The site investigations carried out by the Swedish Nuclear Fuel and Waste Management Co (SKB) at two sites along the Swedish East coast included extensive stress measurements in slim bore holes down to 800 m depth for a repository sited within 400 – 700 m depth. Both sites are composed of crystalline rocks. The objective of the stress estimation campaigns was to contribute to the geoscientific understanding of the sites, as well as to provide input for preliminary design studies carried out at both sites. Triaxial overcoring, hydraulic fracturing and hydraulic tests of pre-existing fractures were used, as well as the indirect observations such as bore hole breakouts and core disking.
The purpose of this paper is to present the strategy applied for assessment of the state of stress and the experiences gained by the extensive stress characterization campaigns at the two sites. An auditing process of measurement results and indirect observations of in-situ stress orientation and magnitudes is also presented, and the outcome of measurements with the two methods in different stress regimes is discussed.
The in situ stress state is a design parameter required for the siting and construction of a repository for final disposal of spent nuclear fuel. This has been one of the focus areas in the recently completed site investigations of the Swedish Nuclear Fuel and Waste Management Co. (SKB). These site investigations were aimed at characterizing the suitability of the Oskarshamn and Forsmark areas, Figure 1. The distance between the two sites is roughly 500 km. A general execution program was presented prior to the start of the investigations (SKB, 2001). The goals were defined to:
present the necessary data on the site for a site-adapted layout of the final repository and assessment of the repository's long-term radiological safety to be carried out,
achieve fundamental geoscientific understanding, i.e. have analyzed the reliability and assessed the reasonableness of the assumptions made with respect to the current states of the site and naturally ongoing processes, and
identify objects that may require special environmental considerations during construction and operation of the deep repository.
(Figure in full paper)
A necessary component of the rock mechanics site descriptive model is the specification of the pre-existing state of stress in the rock mass because knowledge of the stress state is required for both analytical and numerical modelling of the stresses induced by excavation of a repository (Andersson et al., 2002). In this paper the stress measurement campaigns that were used to address objective 1 listed above, are presented and discussed.
The main scientific reference for stress measurements prior to the SKB site investigations.