Liquefaction

Council has commissioned work to assess the likelihood of ground liquefaction following an earthquake. Liquefaction is a process that temporarily turns firm ground into a liquid.

A preliminary geological assessment of our region suggested that the Tahunanui area could be susceptible to liquefaction given an earthquake of sufficient type and magnitude. Council commissioned Tonkin & Taylor Ltd to assess a ‘liquefaction study area’ in Tahunaunui in 2013 (Stage 1) and 2014 (Stage 2). Community feedback was sought on these reports in 2013 and 2017. In Winter 2017, a further desktop assessment was undertaken by Dr Mike Johnston. These assessments are summarised below and the reports can be downloaded by clicking on the links below

The assessment and the subsequent detailed reports for Tahunaui can be downloaded using the links below.

2013 Tahunanui Liquefaction Assessment summary

The assessment suggests that some areas in Tahunanui may be subject to liquefaction following a significant earthquake. However the scale of the liquefaction and the possible effects on homes and services is very difficult to predict.

The assessment used guidance for liquefaction assessment produced by the Government following the Canterbury earthquakes. The investigation has found a definite risk of liquefaction in Tahunanui. The risk across the study area generally increases from the east to the west. However, because of differences in the subsurface materials beneath Tahunanui, there is a big variation in liquefaction potential.

For smaller earthquakes with a return period of 25 years it was found the impacts from liquefaction would be minor. For a 500 year return period earthquake (the event the Building Act requires most buildings to survive) the investigations found the Tahunanui study area would suffer:

  • Widespread sand boils that can damage paved areas.
  • Lifting of buried pipes (water and sewer) and manholes that aren’t properly anchored.
  • Failure of shallow building foundations and subsidence.
  • Ground levels dropping between 130 mm to 290 mm resulting in increased potential for flooding, and damage to underground services, paved surfaces and buildings.
  • Lateral ground spreading within about 100 metres of the shoreline and/or a water course.

2014 Tahunanui Liquefaction Assessment Stage 2 Assessment of Eastern Margin

The 2014 study further assessed the liquefaction potential of sediments in the north-eastern part of the Tahunanui study area where the previous investigation indicated the presence of surficial gravel deposits and a reduced thickness of sediments with a high liquefaction potential. The findings of this Stage 2 assessment generally support the findings of the Stage 1 assessment.

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

In 2017, a further desktop assessment was undertaken by Dr Mike Johnston. The previous Tonkin and Taylor Ltd reports and community feedback helped to inform this assessment. The assessment identifies an area in Tahunanui that should be identified in the Nelson Plan (resource management plan) where the risk of liquefaction should be managed at the time of new subdivision and development. This is a reduced area in comparison to the Tonkin and Taylor Ltd study area, as the assessment recommends exclusion of the north-eastern area which includes surficial gravel deposits.

Liquefaction mapping

Council has mapped the area at risk from liquefaction hazard in Tahunanui.

You can search for liquefaction hazard on a particular property in Tahunanui by using our natural hazards map.

Land Information Memorandum (LIM) and Project Information Memorandum (PIM)

Council has an obligation to make hazard information available to the public under the Local Government Official Information and Meetings Act 1987 and Building Act 2004. Council has placed a note on relevant land and property information files to inform potential buyers about the information it holds in relation to liquefaction hazard in Tahunanui.

The LIM notation reads:

The Council holds information that this property is situated in an area that has been identified as underlain by soils that have the potential to liquefy if a seismic event of sufficient type and magnitude were to occur. The relevant reports can be found at the Council’s website at nelson.govt.nz/liquefaction. The reports recommend that the area be included in a Tahunanui Liquefaction Risk Overlay in future resource management plans. Geotechnical engineering investigation, design and certification is likely to be required for subdivision and new building development within this part of Tahunanui. For more information please contact Customer Services at Nelson City Council.


Frequently Ask Questions

Why is liquefaction a risk in Tahunanui?

The flat land at Tahunanui is comprised of marine sand and sandy gravel. These soil types are the most likely to liquefy during significant ground shaking caused by an earthquake.

What are the effects of liquefaction?

Areas with liquefaction can suffer:

  • Widespread sand boils that can damage paved areas.
  • Lifting of buried pipes (water and sewer) and manholes that aren’t properly anchored.
  • Failure of shallow building foundations and subsidence.
  • Ground levels dropping between 130 mm to 290 mm resulting in increased potential for flooding, and damage to underground services, paved surfaces and buildings.
  • Lateral ground spreading within about 100 metres of the shoreline and/or a water course.

In what situations could liquefaction occur?

Liquefaction is likely to be caused by a magnitude 8 earthquake. The return period for an earthquake of this size is less than 200 years.

An earthquake on the local Waimea-Flaxmore Fault System is most likely to generate enough ground shaking to cause liquefaction in any waterlogged sediments.

How does the Council know this property will be affected by liquefaction when it hasn’t been tested?

The extent of the liquefaction hazard area is based on Tahunanui’s soil types. Testing done on sample sites shows that the liquefaction risk is variable across the wider Tahananui area.

Being in the area identified as being at risk from liquefaction hazard means a more detailed assessment of the property may be needed if building works are proposed.

How does being in the liquefaction area affect insurance costs?

Council cannot advise property owners about the effect this liquefaction hazard information may have on their ability to obtain insurance or on insurance premiums. Different insurance providers will have different policies, and we suggest you contact your insurer directly to discuss your specific policy.

What if I want to sell this property?

Council has an obligation to make hazard information available to the public, under the Local Government Official Information and Meetings Act 1987 and the Building Act 2004. That means people can access information held by the Council about their property, and about any property they are considering buying. The Council has placed a note on relevant land and property information files to inform potential buyers about the information it holds in relation to liquefaction hazard in Tahunanui.

A Land Information Memorandum (LIM) report contains a map which will show where on the property the liquefaction hazard is located.  Even in circumstances where the liquefaction hazard is only identified on a small portion of the site (e.g. driveway or garden), Council is still required to note this on the relevant land and property information files.

I’m currently in the process of selling this property – do I need to tell the new owners?

If you are asked a specific question about it, you must provide the answer. It is important that purchasers do their own due diligence on any property they wish to buy. You should also check what obligations arise under any agreement for sale and purchase you enter into.

Will this property being in the liquefaction area limit the ability to build a new house or extend the existing one?

Council will need to consider liquefaction risk when administering the Building Act 2004 (including compliance with the Building Code). The Building Code (section B1) requires all building works to be designed to accommodate the loads (including earthquake) that they are likely to experience throughout their life without causing risk to life or loss of amenity.

The site investigation recommendations provided by the Ministry of Business, Innovation and Employment (MBIE) and foundation treatments set out in section 6 of the Tahunanui Area Liquefaction Assessment 2013 (pages 16 – 20) are ways to comply with the Building Code.

There are currently no rules in the Nelson Resource Management Plan (NRMP) regarding new buildings in an area at risk from liquefaction hazard. However, this is being reviewed through the development of the Nelson Plan (resource management plan) and it is proposed that new land use rules will apply.

I recently built a house in Tahunanui using an appropriate foundation treatment to mitigate the risk of liquefaction. Why do I still have a LIM notation on my property file?

Since 2013, a number of new developments in Tahunanui have incorporated appropriate foundation treatments into the building design to mitigate the risk of liquefaction hazard. While this has ensured that the liquefaction risk to the building footprint has been mitigated, it is likely that other areas of the property remain at risk e.g. driveways and gardens. On this basis, it is appropriate that a LIM notation remains on the property file to inform potential buyers and to ensure that liquefaction is considered when assessing any future development proposals on the site.

Can this property still be subdivided if it’s in the liquefaction area?

Subdivision requires resource consent. Liquefaction risk is already taken into account when considering an application to subdivide land within this area under the requirements of the Resource Management Act 1991.

Where do I find information about what to do following an earthquake?

Civil defence information and advice on getting prepared can be found on the Nelson Tasman Civil Defence Emergency Management website.

How to I provide feedback to Council about the liquefaction information?

At this time, we are not seeking further feedback from the community on liquefaction hazard. We will release a draft Nelson Plan in August 2018 and this will be your opportunity to provide feedback on the area identified as being at risk from liquefaction hazard and the proposed planning rules. We will let the community know how to get involved in providing feedback on the draft Nelson Plan closer to the time.