(4th November 2011)
Three new categories for residential foundation design required for repairing and rebuilding homes in Canterbury following the earthquakes of 2010 and 2011 was announced on
Friday 28th October 2011. The full media release can be read here http://www.beehive.govt.nz/release/new-foundationdesign-categories-canterbury.
Basically the green zone is now split into three ‘technical categories’. Each category has different requirements regarding foundation design. When processing building consents that include foundations, get into the habit of finding out which technical category the building site is in. This can be done by checking the address on http://landcheck.org.nz/ or to get an idea of areas the new technical categories cover you can use this map and zoom into specific areas http://apps.egl.co.nz/cera/landcheckviewer/Map.htm.
Also the Department of Building and Housing has also released a preview document to the update of the Guidance on house repairs and reconstruction following the Canterbury
Earthquakes which can be found at this link http://www.dbh.govt.nz/UserFiles/File/Publications/Building/Guidanceinformation/pdf/guide-canterbury-earthquakes-preview.pdf.
Please note: This information may be superseded once the new DBH guidance document on House Repairs and Reconstruction is released.
More details on categories
TC1 Area (Grey)
Foundation Options – Technical Category 1 (TC1): Future land damage from liquefaction is
unlikely and ground settlements are expected to be within normally accepted tolerances.
Standard foundations (NZS 3604) are acceptable subject to shallow geotechnical investigation.
•
Options For New Foundations – NZS 3604:2011 timber piles and floor, OR, tied concrete
slabs. Slab on ground floor to follow NZS 3604:2011 as modified referencing B1/AS1 requiring
ductile reinforcing and tied slab.
•
Options For Foundation Repairs – Refer to December 2010 DBH guidance document
“Guidance On House Repairs And Reconstruction Following The Canterbury Earthquake”.
Shallow Geotechnical Investigation For Technical Category 1 (TC1):
Unless being carried out
as a specific design by an appropriately qualified geotechnical engineer, shallow subsurface
investigations to determine the suitability and bearing capacity of the soil shall be carried out by
a soils technician or other suitability qualified person under the guidance of a CPEng qualified
engineer. Shallow subsurface investigations to determine the suitability and bearing capacity
of the soil shall follow the procedure as generally outlined in NZS 3604:2011, with the following
exceptions:
•
While the prescribed depth of investigation of 2 metres is typically acceptable, it is
recommended that 50mm diameter boreholes for the examination of soil materials extend
further, to between 3m and 4m below ground level (where practical). Significant areas of
Canterbury are underlain with organic peat deposits and it is important to check for the
presence of these materials.
• “Soft or very soft peat” in the defined exclusions from ‘good ground’ is to be replaced with
“peat’ in the list of unacceptable materials.
•
For foundation options 1 – 4 in Section 5 of the December 2010 DBH guidance document
“Guidance On House Repairs And Reconstruction Following The Canterbury Earthquake”
SCALA blows per 100mm shall be minimum 2 blows (i.e. 50mm per blow) for ground deemed to have 200kPa geotechnical ultimate bearing capacity. For other foundation types 300 kPa
will need to be confirmed in accordance with NZS 3604:2011.