Subsidence occurs when earth and rock fall into a cavity that has formed below the ground’s surface. It is a natural process, although it can also occur as a result of people’s activities (for example, landfills and mining).
Natural subsidence is not widespread in the Waikato region, although it is sometimes seen in the Waitomo District. Subsidence can also occur where there is thermal power generation or mining activities. For example, in December 2001, 16 houses in Waihi had to be evacuated after old mine workings collapsed beneath them without warning.
Subsidence is the sinking of the ground surface.
Waitomo’s landscape is mainly formed from limestone with underground drainage and many cavities and passages. This type of landform, known as ‘karst’, features many sinkholes or ‘tomo’. Tomo are formed when ground water dissolves underlying limestone rock. The dissolved hole enlarges to a point where it can’t support the ground above it. Rubble falls into the hole initially supporting overlying material, but leaves a visible ‘slump’ on the surface. Water erodes the supporting material so that eventually the overlying material also falls away leaving a hole.
Ground stability is a characteristic of the interaction between water, soil and the underlying geology. Different soils react in different ways to the presence or absence of ground water:
- Soil materials ‘liquefy’ when there is too much ground water present (for instance, some clays). This can happen on sites where landfill has been used, when water from springs or seepage causes the fill to compact. It can also occur in volcanic ash/soil that has filled up valleys over thousands of years. Volcanic ash and soils are easily eroded and are found throughout the region.
- However, ground water filling small spaces between soil particles can help the ground ‘keep its shape’. In this situation, any lowering of the water table may cause these spaces left behind to compact, causing the ground to subside.
As a natural hazard, the early signs of subsidence are not always visible before a major slump occurs. We need to identify and monitor areas at risk to avoid any threat to people’s lives and property due to subsidence.
The effects of subsidence can also be accelerated or triggered by other natural activity in the area, such as volcanic activity, earthquakes and landslides.
- Waikato Regional Council has been involved in mapping the Waitomo ‘karst’ (limestone) area and looking at ground water activity within the Waitomo District.
- We work alongside landowners on initiatives such as pole planting to stabilise hill areas, and encourage production forestry and retiring land around tomo from production.
- We provided assistance to the Hauraki District Council following a major subsidence in the town of Waihi in December 2001.
- We develop risk mitigation plans to minimise the effect of natural hazards on the Waikato economy and community.
For policy information on natural hazards, see our Regional Policy Statement.