Retaining walls play a critical role in shaping and supporting land, especially in hilly or sloped parts of Auckland. But one of the biggest reasons these structures fail isn’t the concrete, timber, or blocks themselves, it’s what’s happening behind the wall.
Inadequate or poorly planned drainage is a leading cause of structural failure in retaining walls. When water builds up in the soil behind a wall with nowhere to go, it creates enormous pressure. Left unchecked, that pressure can push the wall out, cause cracking, or result in total collapse.
At Procut Earthworks, we’ve seen first-hand how preventable mistakes in subsoil drainage and backfilling can derail even well-built walls. Here’s how to avoid the most common retaining wall drainage issues, and ensure your investment stands strong for decades to come.
Why Drainage Is Non-Negotiable
When rain falls, water seeps into the soil behind your retaining wall. Without proper drainage, this moisture builds up, creating hydrostatic pressure that pushes laterally against the structure.
This pressure increases dramatically when:
- The soil is clay-heavy or compacted
- The wall is tall or poorly engineered
- The backfill is saturated and lacks exit routes
- Subsoil drains are missing, blocked, or poorly placed
Even small-scale residential walls can experience failure from trapped water, especially over winter or after heavy rain.
Common Mistakes (and How to Fix Them)
1. No Subsoil Drain Installed
This is the most common issue we see. Every retaining wall over 600mm high should have a proper subsoil drain behind it, often referred to as a “behind-the-wall” drain or an ag pipe.
Best Practice:
- Install a perforated drain coil (ag pipe) behind the base of the wall
- Wrap it in geotextile fabric to prevent clogging
- Lay it on a bed of free-draining gravel
- Ensure it exits to a legal outfall, soak pit, or stormwater system
We include subsoil drainage in all of our retaining wall contractors work unless otherwise specified in an engineering plan.
2. Incorrect Backfill Material
Some contractors or DIY builders backfill with the native soil from the site, especially if it’s clay or heavy loam. This traps water and slows drainage dramatically.
Better Solution:
Use free-draining materials like crushed scoria, builders’ mix, or drainage metal behind the wall. These materials allow water to flow freely down to the ag pipe.
3. No Filter Fabric Between Soil and Backfill
Without a barrier, soil particles gradually migrate into your drainage zone and clog it up. This can completely block the system within a couple of seasons.
Fix:
- Wrap the drainage zone (gravel + pipe) with geotextile fabric
- Fold it over at the top before covering with soil
- This keeps the drainage zone clean and working long-term
4. Improper Drain Outlets or Fall
Even with a subsoil drain in place, it won’t help if the water has nowhere to go. Drains that dead-end or aren’t sloped correctly will eventually back up.
What to Do:
- Ensure at least 1% fall on the drainpipe (1m drop over 100m)
- Outlet the drain to stormwater, swales, or a well-drained area
- Install inspection ports or risers for maintenance access
We handle fall, outfall, and inspection pipe placement as part of our siteworks, making sure your wall drains well not just on day one, but for years.
5. No Surface Drainage Consideration
It’s not just subsurface water that can be a problem, heavy rain, overland flow, and roof runoff can saturate the soil behind a wall if not redirected.
Additions to Consider:
- Surface drains or channel grates at the top of the wall
- Swales or berms to divert surface water away
- Downpipe connections to stormwater
- Tapering the ground behind the wall to shed water outward
Retaining Wall Height and Drainage Risk
The taller the wall, the greater the pressure, and the higher the stakes if drainage is inadequate. 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.
We follow engineering specs closely and integrate drainage planning during the early earthworks for retaining walls phase.
Final Thoughts: Build the Wall Behind the Wall
Most of the critical work in retaining walls happens out of sight, behind the face. Drainage, compaction, and backfill are just as important as the wall material itself.
At Procut Earthworks, we’ve built and prepared dozens of retaining wall sites across Auckland, always prioritising drainage performance. Whether we’re handling the full build or just the excavation and prep, we work to ensure every wall stands the test of time.
