Liquefaction most often occurs as a result of strong ground shaking during an earthquake. There is an increasing understanding and recognition of the potential adverse effects of liquefaction in New Zealand.
What we are doing
Porirua City engaged geoprofessionals (geotechnical engineers and engineering geologists) to undertake an assessment of the liquefaction vulnerability of land in Porirua city.
A fact sheet on the Draft Porirua Liquefaction Assessment can be accessed here [link].
The assessment was undertaken in 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.
The Draft Porirua Liquefaction Assessment report can be accessed here [link].
The Draft Porirua Liquefaction Assessment liquefaction vulnerability map can be accessed here [link].
There is existing regional mapping showing potentially liquefaction prone areas available on the Greater Wellington GIS maps here [link]. The new assessment provides information of the liquefaction vulnerability on land in Porirua that will be used by Porirua city for a broad range of purposes.
What causes liquefaction
Not all soils can liquify. Three key elements are required for liquefaction to occur: saturated soils, soils composed of loose sands and silts, and sufficient ground shaking.
Landform types that are more likely to be susceptible to liquefaction include along rivers, stream and lakes; coastal margins; floodplains; estuaries and wetlands; reclaimed land; and uncontrolled or poorly compacted fill.
Areas of Porirua are potentially susceptible to liquefaction. These areas include the valley floors and wetlands and estuaries where sediment has been deposited in low energy environment and the groundwater level may be high, and the areas of historic land reclamation at the harbour edges.
Once liquefied, the soil loses its bearing capacity. Denser materials will sink, while less dense materials will float upwards. As the liquefied soil is under pressure, it will be forced into any cracks and can flow out of the ground as sand boils and streams of liquified material.
Consequences of liquefaction
The immediate effects of liquefaction primarily relate to land damage. Significant damage to buildings, structures and infrastructure can be caused by liquefaction. This occurs as a result of differential settlement and tilting, loss of support to foundations of buildings, floating of structures such as buried tanks and pipes, and lateral spreading and stretching near streams and rivers.
A rise in sea level as a result of climate change may increase the risk of liquefaction in areas close to the coast where it will influence groundwater levels. In this situation, the thickness of the non-saturated layer of soil would be reduced. Additionally, ground surface damage may be more likely or more severe in areas with an existing high water table.
Changes in rainfall as a result of climate change may also affect groundwater levels, with resulting effects on liquefaction risk. However, this is much harder to predict due to a range of other factors that will also affect groundwater levels in the future.
Porirua City has engaged geoprofessionals (geotechnical engineers and engineering geologists) to undertake an assessment of the landslide susceptibility of land in Porirua city.
A factsheet on the Draft Porirua Landslide Assessment can be accessed here [link].
The assessment is being undertaken in accordance with the guidance document ‘Landslide Planning Guidance: Reducing landslide risk through land-use planning’ published by GNS Science in 2024.
The Draft Porirua Liquefaction Assessment report can be accessed here [link]
The Draft Porirua Liquefaction Assessment liquefaction vulnerability map can be accessed here [link]
There is existing regional mapping showing areas potentially susceptible to earthquake induced landslides available on the Greater Wellington GIS maps here [link]
The new assessment provides information on the susceptibility of land in Porirua to landslides that will be used by Porirua City for a broad range of purposes.
Recent landslides in Porirua
Approximately 60 landslides resulted from the heavy rainfall event in Porirua on 15 November 2016. The majority of these slips occurred due to over-steep cuts in loess or colluvial soil during modification of slopes for residential development.
The rainfall event followed the 2016 Kaikōura earthquake of the previous day. It is difficult to determine whether these slips were predominantly earthquake induced or rainfall induced. It is likely that both events were a factor in many of the landslides, with the shaking caused by the Kaikoura earthquake potentially contributing to the instability of slopes by loosening the soil and weathered rock.
In July 2023 a slip occurred on Spinnaker Drive that resulted in the road being closed at the intersection with SH58. The road currently remains closed. A remediation option is now proposed of a Mass Block retaining wall.
Since then, we have experienced a number of other landslides at a range of scales. Recently these have included a slip on Titahi Bay Road that closed the road for a short time.
What causes landslides
Landslides occur when the stress acting on a slope exceeds the strength of that slope and can range in size from small slips to very large-scale mass movement events. There can be many causes contributing to a landslide event, but generally there is one main ‘trigger’. Landslides can be triggered by natural processes such as intense rainfall events and earthquakes, or by human activities such as earthworks and land development, vegetation removal, or even leaking water pipes. These either increase the stress on the slope or reduce its strength. There are many different categories of landslides based on the type of movement and material involved.
Consequences of landslides
Damage can occur both down slope and up slope of the landslide source area. Down slope, the landslide material can inundate areas in the runout area with rock, debris and soil. Up slope, the movement of the material can undermine and result in the loss of land. Landslides can also cause secondary hazards, such as landslide dams and deposition of large amounts of sediment in rivers and streams.
In Porirua, many landslides have affected Porirua’s land, property and infrastructure over the years causing economic and social disruption. Landslides have affected critical assets such as roads and rail lines. These slips require significant resources, time and effort to clear and in some cases may require costly repairs to be undertaken. Porirua Council is often managing a number of landslides at any one time.
Influence of climate change
Landslides can be triggered by intense rainfall events, with these slips being termed ‘Rainfall Induced Landslides’ (RIL). Climate change is expected to affect rainfall patterns as more moisture is held in a warmer atmosphere, with changes to seasonality, extreme rainfall intensity and persistence. This could increase the risk of landslides, particularly in extreme rainfall events.
By 2090, climate change is expected to result in increases in rainfall in the western part of the Wellington region in all seasons, and by up to 15% in winter. Short return period extreme rainfall events (such as 1-year events) may increase, and larger extreme rainfall events (such as 100-year events) are likely to increase in intensity.
Liquefaction is a phenomenon where otherwise relatively stable soil suddenly decreases in strength and temporarily behaves as a viscous liquid. Three key elements are all required for liquefaction to occur: loose non-plastic soil, saturated soil, and sufficient ground shaking.
Lateral spreading is commonly observed at liquefied sites within close proximity to free-faces A free-face is a river bank, harbour margin etc. Liquefaction induced ground displacements alongside free-faces can generate large cracks in the ground as the ground moves towards a free-face. Lateral spread often causes the most severe damage to structures in an earthquake event.
Liquefaction is a natural hazard that has the potential to affect Porirua. While there is regional mapping available for liquefaction, this mapping is not accurate enough for the range of purposes Porirua City needs it for.
In 2023 the Independent Hearings Panel for the Proposed District Plan recommended that Porirua City undertake a plan change to introduce liquefaction hazard overlays and associate provisions into the District Plan to manage subdivision, use and development potentially at risk from liquefaction. We need accurate mapping of the areas of Porirua vulnerable to liquefaction to inform this work.
The November 2019 Building Code update 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. In order to implement that update to the building code we need to identify liquefaction-prone areas.
The Porirua liquefaction assessments being undertaken will provide liquefaction vulnerability maps of the district. A vulnerability map identifies potentially liquefaction prone areas. These are produced by interpreting a variety of factors to determine if liquefaction could occur in a seismic event.
Porirua City will use this liquefaction assessment to assist with assessing Resource Consent and Building Consent applications. The information will also be used in the consideration of how subdivisions and new building developments are designed.
This information will also allow the Council and others planning, designing and building infrastructure and other assets to make better decisions for a more resilient district.
The information will also help to inform future climate change adaptation planning processes in Porirua.