Ag Pipe Behind Retaining Walls: Installation & Best Practice
Agricultural pipe — commonly called ag pipe or aggi pipe — is the backbone of retaining wall drainage systems across Brisbane. This simple but essential component prevents water buildup behind walls, protecting the structure from the damaging effects of hydrostatic pressure.
What Is Ag Pipe?
Ag pipe is a flexible, corrugated plastic pipe with small slits or perforations along its length. These openings allow groundwater to enter the pipe while keeping soil particles out. The collected water is then channelled to a discharge point such as a stormwater drain, natural watercourse, or soakaway pit.
Why Ag Pipe Is Essential Behind Retaining Walls
Without ag pipe, water trapped behind a retaining wall creates hydrostatic pressure that can exceed the wall’s design capacity. In Brisbane’s climate, where heavy downpours can deliver 100mm or more in a single event, proper retaining wall drainage is non-negotiable. Failed drainage is the number one cause of retaining wall collapse.
Correct Installation Method
Proper ag pipe installation follows a specific sequence:
- Step 1: Lay geotextile filter fabric in the trench behind the wall, leaving enough excess to wrap over the top of the drainage layer
- Step 2: Place a 50mm bed of 20mm clean gravel on top of the fabric
- Step 3: Position the ag pipe (minimum 100mm diameter) at the base of the wall with the slots facing down. Maintain a fall of at least 1:100 toward the outlet
- Step 4: Cover the pipe with 200mm to 300mm of clean drainage gravel
- Step 5: Fold the geotextile fabric over the top of the gravel layer
- Step 6: Backfill with compacted soil
Common Mistakes to Avoid
- Using pipe that is too small — always use 100mm minimum
- Installing slots facing upward, which allows silt to enter
- Omitting geotextile fabric, leading to gravel contamination and pipe blockage
- Failing to provide adequate fall toward the outlet
- Not connecting the ag pipe to a legal discharge point
- Using garden soil instead of clean gravel around the pipe
When to Consult an Engineer
For walls over 1 metre, a retaining wall engineer should specify the drainage design as part of the structural engineering. This ensures the drainage system is adequate for the specific soil conditions and expected water volumes on your site.
Ensure Your Drainage Is Done Properly
Drainage is too important to leave to chance. Connect with licensed retaining wall builders in Brisbane who install comprehensive drainage systems with every retaining wall they construct.