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Peak Shear Strength of Rock Joints – Towards a Methodology for Prediction Based on Field Data

Time: Wed 2022-06-08 13.00

Location: F3, Lindstedtsvägen 26 & 28, Stockholm

Video link:

Language: English

Subject area: Civil and Architectural Engineering, Soil and Rock Mechanics

Doctoral student: Francisco Ríos Bayona , Jord- och bergmekanik

Opponent: Dr. José Muralha, LNEC, Laboratória Nacional de Engenharia Civil

Supervisor: Docent Fredrik Johansson, Jord- och bergmekanik; Adjungerad Professor Diego Mas Ivars, Jord- och bergmekanik; Professor Stefan Larsson, Jord- och bergmekanik

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The rock joint shear strength at field scale is an important design parameter and remains a challenge for rock mechanics engineers. In Sweden, there exist a large number of concrete dams that are founded on rock masses which in many cases contain sub-horizontal rock joints. The action of water pressure and uplift forces makes sliding along these sub-horizontal rock joints one of the most critical failure mechanisms to be considered in a dam’s safety evaluation. Despite the various attempts to develop empirical, analytical, and numerical methods in recent decades, the uncertainty in the prediction of the peak shear strength of rock joints is still significant. None of the existing methods today fully capture the complex interaction between all the relevant parameters.

The overall aim of this research project is to develop a methodology for the prediction of the peak shear strength of rock joints in cases where the whole joint surface is not accessible, such as the foundation under an existing concrete dam. To accomplish this, the prediction of rock joint peak shear strength was studied (1) numerically using discrete element method (DEM), (2) analytically by developing a peak shear strength criterion, and (3) experimentally by characterising the surface roughness and aperture of the tested samples based on high-resolution scanning prior to the direct shear tests.

The results of the numerical study showed that the shear test environment in PFC2D used in this project has the capability of simulating the peak shear strength of actual rock joints both qualitatively, and quantitatively. However, a 3D approach is needed to overcome the limitations of the 2D approach, and to realistically simulate the interaction between the asperities in contact during shearing.

The results of the analytical study showed that the matedness between the contact surfaces of natural, unfilled rock joints needs to be accounted for when predicting their peak shear strength. In this study, the matedness of the tested natural, unfilled rock joints was estimated based on measurements of the aperture between their contact surfaces. The relationship between matedness and joint surface aperture was integrated in a further developed peak shear strength criterion. Furthermore, the performed investigations on two large-size rock joint samples showed that their peak shear strength can be reasonably well predicted based on information from several small-size samples, such as drill cores. In this work, the drill cores were simulated based on the scanning measurements of the joint surfaces at large size. The measured 3D roughness and aperture in each simulated drill core was used to predict their respective peak shear strength by applying the further developed peak shear strength criterion. Each simulated drill core was considered as an independent component of a parallel system. The peak shear strength of the large-size samples was predicted based on the mean value of the predicted peak shear strength of the small-size samples, including the statistical uncertainty due to the number of small-size samples used in the prediction. The main benefit of this approach is that it may enable prediction of the peak shear strength of large natural, unfilled rock joints under conditions of difficult access, such as a sub-horizontal rock joint under a concrete dam. The developed methodology has only been tested on two large-size samples and further research is necessary to verify its applicability.