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New buildings and liquefaction effects

Ensuring new buildings can withstand liquefaction effects

Changes from the November 2019 Building Code update will ensure new buildings are built safe and strong enough to withstand liquefaction effects. Foundations for houses built on ground that has the potential for liquefaction or lateral spread are now outside the scope of B1/AS1 and NZS3604. 

These changes came into effect on 29 November 2021. The change was made as a result of the experience of the Canterbury earthquakes, and subsequent recommendations made by the Royal Commission of Inquiry.

An in-depth summary of the changes is provided on MBIE’s web page: Ensuring new buildings can withstand liquefaction effects

Nelson Regional Liquefaction Assessment

Nelson City Council has commissioned mapping to identify areas that are vulnerable to liquefaction during an earthquake.  The mapping is a desktop assessment and follows the methodology set out in in the joint EQC/MBIE/Ministry for the Environment guidance Planning and engineering guidance for potentially liquefaction-prone land (2017)

Nelson Regional Liquefaction Assessment 23 November 2021 Beca

Report and maps 

Report only

Maps only 

The mapping is colour coded according to liquefaction vulnerability categories assigned in the guidance document.

These are:

Scope of the Assessment

The Nelson Regional Liquefaction Assessment Report is a desktop assessment that uses regional-scale information, simple screening criteria, local knowledge and readily available area-wide information focusing on urban areas without field verification. It is a level A assessment as defined in the guidance document and is not intended to accurately describe liquefaction at an individual property scale. It does not consider ground improvement that may have taken place with existing developments. Detailed site-specific liquefaction assessment will be required for some purposes (for example, design of building foundations).

High-level review of this assessment is being undertaken for the eastern Tāhunanui area, including incorporation of the information below. The amended reporting and mapping will be updated when this is complete, anticipated to be early 2022.

In the meantime, more detailed assessments were undertaken of the Tāhunanui area in 2014 and 2017. These include area specific investigations and analyses and may provide improved information for properties in eastern Tāhunanui.

Tahunanui Liquefaction Assessment – Stage 2 Assessment of Eastern Margin (September 2014) T&T

An Assessment of Areas of Lower Risk of Potential Settlement due to Seismic-induced Ground Shaking, Tahunanui (September 2017) MR Johnston

Reporting levels and liquefaction categories

The guidance defines four levels of liquefaction assessment, ranging from level A for a regional desktop assessment, to level D for a site or project specific assessment.  The possible outcomes from each level of assessment are as follows.

Where no liquefaction assessment has been carried out:

  • the liquefaction category is mapped as undetermined

Following level A and B high level liquefaction assessments, the liquefaction category is mapped as either:

  • Liquefaction damage is unlikely or
  • Liquefaction damage is possible 

Following level C and D more detailed liquefaction assessments, the liquefaction category will be mapped as either:

  • Very Low liquefaction vulnerability
  • Low liquefaction vulnerability
  • Medium liquefaction vulnerability or
  • High liquefaction vulnerability

What does this mean if my property is within an area that is vulnerable to liquefaction?

You are not required to do anything to your existing building, or if you are carrying out building works that are:

  • minor internal alterations that do not significantly load existing or require new foundations, or
  • minor footprint extensions for lightweight residential buildings

What does this mean if I’m considering building work?

An assessment to ascertain ‘good ground’ or specific investigation and design, as applicable, is still required. Owners and Designers should now also:

  • consider specifying a more robust foundation where ground performance is less certain.

Although changing a building’s use may not involve building work, consideration of liquefaction is required where it is proposed to change a building’s use.

A helpful guide to achieve building consent compliance

All building consent applications which nominate B1/AS1 as a compliance pathway must include supporting evidence to show that either liquefaction damage is not likely or has a low probability of occurrence. Regardless of compliance pathway, it is the applicant’s responsibility to provide evidence. The table below and the regional liquefaction mapping can help you with this.

To use the table:

  1. Select the liquefaction hazard category that matches the mapped category at your building site
  2. Look down the column within the heavier borders to see what options you have to demonstrate compliance


Notes:

  1. TC1 to TC3 refer to the ground foundation technical category for land set out in MBIEs Canterbury Guidance Part A Technical Guidance (TC1 and TC2) and Part C Assessing, repairing and rebuilding foundations in TC3. The geotechnical requirements for these are set out in Table 5.2. The Technical Guidance is part of a package of information available at Repairing and rebuilding houses affected by the Canterbury earthquakes
  2. Ground performance levels L1 to L5 refer to the level of land damage expected from a design seismic event as set out in Earthquake Geotechnical Engineering Module 3- Identification, assessment and mitigation of liquefaction hazards.
  3. Geotechnical investigations for foundation design should be undertaken in accordance with Earthquake Geotechnical Engineering Module 2 – Geotechnical Investigations for Earthquake Engineering
  4. Council will not require that liquefaction assessments be supplied for buildings that meet all of the following criteria:
    • importance level 1 (low consequence of failure, non-habitable) as set out in NZS1170.0 clause 3.3
    • lightweight construction less than 40 m2
    • height is no more than 4 m
    • is at least its own height from a property boundary and
    • is at least its own height from another building with importance level 2 or greater

Performance requirements of the Building Code will still need to be met.

What does it mean if I’m applying for subdivision resource consent?

There is a requirement to assess significant risk from natural hazards, including liquefaction when applying for Subdivision Resource Consent.