Site Investigation

As part of the design and approval process of building your new house you will require a ‘Site Investigation Report’ sometimes referred to as a ‘Geotechnical Report’.

For a typical suburban subdivision for a one or two storey house on a standard block around 800m2. these cost in the order of $1,500 – $2,000,

What They Do

The site investigation company will normally send out a truck mounted drilling rig to site.

The rig operator will have a look at the site for any potential problems and drill three holes to a depth of about 2 – 3m, or until they hit rock.

At least one of these holes will include any problem areas identified from the visual inspection such as wet areas and disturbed ground.

For larger blocks, larger houses, and houses with more complex structures (e.g.underground garages) more boreholes, deeper bore holes, and more testing will be required which will all add to the cost.

Samples of the different materials encountered will be classified to be able to assist in assessing the strength of the material

The Results

A report will then be prepared based on the tests, and any previous information about soils in the locality.

The Report will give recommendations about foundations for structures on the site.

The key part of  the recommendation is usually a foundation classification  as this can adds tens of thousands of dollars to the cost of the build

Limitations on Report

A couple of things to remember about these reports are:

  • The recommendations assumes that the boreholes accurately reflect the condition for the whole site.
  • The ground can be exceptionally variable and the borehole are testing much less than a hundreth of one percent of the soil that the house will be standing on. (During excavation the builder may encounter worse conditions that need a stronger foundation with additional costs)
  • If the site has a slope which requires cut and fill the foundation may need to be to a higher classification than the report recommendations . . .  at an additional cost.


For more posts related to land see Blocks


Soil Classification

Before you can build your new house you need to know what sort of foundation is needed, which is based on the ‘Soil classification’

Geotechnical Investigation

A Geotechnical investigations is required to provide a report stating the soil classification.

The Investigation,and the Report should be in accordance with the following Australian Standards;

  • AS2870-1996: “Residential Slabs and Footings – Construction”
  • AS1726-1993: “Geotechnical site investigations”

Standard Classifications

The classification of the site is based on the expected movement of the foundation soils – generally related to the capacity of the soil to shrink or swell.

Your site should be in one of the following classifications:

Class A
Mostly sand and rock sites, with little or no ground movement expected.(see these links: Sand & Rock)

Class S
Slightly reactive clay sites. Only slight ground movement from moisture changes expected.

Class M
Moderately reactive Clay or Silt sites which can experience moderate ground movement from moisture changes (See this link: Building on Clay).

Class H
Highly reactive clay sites. Can experience high ground movement from moisture changes.

Class E
Extremely reactive sites. Can experience extreme ground movement from moisture changes.

Class P
A problem site. This can includes soft soils, such as soft clay or silt, varying depths of Fill (see this link: Fill), loose sands, landslips, mine subsistance, collapsing soils, soils subject to erosion, reactive sites subject to abnormal moisture conditions, or sites which cannot be classified otherwise.

Added category ‘D’

Soil types M, H, and E may also have an added classification of ‘D’. This indicates deep seasonal moisture variation which can mean significant expansion and contraction.
For example, from a dry to a saturated state

  • Class M-D may move up to 40mm,
  • Class H-D 40mm to 70mm
  • Class E-D can move more than 70mm, (up to 250mm has been found in some cases)

Why the report may increase the cost

If you are looking at a Project Builders its worth being aware that their standard price will be based on either an S or M class foundation.

Usually you will be lucky if the foundation is in this range………so you could be up for additional costs.

See the following link to understand why a classification from the developer may be better than the one from the builder: Different Soil Classification Results


 Lots more information in the anewhouse Guide to Buying a Block for only $4

For more posts related to land see Blocks