Explicit modeling of dilation, asperity degradation and cyclic seating of rock joints

This paper presents another development in the discrete element simulation of joint behavior, namely, an appropriate formulation of dilation, asperity degradation, and seating during cyclic motion associated with earthquake and explosive excitation. Nonassociated plasticity is assumed during sliding along local asperity surfaces, however, this causes dilation to occur for the overall joint. The most significant advance is the addition of a continuity condition, the β factor, for normal displacements during joint sliding over local roughness (or asperities) to control seating or bulking during cyclic shear when asperities are degraded or damaged. Three numerical examples are presented to illustrate model calibration and implications for rock mass behavior. The first example involves simulation of a laboratory cyclic direct shear test performed on an artificially-produced tension joint in sandstone. The second example demonstrates use of the seating/bulking parameter, β, to explicitly control continuity of normal displacements for the overall joint. Parameter studies indicate that the value of β should be close to zero and can be uniquely identified from laboratory shear test results. The third example simulates response of a cavern within a moderately jointed rock mass, with and without joint roughness, when subjected to cyclic excitation.