What do the Technical Categories mean for you?

Posted 17 Oct 2012 by GovtDBH Popular

Dave Kelly, Director of the Canterbury Rebuild for the Building & Housing Group explains the technical categories.

Why and how were they created, and what do they mean for you, whether you live in technical category 1, 2 or 3? 

Duration:  9.47 minutes


Hi, my name is Dave Kelly and I’m here to talk about the technical categories within the Green Zone in Christchurch and what they mean for you and why we’ve created them.

Why were they created?

The technical categories are within the Green Zone, they are based on investigation needs for the soil types, particularly focused on the types of foundation that are needed in the different technical categories, known as TC1, TC2 and TC3.

So why did we create these technical categories?

The reason was to give a guide to the geotechnical engineers on the level of investigation required on a particular land so that you get the right solution for your property. It is also a guide for the insurance companies on the level of investigation and the appropriate foundation solution for your land.

Foundations that are designed and built to the right standard will give you confidence that your house will perform better in any future event. If your house is repaired, the foundations are built properly, they’re done once and they’re done well, they will give information both to potential buyers about the quality of the rebuild, they’ll also give you confidence as the homeowner that the redesign/rebuild of your foundations are done to the right level, without cutting costs.

By creating the technical categories, we are able to target those areas that need a greater level of investigation and a higher level of design. If we didn’t have these, we would have a ‘one size fits all’ area where every property potentially would need geotechnical investigation to a higher level and it would make it difficult both for the consenting authorities to know what was required, but also over-design so many more properties than we need to.

By creating these categories, it means that 80% of properties can get on with their repairs without needing detailed geotechnical investigation. It also means from a time scale we won’t have years potentially of further delay while unnecessary geotechnical investigation and design is undertaken.

How did you work out the boundaries of each category?

So let’s have a look at what these categories mean and how we got there.

The areas are based on the best geotechnical information currently available. Experts have for a long time understood what liquefaction means and its effect. However, for most lay people this is something new and New Zealand has never experienced liquefaction on this level – in fact, it could be argued that this is internationally a unique experience.

We now know a lot more about the land in terms of ground water level in certain areas, the quality of the land – whether it’s sand, silt, peat, clay.

There are tonnes of gravel in some areas which allow for easy rebuild because the land is regarded as good. However, some of those other areas that have thick layers of peat or silt mean that we have to do more intensive geotechnical investigation.

Who gathered the evidence about the land?

Geotechnical and structural engineers have examined the data, ground water levels, the types of soils, how deep the layers are, what sort of crust sits over the top of land and how thick that is.

I’m in TC1 – what does this mean?

So now let’s move to what each of these technical categories mean.

Technical category one is land that’s described in New Zealand standards as good ground. You may still need a simple soil test and you’ll be able to use standard foundation solutions that are used across New Zealand. So in this case there is nothing to stop you getting on with the rebuild – simply talk to your insurer and you should be able to proceed.

I’m in TC2 – what does this mean?

The second category is TC two. This is where there is a risk of minor damage of liquefaction during an earthquake. In this case, standard soil tests are required. These are the sorts of standard tests that are used across New Zealand. In this case, the options are to use a standard timber piled foundation with suspended timber floors or enhanced concrete foundations.

I’m in TC3 – what does this mean?

Now let’s talk about TC3. This is land where there is moderate to significant risk of liquefaction. EQC has an area wide drilling programme to determine the extent of TC3 land. It will help engineers to decide which foundations are right for your situation. If you want more information, contact EQC or your insurer. The information gathered from this drilling programme may be enough to determine that no further drilling is required or onsite geotechnical investigation is required. However, the geotechnical engineer will need to make that decision based on the information available. If they decide that further information is required, then they will undertake or arrange to have undertaken further geotechnical investigation on the particular site. They will then be able to advise on what is the best technical foundation solution for your land. Your insurance company or EQC can help you to manage this process.

Why not just put TC3 land in the Red Zone?

There’s some confusion about the difference between TC3 properties and Red Zone properties. Red Zone properties are those areas that are deemed either uneconomical or unfeasible at this time to rebuild. TC3 land on the other hand is economic and is feasible to rebuild, provided that the right foundation design is undertaken, your properties will perform at a similar level to those other properties in the Green Zone either in Canterbury or across New Zealand.

Are individual site investigations a new thing?

Designing foundations to suit the land isn’t in fact new. In Canterbury, areas of the Port Hills have required specific geotechnical investigation and foundation design, areas on the flat within Canterbury with peaty soils in the past have required specific design, and elsewhere in Auckland expansive soils have required specific design and geotechnical investigation.

I haven’t had any foundation damage – why am I in TC3?

There have been a number of queries around what do I need to do if I’m in TC3 land but I’ve had no foundation damage but I do have some other repairs to my house required. In this case you don’t need to undertake any further investigation of the site, because your foundations have not been damaged, you can simply proceed to repair your house, and you just go through the normal consenting process both with the Council and agree with your insurer or EQC about how to go through that process.

Just because you haven’t seen liquefaction manifested on your property does not mean that there isn’t liquefaction potential still in those areas. However, if further geotechnical investigation indicates that a TC2 foundation is sufficient, this can be used as the basis of a consent application – it will be lodged on your property file, which will give confidence both to you as the property owner that you’ve built appropriately and also confidence to any potential future purchaser that the foundation system that you have used is appropriate for the land that your property and your house sits on.

Don’t forget…

So just to recap – provided the right level of geotechnical investigation, design and the right foundations are designed for your situation, your home should perform well wherever you are within the Green Zone.

Secondly, if you want more information please refer to the guidance that was put out earlier this year by Building and Housing and included in there are suggestions around the appropriate use of lighter weight materials which generally will perform better in future events. And finally, make sure that you engage the right professionals whether they are chartered professional engineers, geotechnical engineers or structural engineers and of course Licensed Building Practitioners.

Want more information?

If you want more information, please go to the Building and Housing website or just give us a call. 

0800 242 243

DBH info


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