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ABSTRACT The time-dependent features of soft rock, named rheology generally, should be taken into account in the longterm design and maintenance of mountain tunnels. Based on the classic Burger-MC rheological model, a Burger-Deterioration rheological model is proposed in this paper and is implemented in the numerical codes FLAC3D.A deterioration threshold and two deterioration ratios are introduced in this model to consider the time-dependent strength deterioration aspect of the rock mass. The proposed model is applied to an engineering instance (Ureshino Tunnel Line I, Nagasaki, Japan) to account for the delayed deformations that occurred after its completion. The delayed crown settlement and invert upheaval computed from simulations are featured by an exponential characteristic and a stair-typed characteristic, respectively, which agree well with the in-site monitoring data.
1 INTRODUCTION The New Austrian Tunneling Method has innovated the concept of conventional tunnelling from resisting the passive earth pressure to helping the ground support itself, and has been widely used in mountain tunnelling (Carranzas & Fairhurst 1999, Oreste 2003, Guan et al. 2006). When applying this method in soft rock mass, which exhibits strain softening behaviors and time-dependent features apparently, the tunnels would experience a large delayed deformation that might lead to a delayed failure of structure. Therefore, the time-dependent features of soft rock, named rheology generally, should be taken into account in the long-term design and maintenance of mountain tunnels. According to the results from laboratory (or in-site) tests and the experience from engineering practice, many rheological models have been proposed to account for the time-dependent features of rock mass from manifolds. These models can be generally divided into two categories: the classic viscoplastic models and the viscoplastic-damaged models. The former ones try to relate the current strain rate to the current stress directly. Particularly, the relationship between the deviatoric strain rate and the deviatoric stress can be schematically represented by a series of spring, dashpot and plastic slider that connected in parallel and/or in series (Hudson & Harrison 1997, Itasca Consulting Group 1997). The viscoplastic-damaged models are based on the principle of strain and energy equivalence and are derived from a standard thermodynamic dissipation potential (Pellet et al. 2005, Shao et al. 2006). Generally, the dissipation potential consists of two independent potentials, which correspond to the strain softening process and the damaging process, respectively. When these two potentials arewell defined, by applying the normality rule, the plastic strain rate can be formulated as the potential's first-order difference with respect to the current stress, and the damage evolution rate can be formulated as the potential's first-order difference with respect to the thermodynamic force. The viscoplastic-damaged models can account for the tertiary creep phase phenomena that occur in some creep tests, where the classic viscoplastic models fail to do. However, the viscoplastic-damaged models are generally too complicated to be applied in engineering practice.
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