Mechanical properties of brown coal under different loading conditions

Abstract

The Latrobe Valley in Victoria, Australia has the largest brown coal deposit in the world, where large-scale brown coal mines were established in the early 1970s. The geotechnical stability of mining batters requires a comprehensive understanding of the mechanical behavior of the brown coal. Therefore, an experimental study was undertaken to evaluate mechanical properties of brown coal in different loading conditions. In doing so, effects of intermediate loading histories on the mechanical properties of brown coal in a multistep loading (ML), consolidated undrained (CU), triaxial compression (TC) test using a single specimen were evaluated. A set of single-step loading (SL) tests at different confining pressures, with postpeak ML with unload/reload cycles, were also performed. It was found that peak and residual strength and stiffness values and stress–strain relations of brown coal can be estimated accurately by a relevant ML test. Therefore, cyclic loading and accumulated irreversible axial strain do not have any significant influence on the mechanical properties of brown coal. By observing porepressure behavior of samples during SL loading, it was found that, with an increase in confining pressure, the amount of damage to the specimen as a result of axial loading decreases, and therefore, the sample experiences contraction even after the peak stress is achieved. The ML increasing-confining-pressure test was found to generate more accurate peak and residual strength values as compared with the ML decreasing-confining-pressure test. In addition, it was concluded that, during the ML decreasing-confining-pressure test, maintaining undrained conditions through the entire test will produce more accurate results.