Dealing with subsidence

Subsidence is the excessive movement of the ground supporting a building. If this movement varies from one part of a building to another, cracks may occur.

Subsidence causes

Causes of subsidence include:

• soil compressing under the weight of the house
• soil swelling and contracting due to changes in the moisture content.

Subsidence and footing design

In Queensland, engineers design footings to the Australian Standard AS2870 – 2011 Residential slabs and footings. Footing design and construction involves a number of steps:

• site classification
• selecting the footing system
• structural design
• building in accordance with the required design details and methods
• proper maintenance. 

Choosing appropriate site falls and floor levels should be part of the planning and setting out process during the construction of a new home or renovation. Advice from a qualified Registered Professional Engineer about the installation of an appropriate footing system is essential prior to building your home. Either you or your builder can source this advice. 

Preventing subsidence

• Maintain site drainage 
• Avoid:
  • altering the site drainage provided by the builder. Any drainage changes should direct water away from your house and adjoining properties
  • placing gardens, garden edging or gravel pavements next to your house. These can cause water retention or may change the moisture conditions, particularly on reactive clay sites 
  • installing a sprinkler system next to your house if it is on a highly and extremely reactive site 
  • over watering.
• Plant trees around your house at a distance greater than the fully-grown height of the tree 
• Regularly check and maintain plumbing, drainage and stormwater systems. Immediately repair leaks as well as minor damage and cracking in walls and ceilings.