Rebuilding TC3 properties from the ground up

Last updated 05:00 07/05/2012

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Christchurch Earthquake 2011

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Last week the Department of Building and Housing (DBH) released guidelines for repairing and rebuilding TC3 properties. Three categories of foundation were put forward. Michael Wright sits down with DBH chief engineer Mike Stannard to decipher the engineering-speak.

The introduction gives you your first warning. It's even italicised for effect: "The principal users of this document will be professional geotechnical and structural engineers."

The Department of Building and Housing (DBH) guidelines released last Friday will affect most of the 28,000 Technical Category (TC) 3 or "green-blue" homeowners – those thought to be on the most badly earthquake-damaged land repairable.

Their release was a major step in the lengthy and often fraught insurance settlement process of many of those people.

Few though, will get any real idea of how their homes will be repaired or rebuilt through a perusal of the 106-page document.

DBH chief engineer Mike Stannard, one of the guidelines' authors, doesn't apologise for the jargon:

"This is fundamentally for an engineering audience. We've said that right up front."

There are three broad categories of design: deep piles, site ground improvements, and surface structures with shallow foundations.

Deep pilesProbably the most self-explanatory of the three. It involves sinking piles a long way underground to reach stable ground not susceptible to liquefaction or lateral spreading.

Identifying land suitable for deep piling was not as simple as drilling down until you hit solid ground, Stannard says.

"The concern is if you've got a liquefiable layer [above] and a liquefiable layer [below] any piles.

"There needs to be a decent thickness in the layer you're founding on to."

A single-storey house needs at least three metres of solid ground, and a two-storey house needs four metres.

Deep piling will not be an option on land with a major risk of liquefaction or lateral spread.

"[In an earthquake] the ground will have moved sideways but the founding layer won't have. [The house is] going to tend to want to tilt over."

Lidar (light detection and ranging) testing has identified blocks of TC3 land in eastern Christchurch unsuitable for deep piling: all properties east of Anzac Dr, south of Queenspark Dr and north of New Brighton Rd, and all those bounded by Wainoni Rd, Shortland St, Pages Rd, Kearneys Rd, Cypress St, Ruru Rd, McGregors Rd, Pages Rd and Cuffs Rd.

Site ground improvementsThis technique is new to residential housing but has long been used by civil engineers for road building. Land is strengthened by one of two methods: mixing cement with the ground to make it denser, or compacting up to two metres of earth to give a better crust for building on.

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Once treated the land can be piled or have a concrete pad placed on top of it.

Again, it isn't suitable on land with major liquefaction risk. Anywhere with liquefiable soil deeper than 10m is out, Stannard says.

"All you're doing by improving the ground on the surface is minimising the distortion of the land on the top. If you get very deep layers of liquefiable soils you still will get potentially too much vertical settlement."

In badly damaged areas it could be combined with the third foundation category.

Surface structures with shallow foundationsThere will be many variants in this category but they will have lightweight building materials and shallow foundations in common.

Where houses were skirted with a concrete ring beam, a historically popular building method in Christchurch, lighter material such as plywood will be used.

Heavy building materials on unstable land spell trouble, especially if parts of a house sink more than others in a quake.

"That's potentially going to bow the house," Stannard says.

"It puts a lot of extra load into the bearers and potentially damages the superstructure. We're trying to work it so it's only a little bit of packing underneath in the piles."

He likens it to orthodox light timber construction, which builders are familiar with, but with piles shallower than the standard 1-1.2m.

"These ones, we've got them sitting pretty close to the surface so they're not going down through the crust and potentially penetrating [it] to provide a path for the liquefaction material to come up through."

This option may be used on some of the worst TC3 land. A gravel raft will be laid below the piles, which will be tied together with a concrete underslab.

The technique could be combined with some ground improvements in badly hit areas as well, Stannard says.

"In some of the worst areas that may be a good option. You do your ground treatment and then you put a lightweight structure on top."

The guidelines are available on the DBH website but are not finished.

Stannard concedes there was time pressure to get them out, as evidenced by one page of the document which lists 10 areas where more work is likely to be needed.

The department is open to builders and engineers developing new techniques.

"We're not wanting to exclude innovation," Stannard says.

"It gives people a chance to say 'we've got a better way of doing it than that'. Well, let's have a look at it and see if we can standardise that as well."

t3 foundations

- The Press

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