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Department of Justice

Problem building sites, reactive soils and subsidence

Many Tasmanian homes have suffered structural defects resulting from problem soil conditions.  These problems could have been prevented had the owner, building designer or the builder undertaken a thorough site investigation and site preparation prior to building. Extra costs associated with building on problem sites can be minimised by proper investigation (including a site and soil classification)  before the site-specific design is commenced.

Reactive soils

Reactive clay soils shrink when dry and swell when wet and this movement can cause subsidence or heave, with resultant building damage. Typical damage includes diagonal cracking of masonry walls, slab heave, and windows or doors are often difficult to open. Proper compaction of structural earth fill is also required to prevent subsequent settlement and damage to buildings, driveways and paths. 

In some localities landslips or former mining activities can also cause subsidence.

Soil classification for footing design

Where building work includes footings, the building designer and building surveyor will require a site soil test. Tests will establish the capacity of the site to bear a building and will provide guidance to the designer on the type of footing or slab needed and the number of storeys that could be safely supported.

Soil tests for foundation classification in accordance with the Australian Standard AS 2870 - 2011 Residential slabs and footings are performed by:

  • a Geotechnical Engineer accredited as an Engineer-Civil; or
  • a Soil Scientist; or
  • an Engineering Geologist.

Soil tests may also be used as part of a site evaluation for installation of an onsite waste-water management system in an unsewered area.

Contaminated or undrainable sites

If a council Environmental Health Officer decides that land is unhealthy, contaminated, unsuitable for its intended purpose, or cannot be suitably drained, the general manager may require that the land be cleaned, remedied or drained to their satisfaction before any building work can commence on site.

A person must not erect or place on land a building with habitable rooms, from which effluent normally flows, unless that effluent can be drained to a sewerage system or an onsite waste-water management system.