Phil Glassey, John Begg and Katie Jones
03 Sep 2012 to 03 Sep 2012
Geological maps traditionally have depicted a 2 dimensional view of urban geology because of technology (printing) constraints. Portraying geology in 2D under-represents the insight that mapping geologists have into the subsurface. This 2D constraint has perpetuated into limited representation of geotechnical characterisation and geological hazards that affect urban and peri-urban areas in New Zealand.
In many cases there has been little subsurface input into urban geological maps. Until recently subsurface data has been distributed over a number of geotechnical organisations, developers and resource industries. Local government typically hold most of these data but in relatively inaccessible formats such as hard copy reports provided as supporting documents for development proposals. Extracting useful geological and geotechnical data has been difficult to provide a more comprehensive geological and geotechnical model of what lies beneath a city.
In some countries, such as Britain, the development of standards and capture of subsurface data has been in place for some time, allowing distributed data to be collated into 3D geological models and allowing additional investigations to be targeted where there are gaps. Now in New Zealand, we are seeing the adopting of standards that allow these distributed data to be bought together and utilised in developing 3D geological, geotechnical and resource models of the places we live.
The Canterbury earthquakes of 2010 and 2011 have hastened this process in some respects, with significant subsurface investigation data starting to be bought together using an internationally recognised geotechnical database standard, and being made publically available by CERA. Presently the database contains about 1500 geotechnical probes (cone penetrometer tests), with work on integrating the c.10000 drill holes within the greater Christchurch area, held by Environment Canterbury, still on-going.
These data, modelled by GNS Science, are revealing extraordinary detail and variation in the sediments that underlie Christchurch but also confirming broad geological groupings relating to alluvial gravel influx into a marginal marine setting. This has allowed the development of a detailed 3D geological and geotechnical model of the subsurface that has guided redevelopment in Christchurch, utilising both existing surface and subsurface data. This model is being used to evaluate liquefaction potential and guide redevelopment of the city.