Laboratory tests were performed to explore the effects of frequency on the deformation property of rock salt subjected to cyclic loading under confining stress condition. A two-stage (transient and steady) evolution law of axial cumulative strain was obtained through cyclic stress-strain curves complied from the loci of minimum load of each cycle. The unloading modulus decreases with loading cycles/time as a negative exponential function. The damage of rock salt under cyclic loading is reflected by the degradation of elastic modulus. A unified damage evolution equation considering the effects of frequency and time was established based on the experimental results. A new creep damage model for rock salt was proposed by introducing the non-stationary modular components in Burgers creep equation according to the Lemaitre equivalent strain theory. Comparative study indicates that the new model can accurately describe the creep deformation and damage evolution of rock salt under different loading frequencies. This paper provides a more reasonable model and relevant mechanical parameters for the stability analysis of compressed-air storage cavern in the injection and withdrawal durations.
The time-dependent properties of rock salt play a key role in long-term cavern stability and safety. The typical creep deformation consists of the transient, the stationary, and the accelerated phases. The internal variable theory and elasto-viscoplastic component method have been frequently used to formulate the constitutive equation of the primary creep of rock salt (Aubertin et al., 1996; Munson et al., 1996; Cristescu, 1993; Musso & Vouille, 1974). The steady state creep relationship of rock salt is mainly built based on the creep mechanism of metal material (Pfeifle & Senseny, 1983). In 1990s, continuum damage mechanics (CDM) approach was introduced in the study of the creep characteristics of rock salt (Chan et al., 1992).
Generally, the mechanical parameters of the constitutive models for representing inelastic flow due to coupled creep, damage, and damage healing in rock are assumed to be constant. However, the elastic modulus, strength, and the viscosity of rock mass subjected to multi-field stress and weathering tend to decrease with time (Sun, 2007). The reduction of mechanical parameters objectively reflects the degradation of mechanical properties. Therefore, the damage evolution could be described by introducing variable parameters into the stationary creep constitutive model, which is a new method for investigating the creep damage property of rock material (Zhang et al., 2009; Yan et al., 2010; Zhu et al., 2010; Li & Wang, 2009).
