Geotechnical information

Some parts of Christchurch, including much of the eastern suburbs, are subject to multiple natural hazards, particularly the areas adjacent to the lower reaches of our rivers. Traditionally these hazards have included flooding and coastal hazards, which include coastal erosion and inundation.

Many of these areas were significantly impacted by the effects of the Canterbury earthquakes. Damage was caused not only by the direct ground shaking affects, but more importantly the consequences of the secondary effects of liquefaction and basin subsidence.

These have, in turn, had the cascading effect of making the areas more susceptible to other hazards which include fluvial, pluvial and coastal flooding. Future hazards are expected to be exacerbated by accelerated sea-level rise. While some hazards are independent of each other there is often an overlap or continuum across them- some processes act as a 'feed back' loop that exacerbates the original process.

Potentially these inter-linkages between hazards and their cascading effects over both the short and long term may drive the greatest risk. It is therefore very important to understand the 'multi-hazard' risk from natural processes when making decisions.

There are two geotechnical issues in the residential red zone – liquefaction and lateral spread. Links to more detailed geotechnical information summaries are provided below.


Liquefaction

Liquefaction diagram.Environment Canterbury diagram showing liquefaction

Ground shaking can cause liquefaction. This is the process where, during earthquake shaking, sand and silt grains in wet soil are rearranged and the water in the spaces between the grains is squeezed. Pressure builds up in the water until the silt and sand grains 'float' in the water, and the soil behaves more like a liquid than a solid.

The pressurised water is forced up to the ground surface through the easiest path it can find - often through cracks in the ground or concrete. The water takes silt and sand with it, bubbling to the surface, forming sand boils or sand volcanos, or when there is a lot of it, filling up large areas with sand and silt.

When the ground temporarily loses its ability to carry weight and acts more like a liquid than a solid, heavy structures will tilt and sink while light structures like pipes float. Buildings, roads, pipes and tanks on or in liquefied soil are often damaged.

To liquefy, the soil must be:

  • loose - compacted soils tend not to liquefy
  • sandy or silty - clays and gravels tend not to liquefy
  • saturated - the soil must be below the water table.

Liquefaction usually only happens in susceptible soils in moderate to strong ground shaking (when household items start to fall and break).

Many parts of the eastern suburbs, including the residential red zone, and some of the central city provided the right ground conditions for liquefaction.

Liquefaction can cause considerable damage and contribute to land subsidence, causing increased flooding. Substantial liquefaction took place during the September 2010 earthquake and continued with every major aftershock including 22 February 2011.


Lateral spreading

Lateral spreading diagram.Environment Canterbury diagram showing lateral spreading

During an earthquake, liquefied soils can sometimes move sideways, usually towards streams or rivers, but also if the surrounding land is lower than the liquefied land. This is called lateral spreading. Lateral spreading can cause cracking of the ground surface as the soil underneath it moves sideways and can pull apart any structures on or in it.


CERA geotechnical information summaries for the flat land residential red zone

Geotechnical information summaries for the flat land residential red zone were prepared for CERA in July 2012. Detailed mapping and below ground testing was undertaken primarily to better understand geotechnical properties of the soil, especially the probability of liquefaction during another earthquake and, how the ground behaved in the earthquakes and might behave in a future earthquake.

These documents summarise all the area-wide geotechnical information that was considered by CERA as part of the process for making flat land zoning decisions, and the subsequent zoning review. The following maps are included within each of the summaries:

  • CERA residential red zone and Department of Building and Housing technical categories
  • Observed ground crack locations
  • Ground surface observations of liquefaction and lateral spreading
  • Status of wastewater network
  • Ground surface elevation from the February 2012 Light Detection and Ranging (LiDAR) survey. The Canterbury Maps website provides maps which show LiDAR data
  • Change in ground elevation between LiDAR in July 2003 and February 2012
  • Location of suburb-wide geotechnical ground testing
  • Examples of cone penetration test results
  • Area-wide geotechnical and engineering considerations.

Some parts of Christchurch, including much of the eastern suburbs, are subject to multiple natural hazards, particularly the areas adjacent to the lower reaches of our rivers. Traditionally these hazards have included flooding and coastal hazards, which include coastal erosion and inundation.

Many of these areas were significantly impacted by the effects of the Canterbury earthquakes. Damage was caused not only by the direct ground shaking affects, but more importantly the consequences of the secondary effects of liquefaction and basin subsidence.

These have, in turn, had the cascading effect of making the areas more susceptible to other hazards which include fluvial, pluvial and coastal flooding. Future hazards are expected to be exacerbated by accelerated sea-level rise. While some hazards are independent of each other there is often an overlap or continuum across them- some processes act as a 'feed back' loop that exacerbates the original process.

Potentially these inter-linkages between hazards and their cascading effects over both the short and long term may drive the greatest risk. It is therefore very important to understand the 'multi-hazard' risk from natural processes when making decisions.

There are two geotechnical issues in the residential red zone – liquefaction and lateral spread. Links to more detailed geotechnical information summaries are provided below.


Liquefaction

Liquefaction diagram.Environment Canterbury diagram showing liquefaction

Ground shaking can cause liquefaction. This is the process where, during earthquake shaking, sand and silt grains in wet soil are rearranged and the water in the spaces between the grains is squeezed. Pressure builds up in the water until the silt and sand grains 'float' in the water, and the soil behaves more like a liquid than a solid.

The pressurised water is forced up to the ground surface through the easiest path it can find - often through cracks in the ground or concrete. The water takes silt and sand with it, bubbling to the surface, forming sand boils or sand volcanos, or when there is a lot of it, filling up large areas with sand and silt.

When the ground temporarily loses its ability to carry weight and acts more like a liquid than a solid, heavy structures will tilt and sink while light structures like pipes float. Buildings, roads, pipes and tanks on or in liquefied soil are often damaged.

To liquefy, the soil must be:

  • loose - compacted soils tend not to liquefy
  • sandy or silty - clays and gravels tend not to liquefy
  • saturated - the soil must be below the water table.

Liquefaction usually only happens in susceptible soils in moderate to strong ground shaking (when household items start to fall and break).

Many parts of the eastern suburbs, including the residential red zone, and some of the central city provided the right ground conditions for liquefaction.

Liquefaction can cause considerable damage and contribute to land subsidence, causing increased flooding. Substantial liquefaction took place during the September 2010 earthquake and continued with every major aftershock including 22 February 2011.


Lateral spreading

Lateral spreading diagram.Environment Canterbury diagram showing lateral spreading

During an earthquake, liquefied soils can sometimes move sideways, usually towards streams or rivers, but also if the surrounding land is lower than the liquefied land. This is called lateral spreading. Lateral spreading can cause cracking of the ground surface as the soil underneath it moves sideways and can pull apart any structures on or in it.


CERA geotechnical information summaries for the flat land residential red zone

Geotechnical information summaries for the flat land residential red zone were prepared for CERA in July 2012. Detailed mapping and below ground testing was undertaken primarily to better understand geotechnical properties of the soil, especially the probability of liquefaction during another earthquake and, how the ground behaved in the earthquakes and might behave in a future earthquake.

These documents summarise all the area-wide geotechnical information that was considered by CERA as part of the process for making flat land zoning decisions, and the subsequent zoning review. The following maps are included within each of the summaries:

  • CERA residential red zone and Department of Building and Housing technical categories
  • Observed ground crack locations
  • Ground surface observations of liquefaction and lateral spreading
  • Status of wastewater network
  • Ground surface elevation from the February 2012 Light Detection and Ranging (LiDAR) survey. The Canterbury Maps website provides maps which show LiDAR data
  • Change in ground elevation between LiDAR in July 2003 and February 2012
  • Location of suburb-wide geotechnical ground testing
  • Examples of cone penetration test results
  • Area-wide geotechnical and engineering considerations.