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Tāhunanui Slump geotechnical report

In September 2022, Toka Tū Ake EQC engaged Tonkin + Taylor to undertake drilling and monitoring activities on the Tāhunanui Slump, to support our understanding of the impacts of the severe weather event on the Nelson Port Hills. 

Tonkin + Taylor provided Toka Tū Ake EQC with a geotechnical report on their findings.

This report:  

  • significantly improved the understanding of the depth, extent and mechanisms controlling deep-seated instability in the northern part of the Tāhunanui Slump 
  • improved the understanding of the extent and mechanisms controlling subsidiary areas of movement observed within the Tāhunanui Slump following the event
  • indicated that the area of Tāhunanui Slump affected by deep-seated displacements is not as extensive as previously thought
  • highlighted the relationship of rainfall to groundwater levels in contributing to observed land movement, and the potential influence of mudstone layers in the Port Hills Gravel bedrock in influencing groundwater level and the extent of deep-seated land movement. 

The report does not make recommendations around future land-use with respect to residential land within the Tāhunanui Slump.