Behind a retaining wall, you need a combination of a perforated drain pipe, clean crushed stone or gravel, and a non-woven geotextile filter fabric. These materials work together to relieve hydrostatic water pressure, which is the leading cause of retaining wall failure.
DRAINAGE AGGREGATE (GRAVEL BACKFILL & CORE FILL)
Behind every properly built retaining wall is a layer of free-draining gravel or crushed stone. This drainage aggregate allows water to flow freely rather than getting trapped in the soil. Use 3/4" clean crushed stone (avoid pea gravel or sand, which retain water).
Yes, almost all retaining walls require drainage. Trapped water saturates the soil and creates "hydrostatic pressure," which doubles the weight of the dirt and can crack, lean, or collapse the wall. Proper drainage allows water to escape safely, protecting the wall's structural integrity.
Behind the retaining wall, the fill is placed and compacted in horizontal layers, and there are three categories of fill: granular fill, cohesive fill and other fill. Between the fill and the retaining wall is placed a thin layer (≈ 350 mm) of free draining material called the vertical drainage layer.
The best material for retaining wall backfill is a washed, angular crushed stone (such as ASTM #57 or 3/4-inch clear stone) because it provides structural stability and allows water to drain freely. Proper backfill relieves hydrostatic pressure, which is the leading cause of retaining wall failure.
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.
Retaining Wall Height and Drainage Risk
Generally: Under 600mm: May get by with basic drainage if the soil is free-draining and the site is dry. 600mm–1.5m: Subsoil drainage is essential, even on small residential walls. 1.5m and over: Requires engineered plans, formal drainage designs, and consent.
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.
Retaining wall filtration fabrics provide a separation layer between the soil and drainage rock behind a retaining wall. The non-woven material allows for high water flow and provides dimensional stability. They will not rot or mildew.
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.
Common Drainage Solutions for Retaining Walls
Technically, all retaining walls need some form of drainage to prevent hydrostatic pressure from building up and causing the wall to crack, lean, or fail. However, formal drainage—like a perforated pipe—is generally required for walls 3 feet or taller, but heavily recommended for any wall exposed to heavy water flow.
Inadequate drainage causes pressure to build up behind the wall. This pressure can cause wall tilting or even collapse, leading to costly repairs.
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.
The Golden Rule of Drainage is: “Drain only what is necessary for good crop growth and trafficability, and not one drop more.” “Conservation drainage” goes further, incorporating approaches and practices to minimize the environmental impacts on the downstream environment and ecology.
French drains are a low-cost option for dealing with surface water. These systems consist of a trench which houses a perforated PVC pipe. The pipe is secured in a mesh sock to keep dirt out, that is covered with gravel.
Salt on a gravel road causes maintenance problems as the road begins to thaw out, often making conditions worse. As the salt melts the snow/ice, it causes the salt and water solution to penetrate into the road surface. This causes the road surface to break up and loosens the previously hard packed surface.
DRAINAGE DESIGN
Place the drain tile as low as possible behind the wall so water drains down and away from the wall into a storm drain or to an area lower than the wall. See Diagram 6. Fill in the area behind the blocks with drainage aggregate, at least 12 inches from the wall.
For a French drain, the holes in your perforated pipe should face down (between the 5 and 7 o'clock position). This allows water that pools at the bottom of the gravel trench to easily rise up and enter the pipe through hydrostatic pressure, rather than waiting for the water level to submerge the entire pipe.
The best backfill for a retaining wall is a compacted, angular, open-graded crushed stone or gravel (such as ASTM #57 or #78). This "free-draining" material prevents the buildup of hydrostatic pressure—the leading cause of wall failure—by allowing water to pass through and drain away harmlessly.
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.
For drainage, even though your retaining wall will only be 400mm high, it's important to include proper drainage to prevent water buildup behind the wall. Excess water can create pressure, leading to structural issues over time.
The most affordable and beginner-friendly retaining wall options are interlocking concrete blocks, treated timber, or stacked cinder blocks. These materials typically cost between $2 and $6 per block, making them ideal for DIY projects. Always ensure proper soil drainage and check local building codes for walls over 3 feet tall.