Geotechnical engineering is a branch of engineering concerned with the analysis, design of foundations, slope stability, retaining structures, and other systems that are made or are supported by soil or rock. A good definition is “Geotechnical engineering is the science that explains mechanics of soil and rock and its applications to the development of human kind. It includes, without being limited to, the analysis, design and construction of foundations, slopes, retaining structures, embankments, roadways, tunnels, levees, wharves, landfills and other systems that are made of or are supported by soil or rock. Everything you see around you is supported by soil or rock. Geotechnical engineers are responsible for that. Anything that is not supported by soil or rock, either floats, flies or falls down.”

It is the process in which the physical properties of the sites underlying soil or rock are assessed by types of ground testing, to provide information on the grounds physical properties to provide information for the design of foundation and recommendations to develop the site.

A geotechnical report advisers you on the geological hazards and potential ground risks and constraints to develop your site, such as soft ground, slope stability, settlement risks, site excavations, liquefaction potential etc. A geotechnical report is often required to support subdivision and building consents applications. The geotechnical report is also often a small cost of the overall development, and usually pays for itself in providing for an informed foundation design.

This is a hard question to answer, as each site is different and the scope of geotechnical services is tailored to reflect the site conditions and size of project. As a guide only, for a flat residential site, costs can range between $1000-$2000. For a steep residential site the costs can can start from $2000 upwards. We recommend getting a number of quotes to compare.

The expiry date often stipulated by Councils in New Zealand is 10 years. On a geological time scale this is insignificant, however the report may be outdated for other reasons:

• Firstly engineering ​best practices, codes and standards may have changed.
• If the site has undergone significant natural or man made change within that period.
• A change of engineering consultancy, the new consultancy may not want to use third party information for risk purposes.

This doesn’t automatically mean the site investigation or geotechnical report has to be carried out again, a review and updated advice may be suitable, but should be assessed on case by case basis. ​