Major engineering constructions on top of a high-temperature geothermal system: problems encountered at Tokaanu, New Zealand

The Tokaanu hydroelectric power station, a 2-km long section of the headrace tunnel, and an approximately 0.5-km long segment of the tailrace were sited along the broad, southern resistivity boundary of the active, high-temperature Tokaanu–Waihi geothermal system. Exploration drilling over the tunnel route showed that thermally altered rocks with temperatures up to 42 °C would be found at tunnel level. No toxic gases were detected and excavation of the tunnel went ahead. Temperatures above 99 °C, however, were encountered at shallow depths (<20 m) near the base of the powerhouse and along a section of the tailrace canal. This produced flashing discharges at the wellhead of several test bores. The potential for hydrothermal eruptions, which could be triggered by reducing hydrostatic pressures, was tested by stepwise dewatering of two, max. 10,000 m2, artificial ponds. No eruptions occurred and the canal was excavated after completing the tests. A heat balance analysis of the final test is presented. For safety, the bottom of the canal was raised and widened over the hot ground.