Putting gravel behind a retaining wall is primarily done to manage water and relieve structural stress. It ensures longevity by preventing hydrostatic pressure—the dangerous force of trapped, heavy water buildup that cracks and collapses walls.
Skimping on the base material or skipping the gravel backfill leads to the tilting, bulging, and cracking that homeowners dread. Stick with angular, crushed stone compacted in layers, put a minimum of 12 inches of drainage gravel behind the wall, and your structure will stay strong for decades.
The 1/3 rule is a general guideline for stability: About 1/3 of the wall's total height should be buried below ground. This helps the wall resist the pressure from soil and moisture behind it.
Seven common mistakes when installing retaining wall blocks include poor base preparation, inadequate drainage, not using proper backfill, failing to compact soil, improper alignment of blocks, ignoring wall height limits, and neglecting to use geogrid reinforcement when needed.
Directly behind a retaining wall, you should put a layer of crushed stone or washed gravel (such as ¾-inch angular rock) to provide drainage and structure. Do not use standard dirt or sand, as they hold moisture and can cause structural failure or cracking.
The best gravel for drainage behind a retaining wall is 3/4-inch to 1 1/2-inch angular crushed stone (often called #57 stone). This type of rock has sharp, jagged edges that lock together for stability, and it contains no "fines" (sand or dust), allowing water to flow freely.
One of the leading causes of retaining wall failures is improper drainage. There are many reasons for this, but one main reason is that saturated soils behind walls cause excessive weight. This additional surcharge may cause the wall to lean outward and eventually collapse.
Yes, a 3-foot retaining wall absolutely needs drainage. While it is a relatively short wall, trapping water behind it creates immense hydrostatic pressure. Without proper drainage, this pressure, combined with heavy saturated soil, can cause the wall to bulge, crack, or collapse.
Foundation bearing (gravity case)
For the following simplified procedure, the “middle third rule” is applied, whereby the wall foundation is dimensioned so that the resultant vertical force acts through the “middle third” of the footing.
Yes, a retaining wall should always be built true level horizontally, regardless of the slope of your yard. Failing to level the blocks or stones can cause structural failure, as gravity will constantly push the weight downhill.
If between 1.5-2 metres from boundary, maximum wall height 2 metres. If greater than 2 metres from boundary, maximum wall height 3 metres. If more than 600mm above or below ground level, are contained by a structural support.
Yes, a 2-foot retaining wall generally needs a footing. However, "footing" means different things depending on how the wall is built. Instead of a deep, poured-concrete foundation, shorter walls usually rely on a compacted aggregate base or a single buried row of blocks.
Engineers are required for retaining walls higher than 4 feet. If you need a retaining wall higher than 4 feet, you'll need to employ the expertise of an engineer. After all, it needs to be built with a solid foundation and support to ensure it can do its job without the risk of compromising its structural integrity.
Ensure you use a minimum of 12 inches of drainage gravel directly behind the blocks along the entire height of the wall. For optimal drainage and to prevent structural failure, use a 34three-fourths34-inch angular, washed crushed stone without fine particles (like sand or dust).
Gravel that hardens like concrete is typically a mixture of crushed angular stone and fine rock dust. When compacted, the fine powder acts as a binder, locking the larger stones into a solid, durable surface. Common varieties include Crusher Run, Decomposed Granite (DG), and Crushed Concrete.
Drainage is one of the most important factors in ensuring the longevity and stability of a retaining wall. When water builds up behind a retaining wall with no way to escape, it increases pressure on the structure, leading to bulging, deterioration, and even collapse.
Even a 4-foot wall often requires posts buried at least 2 feet, depending on the material. Getting the depth right ensures your wall structure can handle its own weight and the soil it's holding back over time.
I would recommend you grade it out to 20-25 degrees maximum, with 20 being much safer than 25. Anything over 25 should really have a retaining wall if you plan to maintain the ground. If you aren't going to maintain it, then you need to get an engineering firm to measure your soil's "angle of repose".
Backfill and Compact
Proper backfilling behind the retaining wall is crucial to prevent soil erosion and excess pressure on the structure. Fill the space behind the wall with crushed stone or gravel, which allows for efficient drainage. Compact the backfill periodically to eliminate air pockets and ensure stability.
Yes, you can dig a hole and fill it with gravel to create a drainage system (often called a dry well or a vertical French drain). However, to ensure it works and doesn't quickly clog or fill with mud, you need to follow a few specific steps.
Hydrostatic pressure and freeze‑thaw cycles act on any wall that holds back soil, no matter how low. Building codes also step in around 4 feet, where engineering and clear retaining wall drainage requirements are usually mandatory.
Inadequate Drainage
One of the most common mistakes is inadequate drainage. Water can accumulate behind the wall, exerting pressure that leads to bulging or even collapse. Ensuring proper drainage is critical for a long-lasting retaining wall. To avoid this mistake, install a proper drainage system.
Homeowners insurance covers a retaining wall collapse only if the collapse is directly caused by a "named peril" listed in your policy, such as a vehicle hitting the wall, a lightning strike, or an explosion. If the collapse resulted from normal wear and tear, poor maintenance, erosion, or the weight of saturated soil, it will not be covered.
The longest-lasting retaining walls are poured reinforced concrete and dry-stacked natural stone. Both materials can endure for 50 to over 100 years when properly engineered and constructed.
You need a minimum of 12 inches (1 foot) of drainage rock directly behind the back of the retaining wall units, stretching from the base to within 6 inches of the top surface. However, many professionals recommend increasing this width to 18 to 24 inches for taller walls to maximize structural stability.