A New Rheological Hardening Model for Prediction of Creep Deformation of Rock Samples M. Karami & A. Fahimifar Department of Civil and Environmental Engineering, Amirkabir University of Technology, Tehran, Iran E. Pakniat PhD Candidate, Civil Engineering Department, Shahid-Bahonar University, Kerman, Iran ABSTRACT: Time-dependent deformation may be emerged in rocks and rock masses under different stress levels from relatively low stresses up to stresses near peak strength. Most type of rocks, particularly soft rocks, have both reversible and irreversible creep strains under a wide range of stress levels. Thus, a desirable constitutive model should calculate both elastic and plastic creep strains, simultaneously. In this paper, the concept of hardening behavior of geomaterials is adopted to develop a new elasto-plastic visco-plastic hardening model. The model has a non-linear elasto-plastic hardening behavior for the short term instantaneous deformation. The non-linearity is related to the current confiding pressure and the hardening feature is based on Mohr-Coulomb failure criterion. For the delayed deformations, on the other hand, a viscoplastic hardening unit is used consisting of a frictional hardening Mohr-Coulomb slider and of a viscous dashpot element, connected in parallel. The viscoplastic hardening unit is connected to the ordinary viscoelastic Kelvin model in series. As a result, an elasto-viscoplastic hardening model is obtained for long term deformations. The governing equations of the proposed model are implemented in numerical finite difference code (FLAC) using its built-in FISH language for constitutive models, and then applied to a series of triaxial and uniaxial laboratory tests presented in literature. The potential applicability of the model is examined predicting both instantaneous and creep deformations under different stress levels. A discussion is also presented comparing the numerical results and the test data of the samples. 1 INTRODUCTION Time-dependent deformation behavior of rock masses has a significant impact on the stability of rock slopes and underground structures (Tsai, 2007). Regarding the behavior of geomaterials, various kinds of constitutive equations have been developed following different assumptions and principles. Elastic or reversible deformation is one of the assumptions which is considered in classic models. Many researchers have used such assumption calculating tunnel face convergence (Sakurai, 1978; Zhifa, 2001; Kontogianni, 2006, Dai, 2004; Fahimifar et al., 2010). In contrast with the visco-elastic models and assuming fully irreversible strains for the primary, secondary and tertiary stages of the creep curve, some researchers proposed visco-plastic models to evaluate the long term stability of rocks, mainly rock salts, in underground excavations (Malan, 1999; Wallner, 1983; Liao et al., 2004; Erichsen, 2003). However, the weakness of fully vicso-plastic models have been investigated through the experimental creep tests including unloading and reloading tests at which the reversible creep deformation of rocks occurred (Tomanovic, 2006; Hoxha, 2005; Shao, 2003). Besides the fully elastic or plastic models, a number of rheological models have been introduced considering both elastic and plastic time-dependent features of rocks (Tomanovic, 2006; Sterpi and Gioda, 2007; Karami, 2013). Generally, the purpose of such visco-elastoplasic models is developing an exact description of time-dependent creep behavior of soft rocks under different stress levels, which is particularly significant for the stress conditions in the rock mass surrounding tunnels. Regarding this fact, this paper introduces a non-linear elasto-visco- palstic constitutive model. Potential applicability of the model is also examined simulating the creep behavior of marl samples, presented in literature, under different stress levels.