The green zones

Most of us are Red or Orange, however there is a small portion of Green. Being Green is complicated by the varying risks of liquefaction requiring three Green Zone or TC (Technical Category) classifications. Anyone in our area who goes from Orange to Green will be in TC3.

Wherever you live on the flat (i.e. not the hill suburbs) you can find your TC by clicking on one of these categories:

• Northern Christchurch (Kaiapoi, Pines and Kairaki Beach, Brooklands, Spencerville, Belfast, Styx, Redwood, Casebrook, Northcote, Parklands, Queenspark, Waimairi beach)
• Southern Christchurch (Opawa, St Martins, Beckenham, Somerfield, Sydenham, Spreydon, Hoon Hay, Hillmorton, Halswell, Hornby, Prebbleton, Lincoln, Tai Tapu)
• Central Christchurch (all the inner suburbs not covered by the two previous maps)

The following is extracted from the Department of Building and Housing brochure on the subject. The technical categories only apply to houses that require their foundations to be repaired or rebuilt due to damage from the earthquakes. If your house was not damaged by the earthquakes then you will not need to upgrade your foundations. The full brochure is available from the CERA website here.

TC1 grey

Future land damage from liquefaction is unlikely. You can use standard foundations for concrete slabs or timber floors. Foundation requirements changed in 2011 and information is available on the Department of Building and Housing’s website at: www.dbh.govt.nz/canterbury-earthquake-residential-building

TC2 yellow

Minor to moderate land damage from liquefaction is possible in future significant earthquakes.

You can use standard timber piled foundations for houses with lightweight cladding and roofing and suspended timber floors Or enhanced concrete foundations – i.e. more robust floor slabs that better tie the structure together as outlined in the Department of Building and Housing 2010 Guidance on house repairs and reconstruction following the Canterbury earthquake www.dbh.govt.nz/canterbury-earthquake-residential-building

Homeowners whose land is in TC2 can get on with rebuilding their homes with confidence. The only further site-specific geotechnical investigation required is the simple shallow soil strength testing which is standard for all homes.

If you are carrying out repairs to foundations or other areas of you home these should be carried out in accordance with the Department of Building and Housing’s 2010 Guidance on house repairs and reconstruction following the Canterbury earthquake (NOTE: an emended version will be issued in November)

The average additional cost for homeowners in Technical Category 2 with foundations that need to be repaired or rebuilt due to earthquake damage is expected to be around $5,000. However, depending on site circumstances costs may range from no additional cost to around $10,000.

TC3 blue

Moderate to significant land damage from liquefaction is possible in future significant earthquakes. Site-specific geotechnical investigation and specific engineering foundation design is required.

For houses that have earthquake damage that do not involve repairs to foundations, repairs should be able to proceed without geotechnical assessments. For houses requiring foundation repairs or new foundations, homeowners in Technical Category 3 (TC3) will need site-specific geotechnical assessment and specific engineering foundation design.

If you need to rebuild or carry out repairs to foundations and your house is in Technical Category 3 (TC3) you are advised to talk to your insurer in the first instance. Your insurer should be able to arrange for a geotechnical engineer to carry out an assessment of your property as part of your claims process.

Costs will vary from property to property. The Department of Building and Housing is conducting a research trial to test a range of innovative foundation solutions for houses in Technical Category 3. It will develop further guidance for properties in TC3 when it has tested these foundation options.

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