NEW PEAK SHEAR-STRENGTH CRITERIA FOR ANISOTROPIC ROCK JOINTS

In general, roughness profiles of rock joints consist of non-stationar y and stationary components. At the simplest level, only one parameter is sufficient to quantify non-stationary joint roughness. The average inclination angle I, along with the direction considered for the join t surface, is suggested to capture the non-stationary roughness. Most of the natural rock joint surface profiles do not belong to the self s imilar fractal category. However they may be modelled by self-affine f ractals. Using a new term called specific length, it is shown that eve n though the fractal dimension D is a useful parameter, it alone is in sufficient to quantify the stationary roughness of non-self similar pr ofiles. Also, it is shown why contradictory results for the estimation of D of non-self similar profiles appear in the literature. To estima te D accurately for non-self similar profiles, it seems necessary to u se scales of measurement less than the crossover length of the profile . Because the crossover dimension of joint roughness profiles can be e xtremely small, in practice it may be quite difficult to measure rough ness at scales of less than the crossover dimension and thus to estima te D accurately. To overcome the aforementioned problems, it is sugges ted to combine D with a parameter which is negatively correlated to D and also has the potential to compensate for the errors caused by an i naccurate D, and to use the combined parameter to quantify stationary roughness in practice. Four new strength criteria which take the follo wing general form are suggested for modelling the anisotropic peak she ar strength of rock joints at low normal effective stresses (0-0.4 tim es unconfined compressive strength): tau = sigma tan(phi + alpha(SRP)( c)[log(10)(sigma(J)/sigma](d) + I) where sigma, tau, sigma(j), phi amd SRP denote, respectively, the effective normal stress on the joint, p eak shear strength, joint compressive strength, basic friction angle, and the stationary roughness parameter. The following four options sug gested to represent the term alpha(SRP)(c) : alpha z( )'(c), alpha (Kd Dc)-D-B, alpha (KsDc)-D-b or alpha (KvDc)-D-b. Joint roughness data sh ould be used to estimate the roughness parameters I, z(2)', k(d), K-s, K-v and D in different directions on the joint surface. Parameter D r eflects the rate of change in length in response to a change in the sc ale of measurement r. Because z(2)', K-d, K-s and K-v are scale-depend ent parameters, they can be used to model the scale effect. The coeffi cients a, b, c and d in the strength criteria should be determined by performing regression analysis on experimental shear strength data. In practice, to allow for modelling uncertainties, the new equations sho uld be used with a factor of safety of about 1.5.