Geotechnical Investigations of Subsidence in the Wairakei-Tauhara Geothermal Field

Abstract

The extraction of geothermal fluid from the Wairakei Field in the 1950’s resulted in the formation of a localised subsidence bowl 1km across with maximum subsidence of up to 20m. More recently a number of smaller subsidence features have developed above the adjoining and connected Tauhara geothermal field. Contact Energy Limited required geotechnical investigations sampling and testing to be undertaken both to satisfy conditions attached to its consents for continuing extraction from the Wairakei Field and also to support consent applications for major extraction from the Tauhara field.

Simplified modelling had been previously undertaken on the subsidence by a number of parties using assumed or back calculated soil compressibility parameters. Contact proposed that a 3D Finite Element analysis be undertaken with measured soil properties and recorded or predicted changes in ground fluid pressures.

The geotechnical investigations required continuous coring and the undisturbed sampling of a variety of materials ranging from soft weak surface tephras to hard strong ignimbrites in geothermal conditions to depths of up to 800m.
This paper describes the procedures used to undertake the drilling and coring techniques used and the sampling and testing undertaken to determine the compressibility characteristics of the materials. The drilling equipment and procedures needed to satisfy the requirements for drilling into geothermal conditions as well as achieving the geotechnical objectives of maximum recovery of high quality core. An innovative technique was used to recover undisturbed samples for compressibility testing in special tri-axial cells designed and built for the project.

The investigation programme involved nine boreholes with a maximum depth of 774m and a total length of 4391m. A total of 3928m of core was recovered with 269 undisturbed samples from which 121 were selected for laboratory compressibility testing.