Shear behaviour of rock joints under cyclic loading and constant normal stiffness conditions

Shear behaviour of rock joints under cyclic loading and constant normal stiffness conditions

Shear behaviour of rock joints subjected to cyclic loading was previously studied mostly under Constant Normal Load (CNL) conditions which does not accurately simulate the actual deformation behaviour of field rock joints. Natural joints are often filled with materials such as sand, clay or silt. The shear behaviour of rock joints is affected considerably by the presence of the infill within joints. This poses significant concern for excavations which are constructed in close proximity of jointed rock mass. None of the previous research investigated the shear behaviour of infilled rock joints under cyclic loading. This thesis studies the shear behaviour of rock joints under cyclic loading and Constant Normal Stiffness (CNS) conditions.
Triangular joints asperities with initial angles of 9.5° (Type I), 18.5° (Type II) and 26.5° (Type III) to the shear movement, and replicas of a field rock joint surface cast using high strength Plaster of Paris were tested. Experiments were performed using the CNS cyclic direct shear apparatus updated for this study. The samples were sheared under initial normal stress levels ranging from 0.16 MPa to 2.5 MPa, representing the in-situ stress conditions as experienced in the field. Laboratory test results indicate that, the shear strength and the dilation component decrease with increase in the loading cycles. The asperity damage appeared to be a function of the external energy exerted on asperities during shearing. Thus, the asperity damage was higher for greater initial asperity angles and normal stresses. Furthermore, the effects of shear rate on shear behaviour of rock joints under cyclic loading were investigated. The strength decreased with increase in the shear rate.
The shear behaviour of rock joints infilled with mixture of Clay and Sand at initial moisture content of 12.5% was studied under cyclic loading and various normal stresses ranging from 0.56 MPa to 2.4 MPa while a constant shear rate of 0.5 mm/min and a constant normal stiffness of 8 kN/mm were applied. Types I and II asperity surfaces were selected to prepare infilled joints with infill thickness to asperity height ratio of 0.3, 0.6 and 1. The shear strength of infilled joints was observed to decrease with increase in the number of shear cycles due to asperity damage and deformation of infill material. The variation of normal displacement with shear displacement was dominated by dilation and contraction, depending on the infill thickness to asperity height ratio and initial normal stress.
An analytical model based on the energy balance theory was developed to predict the shear behaviour of clean (non-filled) rock joints under cyclic loading and CNS conditions. An empirical relationship was also proposed to account for the effect of shear rate on cyclic loading shear strength. The concept of Normalised Cyclic Strength Reduction of infilled joints (NCSRi ) was introduced and incorporated in a mathematical model to replicate the reduction in the shear strength of infilled rock joints with increase in the number of shear cycles. Model coefficients were calibrated using laboratory results. In general, the modelled results were in good agreement with the experimental data.
The capabilities of the two built-in constitutive models, simulating the shear behaviour of rock joints under cyclic loading and CNS conditions that are available in Universal Distinct Element Code (UDEC), were investigated. The Coulomb slip model replicates different shear behaviour in forward and backward shearing when the asperity damage is not significant. For the asperity breakage mechanism, the continuously yielding model describes the effects of asperity damage on shear strength and dilation angle.

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