What drainage is needed behind a retaining wall in Ottawa?

Drainage is arguably the single most important element of a retaining wall in Ottawa, and it's the component most often underbuilt in DIY and budget contractor projects. Our climate creates drainage demands that are more severe than most regions, and inadequate drainage is the number one reason Ottawa retaining walls fail prematurely.

The core drainage system behind any Ottawa retaining wall starts with a perforated drainage pipe, typically 100mm (4-inch) rigid or flexible perforated pipe, installed at the base of the wall directly behind the first course of blocks. This pipe sits in a bed of 19mm clear crushed stone, with the perforations facing down. The entire pipe and stone zone should be wrapped in non-woven geotextile filter fabric. That fabric is essential in Ottawa because our prevalent clay soils produce extremely fine particles that will migrate into the stone and clog the drainage pipe within a few years without it. Leda clay, found across much of Ottawa's south and east ends, is particularly notorious for this.

Behind the drainage pipe, the entire space between the wall and the excavated soil face should be backfilled with clear crushed stone, not gravel with fines, and not the excavated clay. This drainage column should extend the full height of the wall and be at least 300mm (12 inches) wide. For taller walls over 0.6 metres, 450mm or wider is better. The clear stone serves two functions: it allows water to flow freely down to the drainage pipe, and it provides a compressible zone that absorbs frost heave pressure rather than transmitting it directly to the wall face.

At the top of the drainage stone, a layer of filter fabric should cap the stone before the final 150mm of topsoil goes on top. This prevents surface soil from washing down into the drainage column and clogging it over time. Some Ottawa contractors also install a strip of impermeable membrane at the top to actively direct surface water away from the wall rather than letting it infiltrate behind.

The drainage pipe must have a positive outlet, meaning it needs to daylight at the low end of the wall where water can discharge freely, or connect to a catch basin and storm drain if available. In Ottawa, running the pipe to daylight is most common, and the outlet should discharge at least 1.5 metres away from the wall base to prevent erosion undermining the foundation. If your wall runs along a property line and there's nowhere for the pipe to daylight, a dry well or infiltration pit at the low end can work, sized to handle the expected water volume.

Ottawa's seasonal drainage loads are intense. During the spring thaw from late March through April, frozen ground above and behind the wall melts from the surface down, creating a perched water table right against the wall. If your drainage system can't handle this volume, hydrostatic pressure builds and either pushes the wall outward or causes water to seep through the wall face, carrying soil particles with it and creating voids behind the wall. After 50-plus freeze-thaw cycles per year, a wall without adequate drainage typically shows visible displacement within three to five winters.

Weep holes through the wall face provide a secondary relief valve. These are simply gaps left in the lower courses, or short PVC pipe sections angled slightly downward through the wall, spaced every 1.2 to 1.8 metres along the wall length. They let trapped water escape through the face rather than building up if the drainage pipe is temporarily overwhelmed during heavy spring melt.

Getting the drainage right is where an experienced Ottawa retaining wall contractor really earns their fee. The materials for proper drainage add relatively little to the project cost, maybe $500 to $1,500 depending on wall length, but the labour and knowledge to install it correctly in our clay soils makes a substantial difference in how long the wall lasts.

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Looking for experienced contractors? The Ottawa Construction Network connects Ottawa homeowners with qualified professionals:

• Justyn Rook Contracting
• JC Carpentry
• Capital Decks.ca
• Jaiko Cleaning Services
• Elie The Carpet Guy Inc.