Retaining Wall Fill – Choosing the Best Materials and Techniques
If you’re building a retaining wall, you might be wondering what to fill it with. Choosing the right fill is just as important as picking the right materials for the wall itself. The right backfill helps with drainage, prevents movement, and makes your wall last longer. If you use the wrong material, your wall could shift, crack, or even collapse over time.
As a retaining wall builder, I’ve seen people make the mistake of using the wrong fill, thinking it will save money or make the job easier. But using the right material from the start saves you headaches (and repair costs) down the track. Gravel and crushed rock are some of the best options because they drain well and keep the wall sturdy. But what if you’re on a budget? Are there cheaper alternatives?
What Should I Fill My Retaining Wall With?
It’s essential to carefully evaluate the slope before beginning any landscaping job. Consider the slope’s qualities and degree, whether mild or steep, since this will greatly impact how you landscape the area. You can properly plan and design your landscape if you know the slope’s special characteristics.
Why the Right Fill Matters
The retaining wall you’re building plays a big role in what kind of backfill you should use. Suppose you’re working with a gravity wall (like timber sleepers or stacked stone). In that case, it relies on weight to stay in place, so you need good drainage to prevent water pressure from pushing it forward. Cantilever walls (like reinforced concrete) need strong, compacted material to support their structure. Segmental walls (interlocking blocks) work best with free-draining gravel to keep them from shifting.
What I Recommend
- Crushed Rock and Gravel – This is my go-to for most walls. It drains well, compacts properly, and helps prevent water buildup.
- Sand—I’ve seen people try to use sand alone, but it shifts too much. Mixing it with gravel can work.
- Recycled Concrete – A good budget option. I’ve used it on a few jobs where clients wanted something affordable but still effective.
- Native Soil—Only use this if it’s well-compacted and not clay-heavy. I’ve had to fix too many walls where people backfilled with clay, resulting in waterlogged, unstable ground.
If you want your retaining wall to last, don’t cut corners with the backfill. A little extra effort in choosing the right material can save you a lot of trouble (and money) down the track.
What Is the Best Filler for a Retaining Wall?
If there’s one thing I’ve learned, it’s that the right backfill can make or break a wall. I’ve seen walls fail because of poor drainage, and I’ve had to fix jobs where the wrong material was used, leading to shifting and cracking. So, what’s the best filler for a retaining wall? Let’s break it down.
How to Choose the Best Fill Material
When selecting backfill, I always consider these three factors:
- Drainage Properties – Water buildup is the enemy of retaining walls. You need a material that lets water flow through instead of trapping it behind the wall.
- Compaction Ability – The fill should settle properly and provide a solid base. Loose or shifting material can cause the wall to move over time.
- Longevity and Durability—You want a material that will hold up over time so you won’t have to deal with maintenance issues down the road.
Comparing Fill Materials
- Gravel and Crushed Rock are my go-to choices for most walls. They drain well, compact nicely, and keep pressure off the wall. I always recommend at least 300mm of crushed rock behind the wall.
- Sand—I’ve seen people try to use sand, and while it drains well, it shifts too easily. Mixing it with gravel can work, but on its own, it’s risky.
- Recycled Aggregates – If you’re looking for a budget-friendly, eco-friendly option, this is a solid choice. It performs well, but make sure it’s properly compacted.
- Native Soil – I only use this when it’s sandy and drains well. If it’s clay-heavy, forget it—it holds water and creates pressure that can eventually push your wall over.
I always tell my clients to invest in good drainage and solid backfill. It’s a small step that can make a big difference in keeping their retaining walls strong for years to come.
How My Team and I Backfill a Retaining Wall
Backfilling a retaining wall isn’t just about throwing in dirt and calling it a day. If it’s not done right, the wall can shift, crack, or even collapse over time. That’s why my team and I follow a step-by-step process to ensure every wall we build is solid, drains properly, and stands the test of time.
Preparing the Base
Before we start backfilling, we ensure the base behind the wall is stable. We compact the Soil properly and add a layer of crushed rock to create a firm foundation. This prevents the backfill from settling unevenly and prevents movement down the track.
Installing Drainage
Water pressure is one of the biggest reasons retaining walls fails, so proper drainage is a must. We always install a perforated drainage pipe at the base of the wall, wrapping it in filter fabric to stop debris from clogging it. Then, we surround the pipe with crushed rock to keep water flowing freely away from the wall.
Layering the Backfill
We never dump all the backfill in at once. Instead, we add it in layers—about 200–300mm at a time—compacting each layer before adding the next. This ensures that the backfill stays solid and doesn’t settle unevenly later on. We mostly use crushed rock or gravel because it drains well and gives the wall proper support.
Final Touches
Once the backfill reaches the top, we add a layer of topsoil if the homeowner wants to do any landscaping. If plants or grass are going in, we make sure the drainage setup will still work to keep excess moisture away from the wall.
How We Do It for Australian Conditions
- Soil Conditions—In areas with heavy clay soil, we use extra drainage and sometimes geogrid reinforcement to prevent the wall from shifting due to water retention.
- Local Regulations – For walls over a certain height, we check council requirements to ensure everything is compliant.
- Geogrid Reinforcement – For taller walls (typically over 1m), we install geogrid layers between backfill sections to add extra strength and stability.
We follow this process every time to ensure the longevity of our retaining walls. It requires more effort upfront, but it saves a lot of trouble down the road.
The Cheapest Way to Fill a Retaining Wall
When building a retaining wall, keeping costs down without sacrificing stability is always a priority. My team and I have worked on plenty of projects where budget was a big factor, and over time, we’ve found a few ways to save money while still doing the job right.
Budget-Friendly Fill Options
Using Locally Available Soil and Aggregates.
The cheapest option is often the one that’s already on-site. If the existing soil is stable and drains well, we sometimes use it as part of the backfill. However, heavy clay or overly sandy soil can cause drainage issues, so we mix it with better materials when needed.
Recycled Concrete and Crushed Brick.
Instead of paying for brand-new crushed rock, we often recommend recycled concrete or crushed brick. These materials are much cheaper, still provide good drainage, and are readily available from demolition yards or landscaping suppliers.
Balancing Cost with Long-Term Performance.
The trick is to save money without creating problems later. For example, using poor-quality soil as a backfill might be cheap at first. Still, if it causes drainage or stability issues, the cost of repairs will be much higher in the long ru
The Cheapest Way to Fill a Retaining Wall
Buying in Bulk vs. Bagged Materials.
We always suggest buying fill materials in bulk rather than in small bags from hardware stores. Bulk deliveries from quarries or landscaping yards cost much less per cubic meter.
DIY vs. Professional Installation.
If you’re comfortable with manual work, backfilling yourself can save labour costs. However, improper compaction or drainage setup can lead to costly fixes down the road. We always tell clients to at least get professional advice before going the DIY route.
Finding the Right Balance.
Sometimes, spending more upfront on quality fill can save money later. A mix of budget-friendly materials like recycled aggregates combined with proper drainage and compaction gives the best results without breaking the bank.
These are the methods my team and I use to keep retaining wall projects affordable while ensuring the walls last for ye
Frequently Asked Questions
Yes, even small walls can suffer from water buildup. A simple layer of gravel and a perforated drain pipe can prevent water pressure issues.
Backfill should be compacted every 200–300mm layer to prevent settling and shifting over time.
Poor fill material can cause drainage issues, wall movement, or even collapse, leading to costly repairs.
Not always, but it’s recommended for walls over 1m tall or in areas with unstable soil to add extra strength.
Conclusion
Choosing the right fill for your retaining wall is just as important as the wall itself. The right material ensures stability, proper drainage, and long-term durability. My team and I always focus on using quality backfill—whether it’s crushed rock, recycled concrete, or locally available soil—to achieve the best balance of cost and performance.
For small projects, DIY can work if done correctly, but for larger or taller walls, getting professional advice can save time and money in the long run. A well-built retaining wall should last for years, and choosing the right fill is a crucial step in making that happen.
