Retaining wall drainage in Australian residential construction

What it is

A retaining wall holds back a mass of earth where the natural ground surface has been cut or filled to create a level building platform driveway pool surround or landscaped step. The wall transfers the lateral earth pressure of the retained mass into a footing or piles. AS 4678-2002 Earth-retaining Structures sets the design loads load combinations and durability classes for retaining walls in Australia. Residential timber masonry concrete and segmental block walls all fall under it.

Drainage is the second half of the design. The wall is engineered against soil pressure plus a small allowance for moisture. If groundwater can sit behind the wall the wall also picks up hydrostatic pressure. Hydrostatic load can double or triple the design load. The wall does not need to fall over to fail. Cracks render the wall non compliant under AS 4678 even where the wall still stands.

The drainage assembly

A compliant retaining wall drainage system has four components. Each one has to be present. Each one has to be installed in the right sequence.

Subsoil drain at the heel

A slotted agricultural drain (aggie pipe) sits behind the wall at the level of the footing or just above. The pipe runs the full length of the wall and discharges to a legal point of discharge. The pipe is wrapped in a geotextile sock to keep fines out of the slots. It is bedded in single sized aggregate not crushed rock with fines.

Free draining backfill

The first 300 mm to 600 mm of fill against the back face of the wall must be free draining material. 20 mm single sized aggregate or 7 mm screenings are the common choices. Where the retained soil is reactive clay the drainage zone widens to the full height of the wall.

Weep holes

Masonry and segmental block walls need weep holes at the base of the wall typically at 1.0 to 1.2 metre centres. The weep hole lets any water that gets past the aggie drain out the front face. A wall without weep holes will build up pressure even where the rear drain is present.

Geotextile separation

A geotextile filter fabric sits between the natural soil and the drainage aggregate. Without the geotextile the fines from the clay migrate into the aggregate and clog it inside a few seasons. A clogged drain is a missing drain.

What AS 4678 actually requires

AS 4678 sets three durability classes for retaining walls based on consequence of failure and design life. Class A is where failure poses risk to human life or significant damage to property. Class B is where failure poses moderate risk. Class C is where failure poses low risk. Most residential walls over 1 metre fall into Class B or Class A.

The standard requires the design to consider surcharge loads from adjacent slabs vehicles trees and structures. A driveway sitting one metre back from a 1.5 metre high wall adds a vehicle surcharge the wall has to be designed for. A residential pool deck on the high side adds dead load plus pedestrian surcharge.

The NCC references AS 4678 indirectly through Performance Requirement P2.1 in Volume Two which requires that a building be designed to sustain the most adverse combination of loads. Where a retaining wall supports or is supported by a Class 1 dwelling it falls inside the NCC envelope.

How retaining wall drainage fails

The failure modes are predictable. The aggie drain was laid without geotextile and silted up inside two summers. The free draining zone was 100 mm wide when the drawing called for 600 mm. The weep holes were filled with mortar drips during construction. The discharge point was the lawn not the stormwater system so water ponded at the base of the wall.

A wall that tips forward after five years is rarely a wall that was structurally underdesigned. It is a wall where the drainage failed and the wall picked up a load it was not built to take.

What an audit looks for

A TradeLens audit on a retaining wall pulls the engineer drawing and matches site photos against it:

• Aggie drain photo before backfill at the correct level along the full wall.
• Geotextile wrap photo showing the sock plus the separation layer.
• Backfill photo showing the drainage zone width matches the drawing.
• Weep hole photo at the base course before render or tile.
• Discharge point photo showing legal connection to stormwater.
• Surcharge zone marked on the as built drawing.

A retaining wall that fails inside the warranty period is almost always a wall where one or more of these photos cannot be produced.