Insights of the asynchronous strain evolution of lean coal in uniaxial compression creep

Owing to the heterogeneity, discontinuity and complex material structure, typical coal exhibits intricate deformation characteristics during long-term creep processes. This study investigates obvious three-stage asynchronous strain evolution characteristics between axial and radial strain of lean coal specimens during uniaxial compression creep tests with staged loading. The radial and axial creep strains are non-synchronous; specifically, the radial creep strain noticeably lags the axial creep strain. Under low-stress levels, both axial and radial strains exhibit instantaneous elastic-plastic behavior with no creep behaviors. At intermediate stress levels, creep strain is not observed before the creep-initiation stress threshold. Axial time-dependent creep strain appears earlier than the radial creep. Under high-stress conditions, although both the axial and radial strains both show pronounced time-dependent creep characteristics, behaving transient and steady creep, the asynchronous strain coefficient and the creep rates of the axial and radial creep differ greatly under identical stress levels. An improved fractional-order creep constitutive model is established and the model parameters were identified. Despite the observed asynchrony, both the axial and radial strains exhibit similar patterns and can well be described by the same creep model, but with different parameters, under identical stress levels. The model parameters of axial and radial strains are independent. These findings can assist predictions of catastrophic failures in geotechnical engineering, contributing to the prevention of geological disasters such as landslides.