Rebuilding Christchurch’s central business district will be “complex and challenging”, according to an official report into February’s earthquake released last week.
This is because of a combination of unstable ground conditions and structural disruption resulting from the 6.2 magnitude quake in the Central Business District.
A geotechnical report commissioned by the Canterbury Earthquakes Royal Commission into February’s earthquake said liquefaction during the quake affected 20m to 25m of top soil.
It is still susceptible to liquefaction and this, combined with the high water table, makes
the ground less stable and more difficult to build on.
Ground investigations will be much more comprehensive in the future because the type of soil changes rapidly over short distances, says the report.
The Institution of Professional Engineers New Zealand (Ipenz) practice manager Cameron Smart told NCE New Zealand’s building code has already begun to adopt changes.
“Soils prone to liquefaction or lateral spread in Christchurch are now disqualified from being ‘good ground’,” said
Smart, meaning that in future more detailed ground investigations will be needed for new projects.
The report also recommends that construction of shallow foundations be accompanied by ground improvement work. No changes to deep foundation designs are proposed because such substructures performed well during the quake.
Another of the commission’s reports, also published this week, doubled the requirements for some types of structural retrofit.
It said unreinforced masonry buildings must be retrofitted to 67% of New Zealand’s building code - or be resistant to at least two thirds of the loads demanded by the earthquake design code - instead of 33% at present.
Ipenz has always advocated 67% building code compliance (NCE 3 March) according to the report’s joint author University of Auckland associate Professor Jason Ingham. But he said building owners saw this as an extra burden.
Official estimates put the retrofitting bill to upgrade all of New Zealand’s 3,867 unreinforced masonry buildings at NZ$2bn (£1.03bn).
A further recommendation is that the improvements should be managed at national level rather than being left to territorial authorities to draft and monitor their own individual policies as they do now.
Ingham said this would help eradicate differences in how local authorities deal with upgrading building stock.
Both reports are out for consultation and are currently being peer reviewed. The Royal Commission will present its final conclusions next February.