FOUNDATION TECHNICAL CATEGORY 3 (TC3) AUGUST 2012

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1 FOUNDATION TECHNICAL CATEGORY 3 (TC3) AUGUST 2012 Building and Housing has published technical guidance for foundation repairs and reconstruction for residential properties in green zone Foundation Technical Category 3 (TC3). The guidance helps engineers, building control officials and insurance assessors to decide if foundations can be repaired or need to be replaced. It provides a range of options for repairing and replacing damaged foundations. The guidance focuses on foundation options for one and two-storey timber-framed dwellings, which are the dominant form of construction in the TC3 area. There is no one-size-fits-all option. You need geotechnical advice and design input from a chartered professional engineer to ensure your foundations are appropriate for your site. The full guidance can be found at Improving building standards after earthquakes All houses in New Zealand must comply with the New Zealand Building Code which sets national building standards. Although there are national standards, design requirements vary from region to region depending on the hazards. In some areas houses are built to a more robust design to resist heavy snow loads or extremely strong winds. There are many parts of the country where, because of ground conditions, local authorities require specific engineering design for foundations. Even before the earthquakes, site-specific foundations were required in the Port Hills and in peaty areas of Canterbury. New Zealand building standards have been upgraded after every major earthquake as knowledge about earthquakes - and designing buildings to resist them has improved. The objective is to build houses that act like crumple zones in modern cars to minimise harm to people in significant earthquakes even if the building suffers some damage. The aim of Building and Housing s technical guidance for rebuilding and repairing foundations is to ensure you have appropriate foundations for your site and your home complies with the Building Act 2004 and the Building Code.

2 Technical categories only apply to foundations The technical categories were established to guide the level of site investigation required and the appropriate foundation system for properties with foundation damage. If you don t have foundation damage the technical categories do not apply. However should you choose to build on an extension to your house in the future you will be expected to comply with modern day building standards. For example, if your house is in TC3 you may be required to engage a Chartered Professional Engineer to design your foundations. Technical categories developed to speed up the repair process Without the technical categories every site in the residential green zone with foundation damage would require further geotechnical investigation and site-specific foundation design. This would slow down the repair and rebuild process considerably. By creating the technical categories, approximately 80% (TC1 and TC2) of houses in the residential green zone with foundation damage were spared the need to have further geotechnical investigation and specific foundation design by Chartered Professional Engineers. Technical categories reflect the soil conditions in the green zone The technical categories reflect the variability of the Canterbury soil conditions. They are based on intensive analysis of scientific and technical data, much of which had been collected following the September and December 2010, and February and June 2011 earthquakes. It included observed land damage, observed property damage, groundwater depth and known underlying soil composition. Engineers applied this knowledge as well as a mix of historical and post-earthquake data, and engineering judgement to determine the areas for each technical category. In TC3 areas, ground conditions can be highly variable over comparatively short distances. Typically the land in TC3 has a thin crust and layers of loose sand and silt which can give rise to settlement and lateral spread which is why foundations in TC3 areas need to be specifically designed to accommodate the soil conditions. 2

3 Liquefaction is not always visible in TC3 areas Although some properties in the TC3 area show no visible signs of liquefaction, liquefaction may still have occurred below the surface. The crust can prevent surface signs of liquefaction (for example sand boils, water ejection) but the area can still suffer liquefaction at layers below the crust. This deeper liquefaction can still cause damage to foundations. EQC s area-wide drilling programme will provide more information about the land in TC3. This information will be critical in determining the appropriate foundation system for properties with foundation damage. Houses repaired to the standards in the guidance will perform significantly better in future earthquakes but there will always be a risk of future earthquake damage. This is due to the variable nature of Canterbury sub-soils and risk of future liquefaction and lateral spreading in TC3 areas. Repairing or replacing foundations The guidance contains information and criteria for establishing: where damaged foundations can be repaired and where replacement foundations are required. Repairs to existing foundations The guidance provides options for repairs to existing foundations. In some cases, heavy roofing material may need to be replaced with lighter-weight roofing materials so the building has greater resilience in future earthquakes. Where there has been damage to heavy wall claddings, the cladding may need to be removed and replaced with lighter-weight material. Replacement or new foundations There is no one-size-fits-all option in TC3. Chartered Professional Engineers (CPEng) will be able to determine what foundation type is appropriate for your property based on information obtained from geotechnical assessments. The guidance outlines three types of new or replacement foundations for homes in TC3. These are: deep piles, site ground improvements, and surface structures with shallow foundations. 3

4 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. 1. Deep pile foundations Deep pile foundations involve drilling or driving piles (timber, concrete or steel) into the ground to a suitable bearing layer. They can take the weight of heavier or more irregular buildings as long as there is a suitable bearing layer that will not liquefy and there is no risk of lateral movement at depth. Deep pile options may not be suitable in areas where there is potential for significant lateral movement of the ground. 2. Site ground improvements Site ground improvements aim to strengthen the land and create a better building platform. A variety of methods can be used. Examples are: compacting the land beneath the building platform to make it denser stabilising the land by adding cement. In most cases the existing house will have to be demolished or temporarily removed to allow ground strengthening work to take place. Once the land is strengthened, standard TC2 foundations can be used (enhanced concrete slab or timber). Site ground improvements will place limits on some two-storey dwellings with heavy walls and irregular floor plans. 3. Surface structures with shallow foundations The guidance describes three types of surface structure. An example is to place shallow or short piles into the ground (similar to NZS3604) and wrap the perimeter of the foundation with plywood sheeting, which braces the structure against movement in earthquakes. Shallow foundation options may be suitable for properties where there is only minor to moderate potential for vertical settlement and lateral spreading. Additional engineering design will be required if there is potential for major lateral spreading on the property. Surface structures with shallow foundations are suitable only for lightweight homes that have standard design features, for example timber piled homes with lightweight cladding such as weatherboards, board and iron roofs. This solution is not appropriate for homes with concrete slab floors or irregular floor plans. 4

5 Ultimately, the decision about foundation repairs or reconstruction will be made by you and your insurer, based on professional advice. Houses without foundation damage If your home is in TC3 and your foundations are not damaged, you can move ahead with repairs such as replacing a chimney, following Building and Housing s November 2011 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 to your home, you may be required to get advice from a chartered professional engineer and construct site-specific foundations for the extension. Any major extension to an existing house must comply with modern day building standards. Lighter-weight construction materials In general, the guidance advocates the use of lighter-weight construction materials for homes in TC3 areas. Lighter-weight materials reduce the weight of your home and the load on foundations. In most cases homes with lighter-weight cladding and roofing materials performed better in the Canterbury earthquake sequence. There is likely to be a significant reduction in the cost of repairing any damage from future seismic events for lighter-weight houses with suspended timber floors. Next steps If you live in TC3 and have foundation damage, you should continue to work with EQC and/or your insurer to progress repairs. Many repairs will include Restricted Building Work which is critical to the integrity of a building. This work must be supervised by or carried out by a Licensed Building Practitioner (LBP). Check with your Project Management Office or builder to ensure the work is supervised by an LBP. If you think that work may be faulty, don t ignore it. Discuss it with the person doing the work or the lead contractor. If you don t have a contract then contact the Building Consent Authority that approved your building consent (usually the local council). 5