Liquefaction

Council has commissioned work to assess the likelihood of ground liquefaction following an earthquake.

A geological assessment of our region suggested the Tahunanui area could be susceptible to liquefaction given an earthquake of sufficient type and magnitude.

The assessment and the subsequent detailed reports for Tahunaui can be downloaded by clicking on 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.

Liquefaction mapping

Council has used the information from the Tahunanui assessment to produce GIS mapping and show the affected area.

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

Next steps

We would like you to give us your feedback on the information, and tell us your thoughts on how Council should use the information to manage risk from liquefaction.  We are holding community information sessions in early May to provide further information on our natural hazards which may help inform your feedback.

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

Properties subject to the Tahunanui study area have had interim notes on the property files (through the LIMs) since 2013. Interim LIM notes will remain on those properties affected by liquefaction hazard while we seek community feedback on that hazard information.


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.

What are the main points in the assessment reports?

The assessment indicates there is a significant amount of variation in liquefaction potential within the Tahunanui study area.

Appendix A of the 2013 Report provides maps of the area, and the results of “core penetration tests” (CPT).  These tests have provided information on the presence of soil layers that have the potential to liquefy.  Appendix B of the 2013 Report provides a detailed description of liquefaction and its effects. 

The 2014 study further assessed the liquefaction potential of sediments in the north-eastern part of the Tahunanui study area and generally support the findings of the Stage 1 assessment.

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

The extent of the study area is based on the type of soils in the Tahunanui area. However, testing done so far on sample sites shows that the liquefaction risk is variable within the study area.

Being in the study area means a more detailed assessment of the property may be needed if building works are proposed.

How does being in the liquefaction study area affect insurance costs?

The 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. Council is not aware of any instances where the interim LIM notices for properties in the study area has impacted on insurance costs.

What if I want to sell this property?

The 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. That means people can access information held by the Council about their property, and about any property they are considering buying. The Council will place an interim note on relevant land and property information files to inform potential buyers about the information it holds in relation to the Tahunanui study area.

What does an ‘interim note’ on the property file mean?

The interim note is based on the Council’s current information. This ‘interim status’ means that there may be some changes in response to further information.

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 study area limit the ability to build a new house or extend the existing one?

The 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) related to liquefaction risk.

Are liquefaction rules likely to be included in the NRMP in future?

Council is in the process of reviewing the NRMP and preparing its replacement, the Nelson Plan. As part of this process we are looking at whether the Nelson Plan should contain requirements for developing land subject to potential for liquefaction. We are seeking your views on whether or not a liquefaction risk overlay on the planning maps and related rules are required.

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

We are seeking your feedback in relation to natural hazards risk management at present, including the liquefaction mapping.  Comments should be provided by 31 May 2017.

You will also have two further opportunities to provide comments on the provisions of the Nelson Plan. Once we have completed the technical information, engaged with the community on new information (such as this liquefaction hazard mapping) and prepared a draft of the provisions, we will release a draft of the Nelson Plan for public comment (likely to be later this year or early 2018). Following this we will prepare a Proposed Nelson Plan which will be publicly notified and you will have the opportunity to lodge a submission.