Source: Auckland Council
Update on GHD’s landslide risk assessment
GHD is still on track to complete their landslide risk assessment and deliver a final report by late September, as previously advised in the timeline shared in the 25 May newsletter. The Recovery Office are currently working with GHD on how results from this work will be shared with affected property owners prior to the final report being made public.
In the past two weeks, GHD has completed its review of the background information (research). These results will support the RBA placard reassessment as well as the government’s property categorisation process. This includes information from property files, LiDAR information (‘Light Detection and Ranging’ – a 3D remote sensing method that uses light to measure and produce pictures of the exact distance of an object), aerial photographs, historical data, anecdotal evidence collected, geological maps and the NZ Geotechnical Database. GHD has also carried out geomorphological interpretation (this is the analysis of the shape of the land to help understand how it formed and how it has changed over time) based on the above review and field mapping.
Geotechnical evidence to support RBA placard reassessments
GHD is also focusing on completing the bulk of the remaining assessments for the presently defined ‘purple zone’ properties in the Piha and Karekare areas. Their assessment will include their site inspections, assessment of damage from landslips and potential remediation options.
Following the outcomes of these assessments, we expect some properties could have their placards downgraded or removed (which will likely begin in the coming weeks). Final inspections of these properties will be carried out by Council compliance staff, and property owners will be informed, including with a formal placard change letter. In some cases, this may mean people can return to their homes sooner than expected in cases where current and future risk can be managed.
Recapping the Colour Zones map from 22 June newsletter
In our 22 June Piha and Karekare newsletters, we included a map with colour zones to communicate which properties GHD is doing site-specific risk assessments on. To recap, the colours do not represent different levels of risk – they are simply to indicate how soon the results are expected to be delivered.
Please note these colours are separate to the RBA placards as well as the government’s property categorisation process.
Because the colour categories are not showing risk, they will not be recorded on properties’ Land Information Memorandum (LIM) reports.
We have grouped properties in Piha and Karekare into three groups – brown, purple and no colour.
Brown – These properties are in an area where the future land instability risk is relatively complex, with more work needed to understand the risk for these properties. The expected delivery date for GHD’s work for the ‘brown zone’ will be in late September.
Purple – These properties are in an area where the future land instability risk is simpler, so results can be delivered before the full area-wide assessment is complete. The expected early delivery date for the risk assessment will be towards the end of August. Preliminary results will be able to be used to support reassessment of RBA placards. Full results will be published in the same report as the rest of the area-wide assessment due in late September.
No colour – These properties are believed to be outside the main large-scale slope instability in Piha and Karekare. Some of these properties have been affected by smaller-scale local landslides, resulting in RBA placards.
We continue to ask all owners of placarded properties to fill in the online form here. This will give us the information needed to apply a risk category to your property.
If you don’t want to take part in this process, then our previous advice still applies – we recommend that you work with your insurers and engage a professional chartered geotechnical engineer or chartered engineering geologist to inform you about the next appropriate steps. The relevant geotechnical documents supporting placard downgrade can be submitted to Council for review, comments, and approval.
Your geotechnical professional is encouraged to contact the council directly to discuss the level of geotechnical investigation and/or assessment they need to undertake in support of the placard downgrade.
You can find advice about how to get a geotechnical report here.
You can also find more details about the process for resolving an RBA placard here.
Background about the geology and landslides across Piha and Karekare
Geology
Piha and Karekare are built on a mixture of sand dunes and rock.
Sand dunes are quite variable, ranging from well cemented (i.e. the sand particles stick together well) to loosely deposited sands. Loosely deposited sands can contain thin layers of clay or silt, which can have variable consistency, strength, and stability.
Most of the rocks seen locally on the slopes in Piha and Karekare are of volcanic origin. These can be exposed at the surface on higher ground or buried beneath the sand dunes in lower-lying areas. These volcanic materials include basalt and tuff, as well as debris from old offshore volcanoes deposited in massive underwater flows creating conglomerates and breccia (coarse grained sedimentary rocks containing individual blocks held in place by a finer grained matrix). These rocks are normally very stable, but over time the surface breaks down through weathering and can turn the outermost layer into a weaker ‘mantle’ of soil that can slide off as a thin layer.
Landslides
Landslides occur when the driving forces that can cause a landslip to occur are greater than the resisting forces keeping the slope in place. Most steep slopes are stable until a change (which might be only a very small change) either reduces the strength of the ground or increases the load on the ground, causing it to fail.
The factors that commonly trigger landslides include pore water pressure (amount of water in the soil), changes in the physical slope (such as excavations or erosion of the surface), or new loads on the slope (including natural loads like earthquakes or water, and man-made loads like buildings).
Rotational landslides have rounded bases, and commonly form where there are relatively deep weak materials.
Translational landslides have a flat base and are more common in sand deposits, and where rocks where they are covered with a thin weathered soil mantle. These translational landslides are particularly common in Piha and Karekare because of the presence of sand dune deposits and the weathered soil mantle over the rock.
Debris flows are fast-moving landslides that are particularly dangerous to life and property because they move quickly, destroy objects in their paths, and often strike without warning. Translational and rotational landslides can turn into a debris flow if enough water is fed into them as they move (for example, where a landslide falls into a stream).
Both translational and rotational landslides can be damaging to homes built on them. Debris flows can be particularly hazardous as a result of the higher speeds at which they move, making it harder to escape their effects.
GHD has completed field mapping assessment in the Piha and Karekare areas and has identified approximately 100 landslides around each community (about 200 landslides overall).
Most of the mapped landslides appear to be ‘shallow seated’ translational landslides where the upper 1 m to 2 m of soil material has mobilised. These often look large and very dramatic, but because they are shallow the amount of material that has moved can sometimes be relatively small.
Several possible ‘deep seated’ landslides were also identified in the Piha and Karekare areas. These commonly move more material, but in many cases do not travel as far.
