TC1, TC2, TC3, the ground, graded
The red zone answered whether a suburb could stay. For the land that stayed, the 2011 grading answered a quieter question: what does this ground ask of the foundations put on it? Three verdicts over 9,603 hectares of flat residential Christchurch, 296 mapped areas, drawn as the council still publishes them.
The grades, drawn
TC1: standard foundations · TC2, enhanced foundations · TC3, site-specific engineering. The grading ran over the flat residential land; the hills and the land outside the zoning carry no grade, and the red zone sits under it all at /redzone.
What each grade meant
TC1, Technical Category 1, Grey. Future land damage from liquefaction is unlikely. The guideline: Standard foundations for concrete slabs and timber floors acceptable. Minor additional foundation works may be required eg more reinforcing. 1,651 hectares in 29 mapped areas.
TC2, Technical Category 2, Yellow. Minor to moderate land damage from liquefaction is possible in future significant earthquakes. The guideline: Standard timber piled foundations or enhanced concrete foundations. 5,768 hectares in 157 mapped areas.
TC3, Technical Category 3, Blue. Moderate to significant land damage from liquefaction is possible in future significant earthquakes. The guideline: Site-specific geotechnical investigation and specific engineering foundation design required eg deep pile foundations, innovative ground strengthening, etc. 2,185 hectares in 110 mapped areas.
Questions, answered briefly
The 2011 grading of the residential land that survived, the green zone’s internal verdict on what each patch of ground asks of a foundation. TC1 means standard foundations are acceptable; TC2 means enhanced timber-piled or concrete foundations; TC3 means site-specific engineering because significant land damage is possible in a future large shake.
2,185 hectares across the mapped TC3 areas in the layer the council still publishes, against 5,768 ha of TC2 and 1,651 ha of TC1. The grading ran over 296 mapped areas of flat residential land in all; the hills and the land outside the zoning carry no grade.
It still shapes how foundations are engineered, the categories were folded into the district plan’s foundation rules after the shakes and the published layer is still the council’s record. Whether a specific site needs TC3-style engineering today is a question for a geotechnical engineer, not a map.
The east and the river margins, the same ground the February survey mapped for liquefaction and the red-zone line crossed: the Ōtākaro Avon’s banks, the estuary edge, and the low eastern suburbs. The ground that earned the grade is drawn patch by patch at /liquefaction.
Also in the earthquakes
- The sequenceevery shake since 2010, counted
- Earthquakes, livethe ones that just landed
- The faultsthe traces under the plains
- Liquefactionthe ground that moved
- The red zonethe land the Crown wrote off
- The rebuild’s receiptsSCIRT’s photograph archive
- The earthquakes, explainedwhat happened and why
- Liquefaction, explainedwhy the east failed
- The rebuild, explainedthe machinery that put it back
The record behind this
Christchurch City Council’s open land layers: QuakeFoundationDesign, the CERA technical categories of 2011, kept as the council publishes them and simplified 30 m for size. A category is the 2011 engineering verdict for a patch of ground, not a finding about any one address’s foundations today, that question belongs to a geotechnical engineer. The ground that earned the grades: /liquefaction; the line drawn over the worst of it: /redzone; the shakes and the fault: /sequence and /faults.
This page as markdown: /technical-categories.md · the grades as data: /technical-categories.json · /technical-categories.geojson · on the live map: /map?layer=tc · the machinery around it all: the rebuild.