THE EFFECT OF FRICTIONAL FILL THICKNESS ON THE SHEAR-STRENGTH OF ROCK DISCONTINUITIES

The results of a series of direct shear tests on models of a sandstone discontinuity of mean roughness amplitude 7 mm and filled with different thicknesses of dry pulverised fuel ash (PFA) are reported. Peak shear strength is shown to decrease with increasing fill thickness (f), expressed as a ratio to discontinuity mean roughness amplitude (a), and approaches a minimum when f/a is between 1.25 and 1.5, depending upon the normal stress. Residual strength decreases less markedly with increasing f/a ratio and tends towards a constant value when f/a > 1.0. For layers of fill thicker than the discontinuity mean roughness amplitude, failure planes may develop partly through the fill and partly along the interface. When f/a > 0.8, the peak shear strength of the discontinuity is less than that of the fill but higher than that of the rock-fill interface. Expressions for prediction of peak and residual friction coefficients for a filled discontinuity are derived from the experimental results: mu = mu(min) + (mu(max) - mu(min))n and n = [1 - 1/c(f/a)]m where mu =frictional coefficient (%), mu(max) is that for the clean discontinuity, mu(min) is the minimum for the system, f/a is the ratio of fill thickness to asperity height and c and m are experimentally derived constants.