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 these cost in the order of $1-2,000,…………… but what is done?… and what do you get?

  1. 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.
  2. Samples of the various materials encountered will be classified to be able to assist in assessing the strength of the material
  3. A report will then be prepared based on the tests, and previous information about soils in the locality. This will give recommendations about foundations for structures on the site.
  4. The key part of  the recommendation is usually a foundation classification .

The above procedure refers to a typical one or two storey house on a standard block around 800m2. 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.

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. Unfortunately soil 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. If during excavation worse conditions are encountered the foundation mzy need to be strengthened at additional cost.
  • If the site has a slope which requires cut and fill the foundation may need to be stronger than the report recommendations at an additional cost.


For more posts related to land see Blocks