Changes to the New Zealand Building Code (NZBC) for dealing with liquefaction-prone land come into effect 29 November 2021.
The following information details the requirements to have specifically designed foundations for buildings on ground identified as liquefaction-prone ground after this date.
If you have any questions about the information on this page please; talk to a building professional (e.g. architect or structural engineer) or contact Building Services on 03 477 4000 or email building@dcc.govt.nz.
Background
Liquefaction is a natural process where earthquake shaking increases the water pressure in the ground in some types of soil, leading to temporary loss of soil strength. It can cause significant damage to land, buildings, infrastructure and the environment, as well as economic and social disruption.
The NZBC definition of ‘good ground’ as defined by New Zealand Standard NZS3604:2011 has now been amended to exclude ‘liquefaction’. The rationale for the change is to support safer and more resilient housing foundations for buildings on liquefaction-prone land.
This change has been made as a result of the experience of the Canterbury earthquakes, which generated widespread liquefaction, and subsequent recommendations made by the Royal Commission of Inquiry.
These regulations are already in place in the Canterbury region and will now be extended to all New Zealand.
Liquefaction risk factors
The three key factors which influence whether liquefaction occurs and how severe the ground damage will be are:
- soil condition (material type and density),
- groundwater depth, and
- earthquake shaking (duration and intensity).
Risk of an earthquake occurring in Dunedin (Seismic risk)
After assessing earthquake risk the Ministry of Business, Innovation and Employment (MBIE) has divided New Zealand into three seismic risk areas - high, medium and low. Most of Dunedin sits in a low seismic risk area, with an area west of Sutton in the medium seismic risk zone.
- Map of seismic risk areas – indicative only (external link, new window).
Dunedin land subject to liquefaction
An Institute of Geological and Nuclear Sciences (GNS Science) report (new window) dated 2014 divided the city into three different domains:
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Liquefaction Domain A
Ground predominantly underlain by rock or firm sediments, with little or no likelihood of liquefaction.
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Liquefaction Domain B
Ground in an area susceptible to amplified shaking in an earthquake and potential liquefaction during a severe earthquake. It is predominantly underlain by poorly consolidated river or stream sediments, with a shallow groundwater table. The likelihood of liquefaction is low to moderate.
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Liquefaction Domain C
Ground in an area susceptible to amplified shaking in an earthquake and potential liquefaction during a severe earthquake event. It is predominantly underlain by poorly consolidated marine or estuarine sediments with a shallow groundwater table. The likelihood of liquefaction is moderate to high.
Dunedin map
Link to the View the Dunedin map in a new window.
The New Zealand Building Code (NZBC)
B1.3.1 of the NZBC requires buildings, building elements and sitework to have a low probability of rupturing, becoming unstable, losing equilibrium or collapsing.
B1.3.2 of the NZBC requires buildings, building elements and sitework to have a low probability of causing loss of amenity through undue deformation. Designers should review the regional seismicity to determine whether liquefaction is likely to be a governing consideration in foundation design for the site. For residential timber framed buildings constructed to NZS3604 on flat sites, amenity is likely to be the governing consideration for the design and specification of foundations.
B1.3.3 of the NZBC requires anyone doing building work to take account of all physical conditions likely to affect the stability of buildings, building elements and sitework, including water, earthquake, differential movement, removal of support and many others.
Please see the MBIE website (external link, new window) for full details. Note the details can be found throughout the website.
What does this mean for my building work?
People applying for building consent post 29 November 2021 will need specifically designed foundations for buildings on ground identified as liquefaction-prone ground. Current ‘deemed to comply’ acceptable solutions, such as NZS3604 for foundations, are unlikely to be able to be used.
MBIE has indicated the technical category foundations used in Christchurch are likely to provide a simple compliance pathway. MBIE recommends designers consider the three technical categories for foundation design options outlined in the Canterbury Guidance:
- Very Low and Low Liquefaction Vulnerability Technical Category 1 (TC1)
- Medium Liquefaction Vulnerability Technical Category 2 (TC2)
- High Liquefaction Vulnerability Technical Category 3 (TC3)
These TC categories were developed from observation of actual earthquake affects in the Canterbury region and don’t directly correlate to Domain A, B and C as described by the DCC’s mapping. This is an issue common to most regions of New Zealand and MBIE has provided guidance to address this - www.building.govt.nz - Changes to foundation design (external link, new window).
Region specific guidance
After considering this information the DCC has provided the following region-specific guidance for:
- New buildings other than importance Level 1 (IL1) buildings - these are typically residential, commercial , industrial, education, health care facilities and other higher-level importance buildings
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Domain A
This land is not considered to be at risk from liquefaction and is potentially ‘good ground’. Other hazards such as low bearing capacity, land instability or uncontrolled fill still have to be assessed and mitigated.
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Domain B
Treatment of Domain B varies depending on the nature of the proposed development. MBIE guidance provides a flow chart for treatment of small-scale urban infill (subdivision where original lot size less than 2500m2).
It recommends the MBIE TC2 foundation design methodology for dwellings, assuming site specific deep ground testing has not been used to exclude the risk of liquefaction.
Further information on the TC2 design process and limitations can be found in Part A: Technical guidance (new window). Figure 5.2 requires the design process to be dependent on the outcome of shallow subsurface investigation. Shallow subsurface investigation is defined in section 3.4.1.
For larger scale urban residential development such as a home in a new subdivision where the original lot size was larger than 2500m2, the owner will need to show evidence of site investigation, possibly to a depth of 15 metres, to either confirm the site is ‘good ground’ or to provide enough information to allow an appropriate foundation to be designed.
It is likely the TC2 foundation design process will be appropriate; however this cannot be determined without deep ground investigation. Guidance on deep geotechnical site investigation can be found in section 3.4.2 of the Technical Guidance found in Part A: Technical guidance (new window). This document provides design guidance for TC1 and TC2.
For commercial or industrial development within Domain B the owner will need to show evidence of site investigation, possibly to a depth of 15 metres, to either confirm the site is ‘good ground’ or to provide enough information to allow an appropriate foundation to be designed.
In all cases the results of the geotechnical investigation will be required to accompany the building consent application. The design engineer will be required to provide a producer statement (PS1) confirming compliance with the technical guidance.
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Domain C
Domain C potentially liquefiable land will require the owner to carry out site investigation, possibly to a depth of 15 metres, to either confirm the site is ‘good ground’ or to provide enough information to allow an appropriate foundation to be designed.
It is likely that the TC2 or TC3 foundation design process will be appropriate for dwellings, however this cannot be determined without deep ground investigation. Guidance on deep geotechnical site investigation can be found in section 3.4.2 of the Technical Guidance found in Part A: Technical Guidance. This document provides design guidance for TC1 and TC2. Please refer to the document titled, Part C: Technical Guidance for TC3 guidance.
In all cases the results of the geotechnical investigation will be required to accompany the building consent application. The design engineer will be required to provide a producer statement (PS1) confirming compliance with the technical guidance.Where TC3 designs are required an external peer review and PS2 will also be required.
Domain C - South Dunedin TC2 area
Deep ground assessment carried out in this area over recent years indicates that the TC2 foundation design process is likely to be appropriate for dwellings.
Please refer to the document titled Part A: Technical Guidance (TC1 and TC2). Figure 5.2 from this document requires that the design process to be dependent on the outcome of shallow subsurface investigation. Shallow subsurface investigation is defined in section 3.4.1.
In cases where shallow subsurface investigation finds a geotechnical UBC of less than 200kPa, specific engineering design is required (see section 5.4 of guidance). Deep ground investigation is at the discretion of the design engineer, unless a deep piled raft solution is proposed in which case deep ground investigation is a requirement.
In all cases the results of geotechnical investigation will be required to accompany the building consent application. The design engineer will be required to provide a producer statement (PS1) confirming compliance with the technical guidance.
- New Importance Level 1 (IL1) buildings - these are typically ancillary buildings not used for human habitation, minor storage facilities, uninhabited detached garages, outbuildings
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Domain A
This land is not considered to be at risk from liquefaction and is potentially ‘good ground’. Other hazards such as low bearing capacity, land instability or uncontrolled fill still have to be assessed and mitigated.
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Domains B and C
Importance Level 1(IL1) structures can be detailed using a TC1 type foundation system, or alternatively by specific design by a Chartered Professional Engineer
- Extensions to Buildings
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Domain A
This land is not considered to be at risk from liquefaction and is potentially ‘good ground’. Other hazards such as low bearing capacity, land instability or uncontrolled fill still have to be assessed and mitigated.
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Domains B and C
Extensions to outbuildings, ancillary buildings and other importance level 1 (IL1) buildings
- Deep ground assessment not required. Apply standard guidance for IL1 buildings.
Extensions to houses and small-to-medium apartment buildings (maximum calculated height of less than 10m)
- Deep ground assessment not generally required. TC2 foundation design acceptable, unless the original building was designed for TC3.
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All other buildings
- Minor Extensions - Deep ground assessment required at the discretion of the design engineer.
- Extensions other than minor - Apply standard guidance for domain B or C as detailed above.
Definition of minor extensions
- The building footprint increase is less than 10% and the value of work (for the extension) is less than $250,000.
Guidance for Engineers
Importance Level 1 (IL1) structures have no seismic load requirements (under AS/NZS 1170.0) at Serviceability Limit State (SLS), and therefore have no amenity requirements relating to liquefaction deformations at SLS levels of shaking.
This leaves a ‘life safety’ design requirement at Ultimate Limit State (ULS) for a 1/100 year event, which should be able to be provided in most cases on a TC2 site by a suitably detailed structure on a TC1 type foundation system.
Alternatively, a specific design can be determined by applying the 1/100 year design event loadings at ULS.
When a building consent is received
Each site will need to be considered on its own merits with the application containing enough geotechnical information to allow the Building Consent Authority (BCA) to determine whether the proposed foundation design is appropriate for that site.
Please note:
- There is no direct correlation between DCC liquefaction mapping domains and the Canterbury technical categories
- The designer or their representative should take reasonable steps to make sure the mapped site domain class is accurate – even for Domain A
- Geotechnical investigations and analysis should be used to inform design
- Designers should consider more robust foundation solutions where ground performance is uncertain
- Site investigation and geotechnical assessment should be tailored to the complexity and risk of the development proposal
- Deep geotechnical testing and liquefaction analysis is likely to be required for high risk sites
- A shallow geotechnical investigation may be sufficient for simple building work on low risk sites
- All geotechnical information supplied as part of either a resource consent or building consent application will be added to a DCC database so enhanced mapping can be developed over time, reducing the need for future site investigation
Useful information
Considerable additional information can be found on the MBIE website with specific guidance found at www.building.govt.nz - Changes to foundation design (external links, new windows).