Retaining walls - Today Tonight horror stories from Brisbane

This reinforced keystone wall was built without the reinforcement geogrid

Retaining walls exposed on Today Tonight Channel 7

Whilst conducting a structural inspection on a sub-standard failing retaining wall at a property north of Brisbane Friday last week, the client mentioned watching this program story from Today Tonight (see link above).

Property managers and owners should be aware of the risks retaining walls can pose to their structures.
Domestic pre-purchase building inspectors typically write-out in their disclaimers ."..the condition of retaining walls that may be present at this site are outside the limits of our inspection expertise.  We refer the buyer to seek further advice.."  This note may be embedded in the reports back page fine print and more often than not its passed over or noted and then just ignored.



Attached is my top four tips for considering the risk of retaining walls;

1. Height and proximity (approval status with local council - worth checking on)
2. Build quality and intent
3. Changing site conditions
4. Drainage concerns and flow paths

A failure zone can extend from the top of the wall back to a distance equal to the height of the wall.  Likewise a slumping failure could threaten structures within the height of the wall.

Boulder walls built in the recent past (90's onward) about South east Queensland have been set using round un-split rocks.  Dry stacking these round marble like stones is very difficult to achieve for a stable wall.  Of late (mid 2000's) split face rocks have been used to better effect.

The basement retaining wall attempted at the start of the TV story appears to be set with a cantilevered reinforced concrete block wall.  These walls will allow some rotation at the top of the wall which appears to be unaccounted for by wall construction details used.  An inadequate footing perhaps unintended for the role of a retaining wall footing may have allowed the wall to rotate over and is now being temporarily propped against the adjacent floor slab.

Maintaining good drainage conditions above and at the base the wall is critical.  Most wall failures are precipitated following periods of sustained wet weather.  Water running off the top or down the face of the wall is not recommended.