Repairing and rebuilding foundations in TC3

Who is the guidance for?

MBIE’s technical guidance for rebuilding and repairing foundations in TC areas is principally written for the engineering design, construction and insurance sectors, local authorities and their professional advisors and contractors.

Foundation repairs and reconstruction in TC3 require engineering input so the principal users of Part C of the guidance document are professional geotechnical and structural engineers.

How does this guidance assist homeowners to progress their repairs in TC3?

The guidance helps engineers, building control officials and insurance assessors decide if foundations can be repaired or need to be replaced. It was updated in September 2015 and now gives engineers more options for repairing and replacing damaged foundations.

Do the updates mean that work already done will have to be revisited?

Work that was done properly, followed MBIE’s 2011 and 2012 guidance and complies with the Building Code should not need to be revisited.

Have the updates removed the need for site-specific geotechnical assessment and specific engineering foundation design in TC3?

No, the guidance says sites in TC3 may need deep geotechnical investigation, depending on the degree of damage, and may need site-specific foundations designed by a Chartered Professional Engineer (CPEng).

Does the guidance help EQC/insurers make decision on my repairs?

The guidance was collated in anticipation of the type of remedial work which will occur following the earthquakes. While it is expected that the guidance will help inform discussions between insurers and homeowners as to whether the proposed foundation solution falls within the scope of the homeowner’s insurance policy, this will ultimately depend on the policy wording in the insurance contract.

What about repairs to existing foundations in TC3?

The guidance provides information about repair work to existing foundations. In some cases, heavy roofing material may need to be replaced with lighter-weight roofing materials. Where there has been damage to heavy wall claddings, the cladding may need to be removed and replaced with a lighter-weight material.

What are my options if I need to replace my existing foundations?

There is no one-size-fits-all option for TC3 properties. Sites in TC3 may need deep geotechnical investigation, depending on the degree of damage, and may need site-specific foundations designed by a CPEng. The December 2012 guidance outlines three types of new or replacement foundations for homes in TC3. These are:

• deep piles
• site ground improvements
• surface structures with shallow foundations.

Each type of foundation has different capabilities to accommodate various levels of vertical settlement and lateral spreading; each places different constraints on the type of dwelling that the foundation can support.

What do you mean by ‘site ground improvements’?

Site ground improvement options aim to strengthen the land, using a variety of methods, to create a better building platform. This typically involves treating the land beneath the building footprint by either compacting the land to make it more dense, or stabilising the land by adding cement. Once the land is strengthened, standard TC2 (concrete slab or timber) foundations can be used.

Site ground improvement options may place limitations on some two-storey dwellings with heavy walls and irregular floor plans. It will depend on the type of ground improvement method that is used.

What are surface structures with shallow foundations?

A shallow pile option may be appropriate for lighter-weight homes with standard floor plans, on sites where there is no evidence of significant lateral spreading. The guidance gives several options. An example is constructing shallow or short piles in the ground and wrapping the perimeter foundation with plywood sheeting to provide bracing against earthquake movement.

For properties where there has been major lateral spreading, additional engineering design will be required.

Will the foundation types place limitations on the type of dwelling I can build on my land?

Each of these foundation types have different capabilities to accommodate various levels of vertical settlement and lateral spreading, and place different constraints on the type of dwelling that the foundation can support. For example:

• deep piles – no height and/or material constraints likely, but they won’t be suitable in some situations
• site ground improvements – will place limits on some two-storey dwellings with heavy walls and irregular plans. For example, this option may not be suitable for a two-storey, brick veneer and tile house
• surface structures with shallow foundations – suitable only for lightweight homes that have standard design features, for example timber piled homes with lightweight cladding such as weatherboards and iron roofs. This solution is generally not appropriate for homes with concrete slab floors or irregular floor plans. However, MBIE has provided guidelines for the possible development of alternative options.

Who decides what foundation type is best for my land?

Chartered Professional Geotechnical Engineers (CPEng) will be able to determine what foundation type is appropriate for each property based on the information obtained from geotechnical assessments. There is no ‘one-size-fits all’ solution.

Ultimately, decisions about foundation repairs and reconstruction will need to be made by EQC/ insurers in consultation with the homeowner. However, in some TC3 areas foundation options may be limited.

How will repaired and replacement foundations perform in future earthquakes?

Houses repaired in accordance with our guidance should perform significantly better in future earthquakes but (as in any part of New Zealand) there will always be a risk of future earthquake damage. This is partly due to the nature of the Canterbury sub-soil conditions and risk of future liquefaction and lateral spreading in TC3 areas, but mainly because the building codes cater for earthquakes only up to a certain size before some damage is expected. (This is normal international practice.)

If damage did occur, the foundation solutions in our guidance are designed to be ‘readily repairable’ in future earthquake events (easy to repair in the future).

How much are these foundation types likely to cost?

This is something that individual homeowners will need to discuss with their insurer. It will largely depend on the extent of the damage, the type of house and the foundation repair option.

If I follow MBIE’s guidance will I meet the requirements of the Building Code?

Our technical guidance for rebuilding and repairing foundations in TC3 areas aims to provide guidance and solutions that will result in compliance with the Building Act 2004 and the Building Code.

Why does the guidance advocate the use of lighter-weight materials in TC3?

In most cases, homes with light-weight cladding and roofing materials performed reasonably well in the Canterbury earthquake sequence.

Lighter-weight roofing and cladding materials will significantly improve the performance of homes in future earthquakes. Lighter-weight materials reduce the weight of your home and the load placed on foundations. There is likely to be a significant reduction in the cost of repairing any damage from future seismic events if you have a lighter-weight home with a suspended timber floor.

Does the guidance apply to garages or outbuildings?

For TC3 properties, TC2 type foundations should be suitable for unoccupied detached garages and outbuildings.

Garages or other outbuildings that are attached to the house need to have foundations designed to be consistent with the foundations of the house.

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Do I have to follow the guidance document?

No, it is only a guide and other repair methodologies may comply with the Building Act. This is a matter that individual homeowners will need to discuss with their insurer/EQC. Building work that follows the design solutions in the guidance will comply with the Building Code, and will likely assist with future insurance and consenting processes.

What if I want to modify my house as part of the rebuild and repair process (for example, add a room)? How will this affect compliance with the guidance document?

If your addition is consistent with the foundation repair and rebuild method suggested by the CPEng for the remainder of your house, then building consent would be likely to be granted provided it met other Council requirements.

Will I need to get any other consents from Councils for the rebuild/repairs to my house foundations?

EQCs or your insurer’s project management office will be able to advise you if you need to obtain resource consent.

My property is in the Christchurch City Council’s flood management area, does the guidance still apply?

Yes, but you or your insurer’s project management office will need to talk to the Council about minimum floor heights and protection of the land to achieve compliance with the Christchurch City Council District Plan and the Building Code, and to reduce the risk of future flooding.

What if I don’t have damage to my foundations?

You may be able to move ahead with repairs, such as replacing a chimney or repairing wall linings by following the December 2012 MBIE guidance. Consult with EQC, your insurance company and the council before going ahead.

If at a future date you decide to build a major extension onto your home, you may be required to get advice from a Chartered Professional Engineer. Any major extension to an existing house must comply with modern day building standards.

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