Effect of a Hole Drilled Under Different Stress Conditions on the Behaviour of a Coal Specimen

Abstract

The drilling hole is an effective measure used to forecast and prevent rock bursts and is widely used during mining operations. To study the effect of a hole drilled under different stress conditions on the behaviour of a coal specimen, a drilling and reloading experiment is carried out. With increasing vertical load, the failure mode of the specimens is transformed from slow deformation to one entailing an undesirable brittle failure when a hole is drilled. The vertical load at the cut-off point between slow deformation and undesirable brittle failure is equal to, or slightly less than, 65MPa. The vertical stress-strain curve of a coal specimen with a hole could be divided into three stages, and its turning point between the first stage and the second stage is related to the stress on the coal specimen after drilling; acoustic emission events seldom occurred in the first stage, rapidly increased in the second stage, and remained at a higher level throughout the third stage. However, the vertical stress-strain curve of that coal specimen without a hole tended to be linear, and AE event counts gradually increased to a maximum. Due to the differences in the various coal specimens, there are two main failure modes seen in samples with a hole upon reloading. The failure mode is determined by the relationship between the reloading initial crack load and the stress after drilling. When the reloading initial micro-crack load is less than the stress after drilling, the coal specimen cracked slowly, otherwise, the specimens ruptured suddenly. We also found that a hole drilled can reduce the difference between the vertical stress and the confining pressure, and increase AE event counts when the coal specimen is reloaded.