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Liquefaction

Source: Te Ara website

Some areas of Nelson are considered susceptible to liquefaction. Liquefaction can occur during earthquakes where strong shaking causes soil to behave more like a liquid than a solid.

Introduction

What is liquefaction?

Liquefaction can occur during earthquakes where strong shaking causes soil to behave more like a liquid than a solid. Loose and saturated sands, silts, and in some cases gravel, are considered susceptible to liquefaction. In some cases, liquefaction can cause lateral spreading, which is where the ground is transported downslope and/or towards a waterway.

Three things need to be present for liquefaction to occur:

  • Sufficient ground shaking during an earthquake (a combination of duration and intensity of shaking)
  • Loose, non-plastic soil (typically sands and silts, and in some cases gravel)
  • Saturated soil (below the groundwater table).

What does liquefaction look like?

The effects of liquefaction can range from minor to severe. Minor effects include small sand boils, where pressure forces sand, silt, and water up through the ground to the surface.

More significant effects include:

  • Uneven ground settlement and subsidence – Areas of ground can settle unevenly, damaging roads and paths, causing heavy objects to sink, and/or increasing flood risk and damaging underground services, paved surfaces and buildings
  • Building foundation damage – Shallow foundations can subside or sink into the ground and/or be stretched as a result of lateral spreading
  • Underground infrastructure movement – Buried pipes (water and sewer), tanks, and manholes can be bent, broken or lift up
  • Lateral spreading and ground cracking – where the ground moves downwards or towards a break in slope (i.e. waterway) and causes the ground to stretch, which can cause cracking and result in uneven settlement.

Liquefaction susceptibility in Nelson

Some areas of Nelson are considered potentially susceptible to liquefaction. These are typically areas where the ground is made up of geologically young deposits (soil, sand, or gravel less than 10,000 years old) and the groundwater table is relatively shallow.

Areas that may be susceptible include:

  • Relatively flat areas close to active rivers and streams, or old waterways
  • Coastal or estuarine areas
  • Areas with uncompacted or poorly compacted fill.

Our understanding keeps improving

Since 2013, Nelson City Council has commissioned several studies on liquefaction that have improved our knowledge of this natural hazard. More recent studies have looked at larger areas or examined areas known to be susceptible to liquefaction in more detail using current methods. The way we assess liquefaction has also improved over time, especially after significant earthquakes like the 2010-2011 Christchurch earthquakes.

Liquefaction assessments

Since 2013 Nelson City Council has commissioned several assessments of liquefaction (described below). These are generally regional or sub-regional scale assessments that do not provide detailed information for individual properties. If detailed information about a specific site is required, a qualified professional will need to carry out a site-specific assessment.

The assessments are listed below, starting with the oldest to the most recent.

Report

Revised Preliminary Assessment of the Liquefaction Hazard in Tasman and Nelson - 18 January 2013

A preliminary assessment of the liquefaction hazard in Nelson and Tasman was completed by Dr Mike Johnson. It identified areas that may contain soils susceptible to liquefaction based on local geology, geomorphology, and local knowledge. Alluvial valleys and coastal deposits, including those in the Tāhunanui area, were identified as potentially susceptible to liquefaction.

This liquefaction assessment was not incorporated into the Nelson Resource Management Plan (NRMP).

Report

Tāhunanui Area Liquefaction Assessment - November 2013

In 2013 Council also commissioned a study of the liquefaction susceptibility in the Tāhunanui area, which was undertaken by Tonkin & Taylor. The assessment confirmed that the Tāhunanui area could be susceptible to liquefaction under certain earthquake scenarios.

This liquefaction assessment was not incorporated into the Nelson Resource Management Plan (NRMP).

Report

Tāhunanui Liquefaction Assessment Stage 2 - Assessment of Eastern Margin - September 2014

This study by Tonkin & Taylor in 2014 (Stage 2) further assessed the liquefaction potential in the north-eastern part of the Tāhunanui area where the previous (2013) investigation had 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 supported the findings of the Stage 1 assessment.

This liquefaction assessment was not incorporated into the Nelson Resource Management Plan (NRMP).

Report

Desk-top assessment of the liquefaction hazard at 'The Glen', Nelson - 26 June 2015

In 2015 Council commissioned Tonkin & Taylor to undertake a desktop assessment of the potential liquefaction hazard at The Glen. The assessment concluded that there is likely to be a generally low susceptibility to liquefaction at The Glen.

This liquefaction assessment was not incorporated into the Nelson Resource Management Plan (NRMP).

Report

An Assessment of Areas of Lower Risk of Potential Settlement due to Seismic-induced Ground Shaking, Tāhunanui, Nelson City - 4 September 2017

In 2017, a further assessment of potential liquefaction in the Tāhunanui area was undertaken by Dr Mike Johnston. The previous reports and community feedback helped to inform this assessment. The assessment identified an area in Tāhunanui where the potential for liquefaction to occur should be managed at the time of new subdivision and development.

This liquefaction assessment was not incorporated into the Nelson Resource Management Plan (NRMP).

Report

Nelson Regional Level A Liquefaction Assessment - 23 November 2021

For context, the Ministry of Business, Innovation and Employment (MBIE) 2017 and the Ministry for the Environment released national guidance in 2017 for planning and building on liquefaction-prone land. More information can be found here. In November 2019, the Building Code was updated to strengthen how liquefaction susceptibility is considered in building and foundation design. These requirements took full effect on 29 November 2021. The changes were made following the Canterbury earthquakes to improve the performance of buildings on land that may be affected by liquefaction.

In 2021 Nelson City Council commissioned Beca to undertake an assessment of liquefaction hazard in the Nelson region in accordance with the national guidance for ‘Level A’ Basic Desktop assessments. The 2021 Nelson Regional Liquefaction Assessment considered information on the subsurface soils from regional and local geologic maps, along with ground surface elevations, aerial imagery and regional seismicity, to identify areas in the region that may be susceptible to liquefaction. The assessment identifies areas in the Nelson region where:

  • ‘liquefaction damage is possible’
  • ‘liquefaction damage is unlikely’
  • there is ‘very low liquefaction vulnerability’.

The assessment is indicative of the potential for liquefaction to affect any individual site or property. To determine the specific liquefaction hazard affecting any individual site or property, a site-specific assessment would need to be undertaken.

The 2021 liquefaction assessment was used to inform the development of a new mapping overlay and associated planning provisions for liquefaction which was introduced to the Nelson Resource Management Plan (NRMP) through Plan Change 29. The overlay and associated planning provisions regulate the new use and development of land within the subject area. These provisions have been operative since 2025 and apply in parts of Residential, Inner City, Suburban Commercial and Industrial zones.

For areas outside the Tāhunanui Area B assessment described below, this 2021 liquefaction assessment is the most up to date information held by Council and is used to inform LIM notations for liquefaction.

Peer review

Nelson Regional Level A Liquefaction Assessment - Peer Review - 21 December 2021

Report

Tāhunanui Level B Liquefaction Assessment - Report only - 13 April 2022

Tāhunanui Level B Liquefaction Assessment - Appendices only - 13 April 2022

Following the 2021 Nelson Regional Liquefaction Assessment (above), Nelson City Council commissioned Beca to undertake a further, more detailed ‘Level B’ assessment of the Tāhunanui area.

The Level B assessment was conducted in accordance with the Ministry of Business, Innovation and Employment (MBIE) 2017 guidance for planning and building on liquefaction-prone land. It is a Calibrated Desktop Assessment, which further refines the 2021 Level A regional assessment (described below). It used quantitative assessments of geotechnical data completed in the low-lying Tāhunanui area between Tāhunanui Beach and the abandoned sea cliff near Monaco.

The Tāhunanui Level B assessment confirms the findings of the 2021 regional assessment, that the part of the Tāhunanui area described above (including the eastern part of Tāhunanui) meets the criteria for being identified as liquefaction damage is possible. It further identifies parts of the Tāhunanui area as medium liquefaction vulnerability, or high liquefaction vulnerability within areas previously identified as liquefaction damage is possible. The assessed liquefaction vulnerability corresponds with the predicted severity of liquefaction damage.

The data contained within the Tāhunanui Level B Assessment report has been generated at a sub-regional scale (1:15,000) using regional, sub-regional, and geotechnical datasets. The assessment is indicative of the potential for liquefaction to affect any individual site or property. To determine the specific liquefaction hazard affecting any individual site or property, a site-specific assessment may need to be undertaken.

The Tāhunanui Level B assessment was not specifically incorporated into the NRMP as part of Plan Change 29, however the area is within in the ‘liquefaction damage is possible’ area identified in the 2021 regional assessment (see below), which was incorporated into the NRMP.

The Tāhunanui Level B assessment is the most up to date information that Council holds on liquefaction hazard for the Tāhunanui sub-regional area, and it is therefore used to inform LIM notations for properties within this area.

Peer review

Tāhunanui Level B Liquefaction Assessment - Peer Review - July 2022

The natural hazard maps

You can view your property on our interactive Liquefaction Map. Check this map to see the most recent mapping of areas potentially susceptible to liquefaction. This map is based on the 2021 Nelson Regional Liquefaction Assessment above, except in parts of Tāhunanui where the 2022 - Tāhunanui Level B liquefaction assessment is shown in the map.

Frequently asked questions

Please note that these FAQs specifically relate to liquefaction hazards. For general FAQs that relate to all hazard types, please see our Natural Hazards Landing Page.

Visit the Liquefaction Map to find out whether the latest information the Council holds indicates your property is in an area that may be susceptible to liquefaction.

Properties within the following mapping overlays are considered to be in areas that are potentially susceptible to liquefaction:

Within the 2021 regional liquefaction assessment

  • Liquefaction damage is possible

Within the 2022 Tahunanui Level B area assessment:

  • Liquefaction damage is possible
  • Medium liquefaction vulnerability
  • High liquefaction vulnerability

Please note that the studies on which the mapping is based are of regional or sub-regional scale and are indicative only of an individual site or property’s susceptibility to liquefaction. If detailed information about a specific property is required, a qualified professional will need to carry out a site-specific assessment.

Natural hazard maps change as our environment changes and our knowledge evolves. The maps reflect our most up-to-date understanding of liquefaction, based on the latest information available to us.

The liquefaction assessment and mapping identify areas that are potentially susceptible to liquefaction in the event of a strong earthquake. Although Nelson has not experienced a local earthquake of the magnitude that would cause liquefaction damage within our lifetime, the council considers natural hazards over a long period of time, and what could happen at any time in the future.

Our understanding of liquefaction in Nelson continues to evolve, as new areas are assessed and we undertake more detailed studies in areas we are already aware of. Methodologies for assessing liquefaction also change over time. The recent 2021 region-wide assessment of liquefaction, and the more detailed 2022 Tāhunanui Level B Liquefaction Assessment, used the methodology outlined in national guidance issued by the Ministry of Business, Innovation and Employment (MBIE) 2017 guidance for planning and building on liquefaction-prone land. This is a different methodology to the one used for previous assessments of Tāhunanui undertaken in 2013 and 2014.

Since 2013, a number of new developments in Tāhunanui have incorporated foundation treatments into the building design to mitigate potential damage from liquefaction.

Lastly and as outlined above, our assessments of liquefaction have been undertaken at a regional or sub-regional scale and do not provide a site specific assessment of any given property. If detailed information about a specific property is required, a qualified professional will need to carry out a site-specific assessment.

Local engineering geologists carried out a Nelson–wide investigation in 2021 to identify areas that are potentially susceptible to liquefaction. This region–wide assessment followed the Ministry of Business, Innovation and Employment (MBIE) 2017 guidance 2017 for planning and building on liquefaction-prone land, and identified the areas as potentially susceptible to liquefaction based on:

  • Regional geology
  • Ground surface elevations
  • Aerial imagery
  • Regional seismicity
  • Previous liquefaction hazard assessments.

The latest sub-regional (Tāhunanui 2022) liquefaction mapping is based on assessments which follow the methodology set out in in the joint Earthquake Commission (EQC)/Ministry of Business, Innovation and Employment (MBIE)/Ministry for the Environment (MfE) guidance. You can view this guidance here.

Plan Change 29 to the Nelson Resource Management Plan (NRMP) aimed to enable housing intensification and choice in the urban zones of Nelson – Residential, Inner City, Suburban Commercial and Industrial zones, while managing natural hazards affecting these zones. A Liquefaction Hazard Overlay with associated planning provisions was introduced to apply only within these zones. The new liquefaction overlay was based on the regional 2021 liquefaction assessment described above (not the 2022 Tāhunanui assessment). This new overlay and associated planning provisions in the NRMP have been operative since 2025 and apply in parts of the Residential, Inner City, Suburban Commercial and Industrial zones.

Other zones were out of scope of the plan change (the out-of-scope zones being the Rural, Conservation, and Open Space and Recreation zones). Mapping of liquefaction susceptibility was therefore not introduced for the zones that were not included in Plan Change 29.

Meanwhile, the hazard maps on our website contain the most up to date information we have on liquefaction, which in this case is the 2021 regional assessment and the 2022 Tāhunanui assessment.

Updated liquefaction mapping and provisions may be introduced in the future through a plan change or when the NRMP is replaced by a new resource management plan.

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