Structural damage presented by flooding in Queensland

Photograph: Blake Bodycote/EPA per Guardian story link below.

Structural damage resulting from flooding events in Queensland.

Aside from the debris and mud laden sludge that follows a severe flood event what serious structural problems may develop for a home building in Queensland?

Earlier Queenslander style colonial houses that are supported on stumps can be subject to uneven settlement following flood events.  These early period homes were originally constructed with buried timber post stumps hand placed into a hand excavated hole and backfilled with rammed soil backfill all by hand.  This construction technique was applied without site soil testing with mixed results as one would expect – one stump depth may not suit all home sites.

A flood can soften the bearing conditions at the base of a stump to precipitate bearing failure.  Later replacement with precast concrete stumps (which are heavier) may also contribute to marginal stump support conditions shifting into a bearing failure mode (large settlement 50-100mm+).

Modern Queenslanders style homes with bored concrete piers would not suffer from these earlier construction techniques.  Likewise the required site soil testing should have alerted the designers to potential problems with soft or very reactive soil conditions if present.

Raft-slab foundations are the most common form of foundation in use since the mid 1980s.  Flooding has the potential to exacerbate poor drainage conditions may be present at the site.  Persisting damp conditions at localised locations adjacent to a side of the house foundation may cause uneven ground movement and attendant wall or floor slab cracking.  Good even drainage is recommended to get the best performance and durability out of a raft slab foundation.

Plywood clad wall bracing if affected by flood water should be inspected to ensure the sheets are not adversely delaminating.  Moisture has been known to affect cheap imported plysheets due to the poor quality adhesives using during their manufactured off-shore.  Newer homes could be at some risk here.  Buying quality QLD manufactured ply offers some better assurance.

Repairing Flood Damage - CSIRO Discussion and Fact sheets

Guardian - Australian military battle floods