Issues related to the long-term stability of unlined water tunnels in Norwegian hydropower projects – a case review

摘要

Most of the waterway system in Norwegian hydropower plants consists of low-, medium-, and high-pressure headrace tunnels, high-pressure shafts, and tailrace tunnels. More than 90% of these plants were built before the 1990s and these water tunnels have been in operation at full hydrostatic pressure for over 30 years. More importantly, these tunnels are built typically in very old (older than 400 Ma) hard rock consisting mostly of high quality rock mass. Therefore, the design philosophy of waterway system in these plants considers the rock mass is capable of absorbing the water pressure exerted on the tunnel wall. Hence, no rock support is applied in these tunnels apart from in critical areas where fractured rock mass, faults, and weakness zones exist. On the other hand, in recent years, some of the high-pressure tunnels and shafts have experienced partial collapses, which forced the stoppage of the power plant and required comprehensive repair work to be carried out to bring the plant in operation once again. This paper will describe one such incident in which the author was directly involved in the inspection and evaluation of the collapse. The paper also briefly highlights geological conditions at the project and evaluates and discusses the chronology of the event. The aim of this manuscript is to increase awareness in the operation of hydropower scheme by presenting a tunnel collapse case of the waterway of a hydropower project in Norway and to highlight on the long-term stability issue.