The Nonlinear Creep Behaviors of Sandstone Under the Different Confining Pressures Based on NMR Technology

In this paper, the creep behavior of sandstone under the different confining pressures is investigated. The nuclear magnetic resonance (NMR) measurements are carried out to detect the micro-pore characteristics of sandstones before and after compression tests and creep tests. The variations of the parameters, including transverse relaxation time (T2) spectra distribution, percentage of the pore distribution and the changes of porosity are analyzed quantificationally. The experimental results show that creep behavior can be regarded as a process of gradual accumulation of damage to rocks. The confining pressure can limit the increase in porosity during the creep stage, and the effect of confining pressure on the incremental value of porosity is more obvious under high-stress conditions than under low-stress conditions. Moreover, the relationship between the creep stain and the incremental value of porosity is established to reveal the relationship between micro-crack expansion of rocks and macro-creep characteristics of sandstone. It is found that, as the loading ratio or confining pressure increases, the variation of the creep strain is similar to the variation of the incremental value of porosity. The larger the confining pressure, the smaller the effects of creep strain on the incremental value of porosity. However, the greater the loading ratio is, the faster the incremental value of porosity increases. The present study can provide a theoretical basis for the deep mining of mineral resources and the structural stability analysis of deep-buried underground engineering.