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Failure of rock specimen in creep tests are known to occur at stresses that are lower than the strength of the intact rock.The time to reach failure depends on the stress levels.Failure in rock specimens occur due to the strength degradation induced by the viscous deformation.In this paper, the parameter-effective viscous strain was introduced to simulate the strength degradation induced during creep.The model was implemented in the three dimensional distinct element code(3DEC).The viscous behavior of rock was characterized by Burgers model and the relation between the effective viscous strain and strength value was determined from laboratory creep test.During creep simulation,the real-time strength value was determined from the effective viscous strain calculated at each time-step in 3DEC.Result from the simulation effort shows that the new model can reproduce the failure of rock specimens undergoing unconfined creep test.