Rocks are often encountered as foundation material for projects such as gravity dams, tunnels, rail and road bridges and underground storage facilities. In the field, the actual stress state is generally polyaxial with different values of minor and intermediate principal stresses. The effect of intermediate principal stress is mostly ignored for simplification while analyzing and designing these structures. In addition, in many situations, the rocks are subjected to loadings which are cyclic in nature. Cycles of loading and unloading induce fatigue damage by altering its mechanical behaviour in the rock which accumulates with increasing cycles. For assessing the performance of a structure with time, it is important to model the evolution of fatigue damage and also consider the effect of intermediate principal stress. The extent of accumulated damage depends on amplitude, frequency, waveform, and confining pressure. The present study discusses results from a series of laboratory cyclic polyaxial strength tests performed on a M20 grade concrete model rock. The tests were performed under monotonic as well as cyclic conditions under load-controlled environment. For cyclic tests, the major principal stress (σz) was varied in cyclic manner from valley stress to peak stress. Complete incremental cyclic scheme was used. In each cycle, the major principal stress (σz) was increased by a fixed amount. Minor and intermediate principal stresses were maintained at constant level during cycles. The cycles of loading and unloading were repeated till failure occurred. The evolution of fatigue damage was modeled through inverted S curve model by using dissipated energy as a descriptor. The effect of the minor and intermediate principal stress on the strength of rock and model parameters has been studied. It is concluded that the intermediate principal stress significantly affects the strength of the rock and fatigue damage model parameters which systematically vary with the stress-state.

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