Liquefaction

Step 1 requires the user to identify the “official” liquefaction vulnerability category of the site which was assigned in the New Plymouth District Liquefaction Vulnerability Assessment (T+T, 2021).

Step 2 requires the user to outline the type of development associated with the resource consent or building consent application. The type of development will need to be defined by one of the following categories:

1. Sparsely populated rural area (lot size >4 ha) – e.g., a new farm building
2. Rural-residential setting (lot size of 1 to 4 ha) – e.g., a “lifestyle” property
3. Small-scale urban infill (original lot size <2500 m2) – e.g., demolish old house and replace with four townhouses
4. Commercial or industrial development – e.g., a warehouse building in an industrial park, or
5. Urban residential development (typically 15 – 60 households per ha) – e.g., home in a new subdivision

The user will need to justify why the chosen development scenario applies to their respective consent application.

Three liquefaction assessment options have been endorsed by NPDC for use in the district and are outlined below:

1. Option 1: Site-specific geotechnical engineering assessment required to support the resource consent or building consent application. This approach requires the user to use fundamental geotechnical engineering principles to assess liquefaction vulnerability. Typically, this would include site specific deep ground investigations and recommendations for site development works and foundation solutions to mitigate the effects of liquefaction (if required). Unless the assessment demonstrated that the site was not prone to liquefaction, every building would require specific engineering design, typically with reference to the MBIE/NZGS Earthquake engineering modules – there would be no reference to NZS 3604:2011 foundation options or the MBIE Canterbury Guidance (2018) foundation options.
2. Option 2: This assessment option requires the user to undertake a site-specific geotechnical assessment (and often deep ground investigations) and as such constitutes a form of specific engineering design. However, the process used by designers to choose appropriate mitigation options would be streamlined with reference to the MBIE Canterbury Guidance (2018). Selection of the foundation options could be further streamlined by undertaking a site-specific liquefaction vulnerability assessment in accordance with the MBIE/MfE Guidance (2017).
3. Option 3: Represents a simplified screening assessment option that allows the user to remove the need for extensive deep site-specific geotechnical investigations for some sites and development scenarios. This assessment option allows the user to transition from sites categorised as Liquefaction Category is Undetermined, Liquefaction Damage is Unlikely, or Liquefaction Damage is Possible in the NPDC Liquefaction Vulnerability Assessment (T+T, 2021) to assumed liquefaction vulnerability categories that can then be correlated to liquefaction mitigation options provided in the MBIE Canterbury Guidance (2018).

This step requires on-site observations and a site walkover to be undertaken to determine if the mapped geomorphic terrain is appropriate for the site of interest. As outlined in the T+T Liquefaction Vulnerability Assessment report, the existing geomorphic map was created at a 1:25,000 scale and as a result it may not identify geomorphic terrains that are apparent at a site-specific level. The design report will need to explain how the geomorphic terrain was assessed, with the evidence that either confirms the original mapped terrain was correct or shows that an alternative terrain is applicable. In undertaking Step 4, the user should reference the terrain mapping methodology and terrain descriptions defined in the Liquefaction Vulnerability Assessment Report (T+T, 2021) report (refer Section 3.2.2 of that report).

The simplified screening assessment tool is based first on assessing the thickness of any non-liquefiable crust and second on factors that would influence the severity of lateral spreading that could occur at the site. Please refer to the notes on the Simplified liquefaction vulnerability flow diagram.

Liquefaction is a natural process where earthquake shaking increases the water pressure in the ground in some types of soil, resulting in temporary loss of soil strength.

The following three key elements are all required for liquefaction to occur:

1. Loose non-plastic soil (typically sands and silts, or in some cases gravel).
2. Saturated soil (i.e., below the groundwater table).
3. Sufficient ground shaking (a combination of the duration and intensity of shaking).

Lateral spreading is commonly observed at liquefied sites within close proximity to free-faces. Liquefaction induced ground displacements alongside free-faces can generate large cracks in the ground as the ground moves towards a free-face (e.g., river bank, harbour margin etc). Lateral spread often causes the most severe damage to structures in an earthquake event.

Liquefaction vulnerability mapping is a type of risk analysis that identifies potentially liquefaction prone areas. Vulnerability mapping interprets a variety of factors to determine if liquefaction could occur in a seismic event.

The liquefaction vulnerability map of the New Plymouth District can be viewed on the NPDC GeoHub viewer.

This assessment was undertaken as a response to the November 2019 Building Code update, which revised B1/AS1 (the design approach used for many simple structures) to ensure that new buildings are built safe and strong enough to withstand liquefaction effects. As a result of this change, MBIE have advised councils across New Zealand to:

“Map their jurisdictions to undertake hazard mapping and identify liquefaction-prone areas. These regional hazard maps should be prepared using the Ministry of Business, Innovation and Employment (MBIE)/Ministry for the Environment (MfE) liquefaction mapping guidance” (Building Performance, 2021).

This liquefaction vulnerability assessment was undertaken in general accordance with the guidance document “Planning and engineering guidance for potentially liquefaction-prone land” published by the Ministry for the Environment and the Ministry of Business, Innovation and Employment in 2017.

The level of detail of this study was Level A – Basic Desktop Assessment.

We are required to make information, such as technical assessments relevant to natural hazards, available upon request. We cannot advise about any effect this information may have on property value or insurance.

It is recommended that professional advice is sought from a property valuation or insurance expert about any concerns you may have regarding these matters.

NPDC will use this liquefaction vulnerability assessment to assist with assessing Resource Consent and Building Consent applications in the New Plymouth District. The information will also be used in the consideration of how subdivisions and new building developments are designed.

This information will also allow NPDC, and others designing buildings and infrastructure, to make better decisions for a more resilient district.

This assessment has been made at a broad scale across the entire district and is intended to approximately describe the range of liquefaction vulnerability across neighbourhood-sized areas. It is not intended to precisely describe liquefaction vulnerability at individual property scale. This information is general in nature, and more detailed site-specific liquefaction assessment may be required for some purposes (e.g., for subdivision consent or the design of building foundations).

A key consideration of liquefaction vulnerability categorisation undertaken in accordance with the MBIE/MfE Guidelines (2016) is the degree of uncertainty in the assessment. Discussion about the key uncertainties in this assessment is provided in sections 3.3 and 3.4 of the New Plymouth District Liquefaction Vulnerability Assessment Report (2021).