Estimation of the joint roughness coefficient of rock joints by consideration of two-order asperity and its application in double-joint shear tests

Joint roughness has a significant influence on the shear behavior of rock joints. Many different statistical parameters have been used to estimate the joint roughness coefficient (JRC) of rock joints, depending on what is most easily available and convenient. Six statistical parameters, Z2, SF, RP − 1, log(Z2), logSF and RP−1 of ten typical roughness profiles were calculated at different sampling intervals (SI). The results indicate that the JRC of rock joints could not be accurately estimated by using only a single statistical parameter. Because the first-order and second-order asperities of joints have different effects on shear behavior, a classified and weighted fitting formula, JRC = 16.09 logZ21st + 12.70 logZ22nd + 33.75 (SI = 5.0 mm & 0.5 mm), is proposed to estimate the JRC. Shear tests on sandstone joints indicate that the maximum JRC along the shear direction is appropriate to represent the total joint surface and estimate the shear strength. This formula was adopted to estimate the JRC in double-joint shear tests, and the results show that the mechanical behavior of double parallel joints is closely related to the interlayer rock and the weaker joint. Under lower normal stress, the interlayer rock does not fracture, and the weaker joint determines the peak shear strength of the rock specimen. In contrast, under higher normal stress, the peak shear strength is attained when the tensile fractures initiate in the interlayer rock, and it has also relevancy to the JRC of double joints and interlayer thickness.