NATURAL FRACTURE PROFILES, FRACTAL DIMENSION AND JOINT ROUGHNESS COEFFICIENTS

Thirteen natural rock profiles (Barton and Choubey, 1977) are analyzed for their fractal properties. Most of the profiles were found to approximate fractal curves but some also showed features of specific wavelengths and amplitudes superimposed. on fractal characteristics. The profiles showed fractal dimensions from 1.1 to 1.5 covering a range of self-similar and self-affine curves. The analysis results suggest a negative correlation between fractal dimension, D, and amplitude, A. Joint roughness coefficients (JRC) show a positive correlation with amplitude, A, and a negative correlation with fractal dimension, D. A numerical model of fracture closure is used to investigate the effects of different profile characteristics (D, A and sample size) on the nature of dilation and contact area, using the natural profiles and synthetic fractional Brownian motion profiles. Smooth profiles (low JRC, high D, low A) display many small contact regions whereas rough fractures (high JRC, low D, high A) display few large contact areas. The agreement with published experimental data supports the suggested correlations between JRC and the fractal parameters, A and D. It is suggested that observed scale effects in JRC and joint dilation can be explained by small differential strain discontinuities across fractures, which originate at the time of fracture formation.