Soil Investigation

A report into the soil of your property is literally a report into the foundation of your building project.

If you are building a house, an extension, an attached garage or even just making significant structural changes to an existing one, you’ll need a soil report as a requirement of the Building Permit.

Before you lay the physical foundation, your plans should take into account the soil of the location.

  • Is it stable?
  • How much weight can it bear?
  • Will it shift under the weight of the building?

The major piece of information to answer these questions is usually referred to as the soil 'classification'.

In Australia, the classification of soil is based on the average amount of shifting, ‘surface movement’ or ‘reactivity’. This is generally the fault of the clay component of the soil which is easily affected by rain, damp and dry.

The more ‘reactive’ the soil is, the more precision is required in the foundation work to compensate.  If the foundations are poor, the building will move and shift with the soil.  A moving, shifting building is a building starting to tear itself apart, and is a constant drain on your finances to repair.

The table below provides measures of soil surface movement from the Australian Standard.

Soil Classifications taken from the Australian Standard 2870 Tables 2.2 & 2.3

Class & Foundation


 

Characteristic Surface Movement

Class A:  A-Grade Soil

Most sand and rock sites with little or no ground movement from moisture changes

0mm (Negligible Movement)

Class S:  Slightly Reactive

Slightly reactive clay sites, which may experience only slight ground movement from moisture changes

 0-20mm

Class M:  Moderately Reactive

Moderately reactive clay or silt sites, which may experience moderate ground movement from moisture changes

 20-40mm

Class H1:  Highly Reactive

Highly reactive clay sites, which may experience high ground movement from moisture changes

 40-60mm

Class H2:  Highly Reactive

Highly reactive clay sites, which may experience very high ground movement from moisture changes

 60-75mm

Class E:  Extremely Reactive

Extremely reactive sites, which may experience extreme ground movement from moisture changes

 >75mm

Class P:  Problem Soil

Sites which include filled sites (refer to AS 2870 2.4.6), soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.

 N/A (Highly Unstable or Abnormal)

Here in Monash, for example, soil tends to have a strong clay component. As such it is often unstable, representing Class S to Classes: H1 and H2 soil.  

Class A is A-Okay and tends to be literally rock steady.

Class P soil on the site of where you want to build means a serious problem.  Class Problem soil is so named because it does not easily fall into one of the soil types, and requires a specialised solution from a structural engineer.

As a general rule, building sites with soil classified as A to M can have generic foundations. However, anything more than Moderately ‘reactive’ is going to require a personal evaluation by a fully qualified structural engineer; and the production of a detailed geotechnical report.

A full geotechnical report provided by a qualified engineering geologist or geotechnical engineer will not only provide you with a classification of the soil where you want to build but building strategies for dealing with it.

If you plan to build on sloping land, and/or land known to fall under a flooding overlay; a geotechnical report may be vital.

Among other things a full geotechnical report may specify:

  • The design and quality needed for an underlying concrete slab
  • Quantity and type of stumps or underpinning needed
  • Any soil contamination
  • Stability of the slope
  • Assessment of any current cracking or damage due to movement on a current building

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Last updated: 21 October 2015