A fully coupled thermo-hydro-mechanical-chemical model for cemented backfill application in geothermal conditions

Abstract

High virgin rock temperature represents one of the greatest bottlenecking constraints for waste tailings recycling in deep mines, and the unique geothermal environment associated with each particular mine calls for customized backfill operation. In this paper, a new fully coupled thermo-hydro-mechanical-chemical model is developed for cemented backfills by invoking basic conservation equations. In addition, the effect of thermal pressurization induced by constrained thermal expansion of pore water that is missing in existing backfill models has been rationally considered in this study based on the generalized Biot's theory. By scrutinizing the complex backfill behavior at various initial and surrounding rock temperatures, novel insights are obtained into the different backfill performance under diverse geothermal conditions. Our numerical investigation has demonstrated that the strong nonlinearity of backfill response stems from the intricate competitions among the multi-physics processes present in the material. In particular, the pore pressure evolution in backfills is shown to be dominated by the interplay among chemical shrinkage, thermal pressurization associated with hydration-heat generation and boundary heating, as well as the resulting fluid migration that modulates the pressure distribution. Moreover, our calculations have also shown for the first time how boundary heating can contribute to backfill instability after an incubation period, highlighting backfill operation in hot mines as the least favorable thermal condition for waste tailings recycling. By examining the effectiveness of different countermeasures in mitigating the impact of boundary heating, guidelines for optimal design are provided in this study to facilitate customized backfill operation under high virgin rock temperatures.