Assessment of rock joint roughness using image analysis of damaged area in direct shear tests

This paper describes the applicability of greyscale images to develop a relationship between the brightness integers in damaged areas subjected to direct shear tests, and asperity angles, which can cause the joint roughness coefficients (JRC) values to be affected. The areas damaged during the shearing stage are formed at contacted areas, which appear to be strongly related to the loss of asperity height of the roughness profiles. The photographs taken before and after shearing stages show differences in brightness and sizes of the damaged areas based on consistent light conditions. An image processing method which converts image types and extracts brightness integers from greyscale images, was employed to define damaged areas on the joint surfaces after the process of shearing. In this study, direct shear tests were performed on four core specimens of sandstone and argillite with joint surfaces, and the JRC values of the joints were calculated using roughness parameters obtained from analyses of the measured profiles. Images of the joints taken before and after the tests were then processed using a MATLAB simulation model. The results recommend that the JRC of rock joint specimens can be estimated by analysing 2D images based on a relationship between brightness integers and asperity angles of the roughness profiles