Liquefaction Prone Land
What is Liquefaction?
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. Liquefaction can cause significant damage to land, buildings, infrastructure and the environment, as well as economic and social disruption.
There are three key factors which influence whether liquefaction occurs and how severe the resulting ground damage is:
Vulnerability to Liquefaction
Tararua District Council commissioned Tonkin + Taylor to prepare a liquefaction vulnerability map which follows the most recent national liquefaction guidance. You can download the full report below:
|Please Note: |
The practitioners report below is in draft format. These reports will feed into the District Plan review process where it, alongside other technical information, will be further refined and tested to informal reviewed policy framework to mange and guide development the Tararua District.
The map included in the report uses the seven categories shown in the table above to describe the vulnerability of the land to liquefaction-induced damage. As well as describing the likelihood and severity of ground damage, the categories also show where there is more or less certainty about the ground condition. As there is less information for our District it is only possible to conclude that “Liquefaction Damage is Possible”. When more detailed information becomes available in future (e.g. new ground investigations), this might show that the actual vulnerability is “Medium” or “High”, or in some cases perhaps even “Low”.
What does this mean for my Building Consent?
In the interim and ahead of the District Plan review Council is still required to give effect to the changes under the Building Act and for clarification, we have outlined a process for building consents for residential dwellings and substantial additions/alternations.
You can download this process below:
National liquefaction guidance
In 2017 the national “Planning and engineering guidance for potentially liquefaction-prone land” was released. It establishes a process to manage liquefaction-related risk in land use planning and development decision-making. A key objective of the guidance is that buildings and infrastructure are located and built with appropriate consideration of the land conditions.
The guidance encourages consistency in how liquefaction is assessed across New Zealand, to make it easier to transfer knowledge and develop efficient standardised solutions. It recommends that maps are prepared using the seven liquefaction vulnerability categories shown above.
Tonkin & Taylor has completed a Level A – Basic Desktop Assessment to determine the liquefaction vulnerability of the Study Area outlined by TDC in accordance with the MBIE/MfE Guidelines (2017).
The key conclusions and recommendations are:
- The land within the Study Area has been classified into one of three liquefaction vulnerability categories: “Liquefaction Category is Undetermined”, “Liquefaction Damage is Unlikely” or “Liquefaction Damage is Possible”. The currently available information does not support further classification of the land into the other more precise categories of “Very Low”, “Low”, “Medium” and “High”. This degree of liquefaction vulnerability categorisation precision is consistent with a regional scale study (such as this) undertaken to a Level A level of detail.
- The liquefaction outputs of this study provide a regional base layer which will be useful for Resource Management Act (RMA) applications within the Tararua District. In particular, the outputs of this study relate to the Horizons Regional Council Regional Policy Statement (RPS), which outlines areas within the region that are prone to natural hazards. In some areas where liquefaction damage has not been ruled out, it is likely that liquefaction vulnerability studies will need to be completed to a higher level of detail to satisfy RMA requirements.
- TDC can also use the outputs of the study to inform building consent applications. In some cases, where liquefaction has not been ruled out, it is likely that liquefaction vulnerability assessment will need to be completed to a higher level of detail to satisfy Building Code requirements.
- Regardless of the vulnerability classification given, any proposal for development within the Tararua District should be accompanied by a statement that either confirms or updates the vulnerability classification assigned in this report. Section 4.4 provides guidance on approaches for efficiently assessing liquefaction vulnerability for each geomorphic terrain.
To improve the resolution of the liquefaction vulnerability output to promote additional uses of the liquefaction vulnerability information, further information will need to be collected. The two main areas where additional base information is required to support higher level of detail studies include geotechnical investigations and groundwater information. Potential steps to address this information limitation are as follows:
A key source of uncertainty in this liquefaction assessment is the relatively limited amount of geotechnical investigation data in the Study Area. This information is important for both the assessment of liquefaction vulnerability and for other future applications. To help facilitate the collection of more geotechnical investigation data, TDC may wish to undertake the following:
- Identification of geotechnical investigations from historic projects and uploading of these investigations onto the NZGD.
- Advocation of uploading supporting geotechnical investigations onto the NZGD as part of the process of evaluating resource and building consent applications. Local engineering and scientific practitioners may need to be educated about why this uploading process is important.
- Engagement of suitably competent geo-professionals to undertake geotechnical investigations within given areas where more information about the ground conditions is required (e.g. areas where a Level B, C or D level of detail is targeted).
A key source of uncertainty in this liquefaction vulnerability assessment is the limited amount of groundwater information in the Study Area. While not critical for this Level A study, detailed information about shallow groundwater levels becomes increasingly important when targeting higher level of detail liquefaction vulnerability studies. It also provides a valuable data source for other purposes such as asset management. To help facilitate the collection of more detailed groundwater data, TDC could consider installing a network of piezometers to monitor groundwater level fluctuations over time. This data could also be used to develop depth to groundwater surface models.