A METHOD FOR DETERMINING THE JOINT ROUGHNESS COEFFICIENT (JRC) OF FRACTURES WITH AN APPLICATION TO THE SHEAR STRENGTH OF CONCRETE/ROCK INTERFACES

The method is based on the dimensionless measure of the roughness angle as defined by the relationship, cos(i)=LD/LT, where i is the roughness angle. LD is the direct length of the fracture (end to end), and LT is the trace length of the fracture. The angle i was determined for the 10 standard JRC profiles (0 to 20 in steps of 2; Barton and Choubey, 1977). The standard profiles were digitized 18 times each to minimize digitization errors and the angle i calculated. The best-fit relationship determined by regression between i and JRC is: JRC=-6.827 + 1.815(i) - 0.028(i^2), with a coefficient of determination of 0.97. The fractal nature of the measurement interval is considered in applying the results to fracture lengths greater than 10 cm, the length of the Barton and Choubey (1977) profiles.

The undulations and asperities along the interface of a prepared rock foundation and the concrete section of a dam have a significant influence on and are an important component of the shear strength of the interface surface. Cross-sections of a dam foundation were constructed every 15 feet (resulting in 89 sections), digitized, and the i angle calculated. From the above relationship, JRC values were calculated for each section. Average JRC values were then calculated for the different integral concrete water retaining structures (bulkheads, spillway, and powerhouse). The average JRC value for the particular structure and other needed parameters (such as Joint Compressive Strength) are then substituted into Barton and Choubey’s (1977) empirical shear strength equation and a non-linear shear resistance function (SRF) is calculated for each structure. The resulting SRF’s are input into the commercially available CGDAMS program, developed specifically for the stability analysis of concrete dams.