CS5B - Concurrent Session 05B: Alternative Method to Model Dam Concrete-Rock Foundation Interface Shear Strength

Abstract Description: The Barton-Choubey method is used to estimate the shear strength of rock joints based on their roughness and material properties. It was proposed by N. Barton and V. Choubey in 1977 and is widely used in rock mechanics. In this paper, it is shown how this method has been applied at several hydroelectric dams in Brookfield Renewable’s (Brookfield) portfolio for the assessment of foundation interface shear strength of concrete gravity dams to meet regulatory requirements with regard to sliding factor of safety.

For the Brookfield dams, the method is applied by field surveying of the asperity of rock outcrops directly downstream of the dam. This is done using two techniques that will be described. The measured asperity is used to estimate the Joint Roughness Coefficient (JRC) which will be discussed in detail. Using the unconfined compressive strength of the dam concrete as the JCS, and the estimated effective normal stress on the base of the dam in contact with rock, the governing equation (see paper) can be solved for shear resistance. The advantage of this method is that it provides a non-linear strength relationship between normal stress and interface shear resistance that better represents the mechanism for development of concrete-rock interface strength compared to the Mohr-Coulomb model or friction alone. This method considers the two mechanisms of strength at the concrete-rock interface where asperity exists, both of which are dependent on the normal stress on the interface. Namely, these two mechanisms are a linear interface friction at lower normal pressures (riding up and over asperities with no material shear) followed by a non-linear transition to asperity shear through the dam concrete at higher normal pressures. This latter component transitionally occurs when the bedrock asperities begin to preferentially shear through dam concrete as the path of least resistance versus overcoming the frictional resistance and riding over the asperities at higher stresses.

Based on the use of this method for several small to medium size regulated dams, this paper will discuss the techniques used to determine the equation terms (Phi(), JRC, JCS) and provide discussion on: selection of the downstream surfaces selected for survey of asperity; techniques of the survey; application of the interface shear strength in sliding stability analysis and; questions regarding these method that have been raised in past regulatory reviews.