Geotechnical requirements for subdivision applications
With the introduction of the geotechnical assessment guidelines in 2011, the Council has been requiring geotechnical reports for all subdivision applications.
More information is now available about liquefaction risks including Environment Canterbury’s recent liquefaction hazards study.
Recent changes made to the subdivision guidelines in light of that Environment Canterbury study mean that the Council now has more discretion about what sort of geotechnical information it will require with subdivision applications.
At the subdivision consent stage, the Council has a duty to ensure that there is a stable building platform on all new newly created lots. For a large part of Selwyn district the additional information now available has demonstrated that the land is stable without the need for a geotechnical investigation.
The map of where liquefaction assessment is required in Selwyn District.
Changes to reporting in the low geotechnical risk area
In the orange thatched area shown in the map, damaging liquefaction has been assessed as being unlikely as it has mostly deep gravel soils and, in many areas, also deep ground water levels. The possibility of liquefaction over much of this area is extremely low. In addition, this area is also free of other geo-hazards.
For smaller subdivisions in this area the Council is not going to require, in the first instance (there may be some special circumstances that result in the need for some geotechnical investigations), geotechnical investigation reports for the smaller subdivision consent applications from 1 August 2013.
For larger subdivisions within this area, ie those exceeding 15 lots, a geotechnical investigation will still be required, given the scale of development and size of investment.
In other parts of the district
Geotechnical reporting is still required for all subdivisions outside the orange thatched area.
Potential Liquefaction Zones and Ground Stability Issues
There is potential liquefaction hazard in the eastern areas of the district (Lincoln, Tai Tapu and Prebbleton area), possible slope stability issues along the high terrace faces on the Rakaia and Waimakariri Rivers, and potential slope stability and foundation bearing issues within the foothills and mountainous areas. Geotechnical investigations to address these and any other ground associated issues are needed.
Plan Change Stage
A geotechnical report is also necessary at the plan change stage in any part of the district.
Geotechnical reporting should follow the requirements of the current Ministry of Business Innovation and Employment guidelines and include subsurface testing.