Soil Investigation
If you are building a house, an extension, an attached garage or even just making significant structural changes to an existing building, a soil report is a requirement of a building permit.
Before you lay any physical foundations, your plans should take into account the soil at 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 and can start to crack.
Soil Classifications
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)
|
In the City of Monash, soil tends to have a strong clay component. So, it is often unstable, representing Class S to Classes: H1 and H2 soil.
Class A is top class and tends to be rock steady.
Class P soil means a serious problem. Class Problem soil does not easily fall into one of the soil types, and requires a specialised solution from a structural engineer.
Getting a Soil Report
As a general rule, building sites with soil classified as A to M can have generic foundations. But anything more than moderately ‘reactive’ will need 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 provide you with a classification of the soil where you want to build and building strategies to deal 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.
A full geotechnical report may also 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 building