ORIGINAL ARTICLE Effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis Qingquan Liu 1,2,3 • Yuanping Cheng 1 • Kan Jin 1 • Qingyi Tu 1 • Wei Zhao 1 • Rong Zhang 1 Received: 1 August 2016 / Accepted: 14 February 2017 Ó Springer-Verlag Berlin Heidelberg 2017 Abstract Underground borehole drilling usually causes instability in the surrounding coal due to in situ stress redistribution (including stress concentration and stress release). However, the mechanisms of unloading-induced coal strength reduction are still poorly understood. The primary objective of this study is to investigate the effect of confining pressure unloading on soft coal strength reduc- tion for borehole stability analysis. A series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experi- mental stress paths, including conventional uniaxial/triaxial compression and triaxial compression with confining pressure unloading. The unloading stress path was obtained by analyzing the stress redistribution around a borehole, to capture a more accurate coal mechanical response. According to our experimental results, plastic deformation generated before failure under the unloading stress path is smaller than that generated under the conventional loading stress path. Furthermore, the cohesion of the traditionally and newly reconstituted samples diminishes approximately by 44.77 and 29.66%, respectively, with confining pressure unloading, indicating that there is a significant reduction in coal strength due to confining pressure unloading. The mechanism for unloading-induced coal strength reduction comes from confining pressure unloading-induced increase in shear stress on the fracture surface and a decrease in shear strength. This effect increases the shear slipping potential, whose driving force generates tension fractures at both ends of the preexisting fractures. Keywords Soft coal  Borehole stability  Strength reduction  Loading condition  Unloading condition List of symbols P 0 Initial in situ stress (MPa) r r Tangential stress (MPa) r r p Tangential stress in the plastic zone (MPa) r r e Tangential stress in the elastic zone (MPa) r h Radial stress (MPa) r h p Radial stress in the plastic zone (MPa) r h e Radial stress in the elastic zone (MPa) r Distance from the center of a borehole (m) r a The borehole radius (m) R p The radius of the plastic zone (m) c Cohesion (MPa) u Internal friction angle (°) r Principal stress (MPa) s Shear stress (MPa) r p Peak strength (MPa) r 1 Compressive strength (MPa) r 3 Confining pressure (MPa) r 3 0 Initial confining pressure (unloading condition) (MPa) r 3 p Confining pressure measured at the peak strength (unloading condition) (MPa) e 1 p Axial strain measured at the peak strength w Fitting coefficient f Fitting coefficient & Yuanping Cheng ypcheng@cumt.edu.cn 1 National Engineering Research Center for Coal Gas Control, China University of Mining and Technology, Xuzhou 221116, China 2 State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China 3 School of Civil, Mining & Environmental Engineering, University of Wollongong, Wollongong, NSW 2522, Australia 123 Environ Earth Sci (2017)76:173 DOI 10.1007/s12665-017-6509-9