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).

These elements are shown in Figure 1, Figure 2, and Figure 3. Figure 4 summarises the process of liquefaction with a schematic representation.

Three key elements are all required for liquefaction to occur

Figure 1: Three key elements required for liquefaction to occur (MBIE/MfE Guidance, 2017) (Figure 2.4 found on page 8 of MBIE/MfE Guidance).

The liquefaction process

Figure 2: Schematic representation of the process of liquefaction (MBIE/MfE Guidance, 2017) (Figure 2.3 found on page 8 of MBIE/MfE Guidance).

Process and manifestation of liquefaction

Liquefaction can give rise to significant land and building damage through, for example, the ejection of sediment to the ground surface, differential settlement of the ground due to volume loss in liquefied soil and lateral movement of the ground (known as lateral spreading). These effects are schematically presented in Figure 4 and summarised in Table 1.

The process of liquefaction

Figure 3: Schematic representation of the process of liquefaction.

The consequences of liquefaction

Liquefaction can have a variety of consequences that relate to land, the environment, buildings, infrastructure, the economy, and the community. These consequences are described in detail in the MBIE/MfE Guidance (2017) and are summarised in Table 1.

The consequences of liquefaction

Figure 4: Visual schematic of the consequences of liquefaction (MBIE/MfE Guidance, 2017) (Figure 2.6 found on page 11 of MBIE/MfE Guidance).

2021 Assessment of liquefaction potential throughout the New Plymouth District

Tonkin & Taylor Ltd (Tonkin + Taylor/T+T) was engaged to assess the liquefaction potential of the New Plymouth District in general accordance with the guidance document ‘Planning and engineering guidance for potentially liquefaction-prone land’ published by the Ministry of Business, Innovation and Employment (MBIE) and the Ministry for the Environment (MfE) in 2017. This assessment was a Level A – Basic Desktop Assessment as outlined in the MBIE/MfE Guidelines (2017).

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.

This assessment produced a liquefaction vulnerability map for the New Plymouth District that can be viewed on the NPDC GeoHUB viewer:

View the map

This liquefaction vulnerability map categorises the land within the district into one of three categories:

  • Liquefaction Damage is Possible
  • Liquefaction Damage is Unlikely, and
  • Liquefaction Category is Undetermined.

Please refer to the New Plymouth District Council Liquefaction Vulnerability Assessment Report (T+T, 2021) for further information on this study.


Assessing liquefaction vulnerability in the New Plymouth District during Resource Consent and Building Consent applications

Following issue of the Tonkin + Taylor liquefaction vulnerability assessment (October 2021), NPDC and T+T have worked together to develop a flow diagram and screening tool that can be used by local practitioners to assess liquefaction vulnerability in the New Plymouth District (for Resource Consent and Building Consent applications). The simplified liquefaction vulnerability flow diagram requires the user to move through the following steps:

Liquefaction Frequently Asked Questions