Liquefaction
Liquefaction is a process which causes soil to behave more like a liquid than a solid during, and following, an earthquake. Loose and saturated sands, silts, and in some cases gravel, are considered susceptible to liquefaction. Nelson contains areas with deposits that are considered vulnerable to liquefaction during an earthquake.
Our understanding of liquefaction in Nelson continues to evolve as new areas are assessed and we undertake more detailed studies in areas known to be susceptible to liquefaction. Methodologies for assessing liquefaction also change over time especially in response to recent earthquakes, such as the 2010-2011 Christchurch earthquakes. Liquefaction studies commissioned by Nelson City Council are described below.
Liquefaction assessments and mapping in Nelson from 2013
2013
A preliminary assessment of the liquefaction hazard in the Nelson and Tasman areas was completed by Dr Mike Johnson. The assessment 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.
The council commissioned a study of the liquefaction susceptibility of deposits in the Tāhunanui Area which was undertaken by Tonkin & Taylor. The (Stage 1) assessment confirmed that the Tāhunanui area could be susceptible to liquefaction under a particular earthquake scenario.
Tahunanui Area Liquefaction Assessment – Tonkin + Taylor, 2013
2014
A subsequent 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.
Tahunanui Liquefaction Assessment Stage 2 – Assessment of Eastern margin, Tonkin + Taylor, 2014
2015
In 2015 Tonkin & Taylor undertook a desktop assessment of the potential liquefaction hazard at The Glen. The assessment concluded that there is likely to be a generally low risk of liquefaction at The Glen.
Desk-top assessment of the liquefaction hazard at ‘The Glen’, Nelson, 2015
2017
In 2017, a further assessment of liquefaction was undertaken by Dr Mike Johnston. The previous Stage 1 and 2 reports and community feedback helped to inform this assessment. The assessment identifies an area in Tāhunanui where the risk of liquefaction should be managed at the time of new subdivision and development. This assessment recommended exclusion of the north-eastern part of Tāhunanui.
2021
Updates to the Building Code in November 2019 required local authorities to undertake regional mapping to identify areas potentially susceptible to liquefaction by 29 November 2021. The Building Code updates were made to ensure that new buildings are built safe and strong enough to withstand liquefaction effects and came in because of the experience of the Canterbury earthquakes and subsequent recommendations made by the Royal Commission of Inquiry.
The liquefaction hazard in the Nelson region was assessed in accordance with the Ministry of Business, Innovation and Employment (MBIE) 2017 guidance for planning and building on liquefaction-prone land. The assessment was completed by Beca in 2021 and identifies areas in the Nelson region where ‘liquefaction damage is possible’ and where ‘liquefaction damage is unlikely’ and/or has a ‘very low liquefaction vulnerability’.
The report can be downloaded here:
Locate your property on the Geotechnical Hazards Map
The MBIE guidance Planning and engineering guidance for potentially liquefaction-prone land can be found here.
Please note that the data contained within the Nelson Regional Liquefaction Assessment report has been generated at a regional scale (1:25,000) using regional datasets. Site-specific liquefaction assessments were not considered. 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.
Further refinements to the geographic extent of the identified area where ‘liquefaction damage is possible’ may occur in the future, for example as the Nelson Resource Management Plan (NRMP) is updated.
2022
Following the 2021 Nelson Regional Liquefaction Assessment, Nelson City Council commissioned a further, more detailed ‘Level B’ assessment of the Tahunanui area.
The Level B assessment, by consultants BECA, was also 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 Basic Desktop Assessment undertaken at Level A for the region using geotechnical investigation data and quantitative liquefaction assessments.
The Level B assessment confirms the findings of the 2021 regional assessment, that the Tahunanui area (including the eastern part of Tahunanui) meets criteria for being identified as liquefaction damage is possible. It further identifies parts of the Tahunanui area as medium liquefaction vulnerability, or high liquefaction vulnerability.
The Level B assessment of Tahunanui was peer reviewed by consultants WSP.
The report can be downloaded here:
