Parametric Analysis of Borehole Breakout and Spalling Phenomena: An Experimental Investigation

Abstract

AbstractThis study investigated borehole breakout and spalling phenomena through true-triaxial experiments conducted on cubic Gosford sandstone and yellow mudstone specimens. The experiments were carried out based on the custom-developed borehole breakout testing system, which could enable the pre-stressed (PS) tests (applying stresses onto intact specimens and then drilling). In comparison to the pre-drilled (PD) tests (applying stresses onto specimens with pre-drilled boreholes), the PS tests consistently resulted in wider and deeper breakouts under the same far-field stresses in both rock types, which are attributed to the impacts of excavation-unloading and removal of rock debris. The results from 36 PS tests indicated that the breakout and spalling extent are affected by all three principal stresses, and the impact of each stress was also quantitively assessed and compared. Moreover, analysis of experimental data from this study and the literature revealed that a unique correlation between breakout angle and normalised breakout depth could not be obtained, suggesting deriving two principal stress components from two breakout geometries should be viable. Furthermore, empirical correlations based on the maximum tangential stress (σmax) and out-of-plane stress were developed to characterise the breakout geometries, which outperformed those previous relations based solely on σmax across all datasets. This suggests that while the breakout and spalling phenomena are primarily controlled by σmax, a more accurate representation of failure extent can be achieved by considering the out-of-plane stress.