Application of Generalized Rheological Model to Creep Behavior of Rock

Abstract

Creep tests were performed on Kobe layer tuff under seven uniaxial compressive stress states. It was found that the creep strain varied in proportion to stress up to the 50% point of unconfined compressive strength (σ_s), and that the creep Poisson's ratio did not vary with time at low stress levels. These results indicate that the time dependent behavior of ground structure consisting of this rock may be analyzed by the linear viscoelastic theory if the design stress is below 0.5σ_s.
It is convenient to use a rheology model when the linear viscoelastic theory is applied. For this purpose, the evaluation technique of rheology model constants must be considered. In this paper, the generalized Voigt model was introduced and the determination method of its constants was discussed.
As the first step of this method, the approximate retardation spectrum was obtained by numerical differentiation of the experimental creep curve. Second, the number (n) of Voigt elements in the generalized Voigt model and the retardation times (T_i, i=1, 2, ……, n) were determined from the number of peak and the peak position in spectrum. Finally, the overall creep process was divided into several time ranges and the compliance J_i in each time range was determined by the method of least squares.
It was proved that the experimental results could be interpreted reasonably by the creep function determined by applying the present method.