The prediction of a rock joint's peak shear strength becomes complex when its joint surfaces are not fully accessible, such as the rock foundation under an existing concrete dam. This paper presents a case study with the application of a newly developed methodology that uses objective observations of the 3D roughness and joint aperture from drill cores to predict the peak shear strength of large natural, unfilled rock joints. Eight rock joint samples obtained after core-drilling an existing rock joint adjacent to the foundation of Storfinnforsen dam in Sweden were used. The main benefit of this approach is that it enables the prediction of both the mean value and the statistical uncertainty of the peak shear strength under conditions of difficult access. This information may be of importance in order to assess the safety in the foundation of an existing concrete dam.
The hydropower industry in Sweden is responsible for approximately 45% of the total energy production in the country. The fact that many of these dams have been in service for over 50 years may raise concern regarding their structural safety, due mainly to adaptation to new regulations and the impact of climate changes (Krounis 2016). In recent years, the safety evaluation of various existing concrete dams have presented some difficulties due to uncertainties associated to the assessment of the shear strength of horizontal or sub-horizontal rock joints present in their rock foundations (Johansson 2009).
The mechanical behavior of rock joints is complex and is affected by a number of different parameters that are difficult to estimate in the field (Ríos Bayona 2022). Various attempts have been made in recent decades to develop both empirical and analytical criteria to increase the understanding of how the roughness, matedness, and surface area of a rock joint interact and contribute to its peak shear strength (e.g., Barton & Choubey 1977; Barton & Bandis 1982; Johansson & Stille 2014; Johansson 2016; Casagrande et al. 2018). However, the majority of the proposed criteria fail to account for the complete interaction of the aforementioned parameters (Ríos Bayona 2022). Additionally, the application of available techniques to predict the peak shear strength in the field becomes complex when the rock joint surfaces are not fully accessible, such as the rock foundation under an existing concrete dam. In these cases, to reduce the uncertainties in the prediction of the peak shear strength, it requires that a representative number of observations of both 3D roughness and aperture characteristics are performed.
