A new index to describe joint roughness coefficient (JRC) under cyclic shear

• The JRC has a strong directivity for irregular undulated joints.
• Surface inclination can reflect the roughness difference under the condition of cyclic shear.
• β100% is suggested as a new index to describe JRC considering shear direction.
• The order of the Barton standard profiles should be adjusted under backward positive shear process.

Rock joint roughness coefficient (JRC) is an important parameter to determine the shear strength of unfilled hard rock joints. It is of great significance to acquire a representative value of JRC. For irregular undulated joints, the JRC has a strong directivity and surface inclination can reflect the roughness difference under the condition of cyclic shear. Considering Barton's standard joint roughness profiles as an example, the secant angles and tangent angles of surfaces dipping opposite to shear direction were calculated and then the mathematical relationships with JRC were investigated. Based on the results of the study, it can be said that all mathematical relationships of surface inclination of surfaces dipping opposite to shear direction with JRC satisfy the power law equations. The average value of entire secant angles of all surfaces dipping opposite to shear direction (β100%) has a higher sensitivity to evaluate JRC comparing with other parameters. It (β100%) is recommended as a new index to describe JRC considering the directivity. Under cyclic shear, it shows that the JRC of forward positive shear process (JRCfp) is basically larger than that of backward positive shear process (JRCbp) except the first and the fifth standard profile. Moreover, the JRCbp value of the seventh standard profile is smaller than that of the fifth and sixth one. With the digitized sampling interval increasing, the correlation coefficient of the relationships between β100% and α100% (average value of entire tangent angles of surfaces dipping opposite to shear direction) and JRC of Barton standard profiles both present a trend of decreasing and the former is a little larger than the latter under the condition of equal sampling interval.