Rock compressibility at the Wairakei–Tauhara geothermal field, New Zealand

Geothermal fluid has been extracted from the Wairakei field in New Zealand since the mid-1950s for electricity generation. This has induced regional subsidence of more than 1 m; in addition, there are a few localised bowls with much greater subsidence. A comprehensive geotechnical investigation with recovery of undisturbed samples from depths of up to 774 m was undertaken with testing of samples from within and outside subsidence bowls to determine material properties, with the aim of improved understanding of subsidence within the Wairakei–Tauhara geothermal system. Results from more than 130 K₀ triaxial compression tests on core samples are discussed. Given the volcanic origins of most of the material present, a wide variation in the measured constrained modulus values is not surprising. Scanning electron microscope images reveal differences between the texture of the soft materials and the very stiff materials; these correlate with the constrained modulus values. The applicability of the Terzaghi effective stress equation is considered in relation to the measured property values; the conventional equation remains valid even for the least compressible materials tested.