Installing a concrete block retaining wall involves careful preparation of a gravel base to ensure the first row of blocks is perfectly level. This critical step establishes the proper foundation for building a structurally sound wall that can withstand time and weather.
For this reason, our experts created this guide to help you in the process.
Assessing the Gravel Base Site
Before beginning a retaining wall project, there are a few steps you must follow:
- Evaluate your site conditions and terrain
- Excavate down to solid, undisturbed soil if needed
- Remove any topsoil, organic matter, sod, and debris. Retaining wall blocks must sit on a solid, compacted gravel base.
- Consider the slope and geography of the site. Retaining walls on steep slopes or hills generally require deeper gravel bases and more advanced engineering techniques like geogrid soil reinforcement.
For simpler landscape projects, aim for a relatively flat site with a modest slope if possible.
Leveling and Compacting the Gravel
Once you prepare the site, it’s time to level the gravel base. This prep is one of the most important steps, as any mistakes here will throw the entire wall out of plumb.
Spread 4 to 6 inches of dense graded base gravel across the area, using a rake to roughly level it out. Use a hand tamper to start compacting the gravel. Make repeated passes across the area, tamping down about 2 to 3 inches at a time before adding more gravel.
Compact the gravel thoroughly, as any soft spots allow blocks to settle and shift out of position. Proper compaction is critical for DIYers, since uneven settling of the gravel base is a common issue that can undermine the structural integrity of the entire retaining wall. Taking the time upfront to thoroughly compact the gravel will save you headaches.
A plate compactor is ideal for larger retaining wall projects. For smaller DIY walls, a hand tamper can achieve adequate compaction with some elbow grease. The key is taking the time to tamp repeatedly until the gravel feels extremely firm.
Leveling the Compacted Base Layer
Once the gravel base is compacted, you must level it precisely. Set up two parallel rows of stakes or batter boards with a taut mason’s string stretched between them. This string establishes your reference line for leveling the gravel and positioning the first course of blocks.
Use a straight board as a screed to drag across the gravel and scrape off any high spots. Check for level frequently with a 4-foot level placed against the mason’s string. Make multiple passes with the screed, adding or removing gravel as needed to fine-tune the slope.
The flatter the site, the easier to achieve a perfectly level gravel base. For sites with steeper slopes, it may help to level and compact the gravel in smaller sections as you step down the slope.
Topping With Sand for Precision Leveling
Some manufacturers recommend topping the compacted gravel base with a thin 1/2″ to 1″ layer of coarse builder’s sand for the most precise leveling possible. This allows you to fine-tune any minor uneven spots for flawless block alignment.
Be sure to use coarse sand that compacts well. Decomposed granite and stone dust also work. Spread and screed the sand just like you did with the gravel. Use a hand tamper to compact the sand so it does not shift under the blocks.
The downside to sand is that it does not offer as firm of a base as gravel alone. If you can level the gravel precisely, opting out of the sand layer is fine. But sand gives you some insurance during the tricky block leveling process.
Tips for Keeping Blocks Level
Leveling and aligning the first row of blocks on a gravel or sand base takes precision. I have learned from experience that one small rock out of place can throw blocks out of level. Here are some tips to make this process easier:
- Use block leveling pads or sheets of polyethylene under each block for a more uniform base.
- Check for level. Plumb every 1 to 2 feet as you position blocks. Check all the retaining wall corners for levelness. If you’re building a curved wall or one that’s extra long or large, consider using a laser level to ensure integrity.
- Make minor adjustments by nudging gravel or sand underneath too-low blocks.
- Avoid stepping directly on the gravel base to prevent shifting.
- Set up additional Mason’s strings farther back as guides if the wall is very long.
- Begin your project with a small starter section to get the hang of leveling on gravel.
Proper base preparation is the foundation of every successful retaining wall project. Investing ample time up front to level and compact the gravel will prevent headaches once you start stacking blocks. Taking steps to level more finely with sand or pads gives an added advantage.
Adding Gravel or Drainage Material Behind the Wall
Once you install your retaining wall, you need to backfill behind it with drainage material. Spread 4 to 6 inches of gravel behind each course before backfilling with soil. The gravel creates a drainage zone to prevent water pressure buildup behind the wall.
You can also install perforated drainage pipes along the base of the retaining wall gravel that daylight downhill. This added drainage precaution is recommended for walls in rainy climates or clay soils. Don’t forget to account for future plantings and soil depth when planning your gravel and drainage layers.
Choosing the Best Retaining Wall System
With the wide variety of retaining wall materials and systems available, I know selecting the right solution for your property can be daunting. Carefully consider these key factors when weighing your options:
Gravity walls less than 3 feet tall have fewer structural demands than taller reinforced walls.
Retaining walls along a driveway and other high-traffic zones need durable block finishes that resist chips, scrapes, and impact damage. Natural stone faces are more elegant but more prone to blemishes with age.
Retaining walls containing backfill soil in wet climates may require built-in drainage features or dense block construction that resists moisture damage. Not all wall types handle frequent water exposure well.
From the sleek facades of modern cellular concrete blocks to the natural beauty of quarried stone blocks, aesthetics often drive retaining wall selection. Choose a look that complements your landscape design style.
Many new retaining wall blocks are designed for fast DIY assembly with unique shapes that align automatically. Older block systems rely on pins or adhesives for a secure bond, which complicates installation.
These practical factors will guide you toward the ideal retaining wall for your property’s needs and personal tastes. Don’t be afraid to splurge for a higher-end wall system if the added durability and curb appeal enhancements are worth the investment.
Consulting With Retaining Wall Experts
For those in over their heads with a major retaining wall installation, consulting local retaining wall construction experts is wise. Professional landscape contractors and masons evaluate your site conditions and recommend the optimal retaining wall design. They also handle heavy excavation work and compact any needed fill dirt.
Thoroughly explain your retaining wall goals and explore all the design possibilities. Many retaining wall contractors have intensive product training with leading brands of wall systems.
This inside knowledge can prove invaluable when selecting blocks best suited for your project site. Qualified professionals also know engineering techniques to reinforce taller gravity walls or establish proper drainage.
Don’t hesitate to get multiple retaining wall bids and ask each contractor plenty of questions. Confirm whether their quotes include preparation work and materials. A modest investment in expert consultation during planning can pay big dividends through a wall that meets your needs for years.
So, Is Leveling Gravel for Retaining Walls Difficult?
When installing a retaining wall, leveling the gravel base is often one of the trickiest parts. However, the right preparation and tools can make achieving level blocks much more manageable.
Staking out clear level lines, using a long screed, and thoroughly compacting the gravel are key first steps. Topping with sand or block pads adds insurance against stray rocks under blocks.
For first-time DIYers, beware of biting off more than you can chew with a large or complex retaining wall on a steep slope. Choose a flatter site if possible, and start small to hone your leveling skills before attempting long walls.
Having an extra set of hands helps too. Patience and periodically double-checking for level will pay dividends for a wall that stands straight and true for decades.
FAQs About Leveling Gravel For Retaining Walls
How much gravel do I need under a retaining wall?
Recommendations vary by manufacturer, but usually, you need a 4- to 6-inch-deep layer of compacted gravel under retaining wall blocks. You should build a deeper base of around 8 inches for walls taller than 4 feet. Check your local building codes.
Should I use sand or gravel under patio blocks?
Compacted gravel makes the best base under retaining walls and pavers for strength and stability. Adding a thin ½- to 1-inch layer of sand atop the gravel can help with precision leveling when aligning the first blocks.
What size of gravel is best for a patio base?
Use dense graded base gravel that compacts well, typically with stones between half an inch and 1 inch in diameter. Larger gravel won’t be compact enough to use. Granite screenings or crushed stone also work well.
How do I ensure my first row of blocks is level?
Accurate leveling of the gravel base is crucial. Screed and smooth the gravel, then top with sand and re-screed if needed. Use a mason’s string line stretched between batter boards as your reference for level. Check block level every few feet.
Should you tamp gravel for a patio?
Yes, compacting the gravel base under a patio or retaining wall is extremely important. Use a hand tamper, plate compactor, or jumping jack compactor machine. Compaction prevents settling issues.
