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Canterbury receives three new foundation design categories

Canterbury has been given three new residential foundation design categories following the earthquakes of 2010 and 2011. The categories, and the areas they apply to, are based on ground conditions, including the susceptibility to liquefaction, and the extent of land and building damage caused by the earthquakes. 

Canterbury Earthquake Recovery Minister Gerry Brownlee said experts had undertaken extensive scientific and geotechnical investigation and research into land issues and ways to reduce the risk of injury to people and damage to homes in future earthquakes. 

The foundation designs apply to the flat land in the residential green zone in Canterbury, which has now been divided by engineering experts into three new technical categories (TC1, TC2 and TC3). 

• Technical Category 1 (TC1)

Properties in TC1 (grey) are unlikely to experience significant land damage from liquefaction in future earthquakes. Standard concrete slabs and timber floors are acceptable for foundation repairs or rebuilds. 

• Technical Category 2 (TC2)

For properties in TC2 (yellow), minor to moderate land damage from liquefaction is possible in future significant earthquakes. Lightweight construction, for example corrugated iron not tiled roofs, or enhanced foundations such as more robust floor slabs that better tie the structure together will be required for foundation repairs or rebuilds. 

• Technical Category 3 (TC3)

For properties in TC3 (blue), moderate to significant land damage from liquefaction is possible in future significant earthquakes. Foundation solutions should be based on site-specific geotechnical investigation and specific engineering foundation design where foundation repairs or rebuilds are needed. This might involve deep pile solutions. 

These categories form part of the Department of Building and Housing updated guidance for repairing or rebuilding houses in Canterbury following the earthquakes.

A summary of the DBH guidance for repairing or rebuilding houses following the earthquakes in TC1 and TC2 categories is available on www.dbh.govt.nz/canterbury-earthquake-residential-building