We report on two sites in Iceland, Blanda and Kárahnjúkar, where hydraulic fracturing stress measurements were performed in the initial stages of pre-excavation design of two underground hydroelectric projects. The dominant rock is young basalt, naturally fractured and blocky. The measured stresses were used in both cases to define the boundary conditions in the modeling of the stability conditions around the powerhouse caverns. Both caverns have been successfully constructed.
The advent of the hydraulic fracturing (HF) method of deep in situ stress measurements some 35 years ago has been of particular importance to the hydroelectric power (HEP) industry. Until then the state of stress at the site and depth of the major underground components of a planned project was typically unknown in advance of excavation. In the absence of that unique parameter the design of cavern stability, orientation, and support, among others, had to be mostly guessed until excavation, at which time any needed corrections would be very expensive. The ability to use HF at any depth that a borehole could reach enabled the measurement of in situ stress early in the site investigation stage, and thus provided crucial information for the pre-excavation design of the underground works. The first projects that took advantage of the new stress determination technique were the Helms and Bad Creek hydroelectric schemes in California and South Carolina, respectively (Haimson 1977). Since then, the use of HF ahead of excavation has become routine all over the world. In Iceland, starting in the late 1970’s, HF stress measurements were conducted at several potential hydroelectric sites. This paper reports on two case histories, Blanda and Kárahnjúkar, where HEP projects were constructed. The Blanda Hydroelectric Project is located in northern Iceland, some 340 km from the capital city of Reykjavik (Fig. 1).