| Title | SUBSIDENCE AT KAWERAU GEOTHERMAL FIELD, NEW ZEALAND |
|---|---|
| Authors | S. Kelly, P.A. Siratovich, J. Cole, J. Clark |
| Year | 2015 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Kawerau Geothermal Field, Subsidence, 3D Modelling, Laboratory testing. |
| Abstract | Subsidence at Kawerau Geothermal Field (KGF) is closely monitored due to the presence of the Carter Holt Harvey (CHH) and Norske Skog Tasman (NST) pulp and paper mills which are located within the operating field and have the potential to be affected by subsidence. Spatial analysis of re-levelled benchmarks from 2007 – 2013 revealed two scales of subsidence: 1) Field wide subsidence currently covering ~17 km2; and 2) Four localised subsidence anomalies covering 150 – 400 m2 each. This paper focuses on two of the localised anomalies north of the CHH and NST mill site, and the mill site itself. Three dimensional modelling of KGF was completed to determine the stratigraphy below the localised subsidence anomalies and whether the same conditions exist beneath the pulp and paper mills. Samples of Recent alluvium were taken from three geotechnical boreholes drilled in the mill complex and analysed for materials such as smectite clay that could contribute to subsidence across the mill site. Samples of Tahuna and Caxton formations were also taken from boreholes KAM11 and KA37a to investigate the hypothesis that an anomalous thickness of Tahuna Formation could be responsible for the two localised subsidence features studied and that the Tahuna Formation is more compressible than the overlying Caxton Formation. To do this, the physical properties of both formations were compared and a new methodology utilising a soil shear box to measure compressibility was developed. Results show that the recent alluvium sampled from the mill site does not does not contain smectite clays and is unlikely to contribute to the formation of a subsidence anomaly. Compressibility measurements of Tahuna and Caxton Formations show that the Tahuna Formation is twice as compressible as the Caxton Formation when saturated. However further investigation would be necessary to confirm whether this contributes to the mechanism for the localised subsidence features. |