pros, build with a step-back, meaning every new row of blocks is set back about a half inch from the one below it. This creates a slight lean, significantly increasing the wall’s strength.

Signs a Retaining Wall Was Built Wrong

Early detection can save you thousands in total replacement costs. So, watch for these red flags:

• Gaps at the Corners: If the sides of your wall are pulling away from the front face, the structure is struggling to hold the weight of the hill. Gaps like this show that the wall is shifting outward because it isn’t properly anchored.
• The “Belly” Bulge: For whatever reason, if you notice a bulge in the middle of your wall, you have a major water problem. This “belly” means water is trapped behind the blocks, creating heavy pressure that is physically pushing the stones out of alignment.
• Cracked or Snapped Blocks: While small cracks from settling are typical, extensive horizontal cracks are a cause for concern. This usually means the weight of the soil is simply too much for the blocks to handle, and the wall is at risk of snapping.
• Dipping Soil at the Top: When the earth just behind the top of your wall is settling or slumping, it’s a sign your soil is probably being eroded. This occurs when the wall lacks adequate backing material to hold the soil, allowing it to seep through the fissures whenever it rains.

Protect Your Landscape Investment

A failed wall is more than an eyesore; it’s a hazard that can lead to Wisconsin tree diseases on a steep grade. It can drown or crush root systems. To protect your property, keep these three takeaways in mind:

• Check Your Drainage: Ensure your drain holes aren’t clogged and water isn’t pooling at the base of the wall.
• Monitor After Storms: The best time to spot a failing wall is right after a heavy downpour. Look for new bulges or sinkholes in the soil behind the wall.
• Prioritize the Foundation: Whether you’re planting privacy trees or building a stone terrace, the structural integrity of your soil determines the health of everything above it.

FAQs

Do I need a permit for a retaining wall?

In most municipalities, any wall over 4 feet tall requires a permit and a structural engineer’s stamp. Always check local building codes before digging.

Can I use wood for a retaining wall?

Well, timber is cheaper but has a shorter lifespan (usually 10–15 years) and is prone to rot. Stone or concrete blocks are best for permanent structures.

What is the best stone for drainage?

Always use 3.4-inch clean angular stones, and avoid round river rocks or pea gravel. They don’t lock together to provide structural stability.

Should I use mortar?

Modern interlocked retaining walls are designed to be dry-stacked. This allows the wall to adapt slightly with the ground’s natural movement without cracking the way mortar would.