Assessment of coal pillar strength under the influence of sand stowing in deep coal mines

Backfilling in goaf after pillar extraction seems to be a promising technology to help exploit these standing pillars. This paper presents the assessment of the impact of sand-stowing-based backfill on the peak strength and post-failure behavior of coal pillars. Rectangular coal samples were prepared and tested with different proportions of sand stowing, and a 3D finite-difference code was used for the numerical simulation to achieve the objective of the study. In laboratory experiments, the proportion of sand stowing was set at 0%, 60%, 70%, 80%, 90%, and 100% and tested in a servo-controlled machine, whereas, in numerical simulation, the percentage of sand stowing varied from 9 to 91% along with the coal pillar width-to-height ratios ranging from 1.5 to 5. The experimental and numerical results demonstrated that the average pillar strength increased as the proportion of stowing increased. The overall strength increased by 22% in the experimental study and about 28% to 32% in numerical simulation. The load vs. displacement characteristic changed from brittle to ductile as the sand proportion increases and the width-to-height ratio increased.