Environmental Earth Sciences

, 76:173 | Cite as

Effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis

  • Qingquan Liu
  • Yuanping ChengEmail author
  • Kan Jin
  • Qingyi Tu
  • Wei Zhao
  • Rong Zhang
Original Article


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 reduction for borehole stability analysis. A series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experimental 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.


Soft coal Borehole stability Strength reduction Loading condition Unloading condition 

List of symbols


Initial in situ stress (MPa)


Tangential stress (MPa)


Tangential stress in the plastic zone (MPa)


Tangential stress in the elastic zone (MPa)


Radial stress (MPa)


Radial stress in the plastic zone (MPa)


Radial stress in the elastic zone (MPa)


Distance from the center of a borehole (m)


The borehole radius (m)


The radius of the plastic zone (m)


Cohesion (MPa)


Internal friction angle (°)


Principal stress (MPa)


Shear stress (MPa)


Peak strength (MPa)


Compressive strength (MPa)


Confining pressure (MPa)


Initial confining pressure (unloading condition) (MPa)


Confining pressure measured at the peak strength (unloading condition) (MPa)


Axial strain measured at the peak strength


Fitting coefficient


Fitting coefficient



Coal mine methane


International Society for Rock Mechanics


Acoustic emission


Unconfined compressive strength


Scanning electron microscope



The authors are grateful to the financial support from projects funded by Natural Science Foundation of Jiangsu Province (No. BK20160253), China Postdoctoral Science Foundation (No. 2016M590519), the State Key Laboratory of Coal Resources and Safe Mining (No. SKLCRSM16KFB01) and the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Fundamental Research Funds for the Central Universities (No. 2013QNA03).


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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • Qingquan Liu
    • 1
    • 2
    • 3
  • Yuanping Cheng
    • 1
    Email author
  • Kan Jin
    • 1
  • Qingyi Tu
    • 1
  • Wei Zhao
    • 1
  • Rong Zhang
    • 1
  1. 1.National Engineering Research Center for Coal Gas ControlChina University of Mining and TechnologyXuzhouChina
  2. 2.State Key Laboratory of Coal Resources and Safe MiningChina University of Mining and TechnologyBeijingChina
  3. 3.School of Civil, Mining & Environmental EngineeringUniversity of WollongongWollongongAustralia

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