Strong timeliness characteristics of coal and rock based on nonlinear rheological theory

The environment of deep resources is characterized by high in⁃situ stress，high seepage pressure and high temperature. The subsidiary characteristics of strong disturbance and strong timeliness are of great significance to the seepage theory of coupling multi⁃field and multi⁃phase at depth. The study of deep rock mechanics often considers the aging characteristics，thus it is necessary to define the strong timeliness scientifically. Combining with the occurrence characteristics of deep rock mass and the process of resource exploitation，the meaning of deep strong timeliness is given. It is proposed that strong timeliness includes three aspects:accelerated creep，brittle⁃duc⁃ tile transition and weakening of physical and mechanical parameters. The onset of tertiary creep is determined by the non⁃linear rheological theory， and the strong aging characteristics of deep rock mass are studied. According to the existing rheological theory，it is concluded that the critical damage threshold for the accelerated creep of rock mass should be reached，and the critical damage threshold can be determined by non⁃elastic volume strain，po⁃ rosity change or acoustic emission data. The creep test of rock under uniaxial or triaxial stress can distinguish the start⁃ ing point of strong timeliness from strain acceleration. According to the rheological constitutive model of rock，the strong timeliness condition can be determined by the stability of rheological differential equation. Based on the analysis of rock non⁃linear rheological differential equation，a power⁃law relationship between strain acceleration and strain rate is proposed under strong aging characteristics. The specific parameters can be derived by experimental data. The anal⁃ ysis of rock strain， strain rate and strain acceleration curve shows that the two inflection points of strain acceleration curve can be used as the critical values of creep attenuation⁃steady⁃state and steady⁃tertiary state phase transition. Based on the triaxial rheological experimental data of deep coal，the onset of strong timeliness is de⁃ termined，and the parameters in the equation of strain acceleration and strain rate under strong aging condition are fit⁃ ted. Based on the mining stress path of deep coal，the creep⁃seepage experiment of unloading confining pressure is carried out，and then the characteristics of coal permeability increasing sharply under strong timeliness are ana⁃ lyzed.