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

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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 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.

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Abbreviations

P 0 :

Initial in situ stress (MPa)

σ r :

Tangential stress (MPa)

σ p r :

Tangential stress in the plastic zone (MPa)

σ e r :

Tangential stress in the elastic zone (MPa)

σ θ :

Radial stress (MPa)

σ p θ :

Radial stress in the plastic zone (MPa)

σ e θ :

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)

φ :

Internal friction angle (°)

σ :

Principal stress (MPa)

τ :

Shear stress (MPa)

σ p :

Peak strength (MPa)

σ 1 :

Compressive strength (MPa)

σ 3 :

Confining pressure (MPa)

σ 03 :

Initial confining pressure (unloading condition) (MPa)

σ p3 :

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

ɛ p1 :

Axial strain measured at the peak strength

ψ :

Fitting coefficient

ζ :

Fitting coefficient

CMM:

Coal mine methane

ISRM:

International Society for Rock Mechanics

AE:

Acoustic emission

UCS:

Unconfined compressive strength

SEM:

Scanning electron microscope

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Acknowledgements

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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Authors and Affiliations

  1. National Engineering Research Center for Coal Gas Control, China University of Mining and Technology, Xuzhou, 221116, China

    Qingquan Liu, Yuanping Cheng, Kan Jin, Qingyi Tu, Wei Zhao & Rong Zhang

  2. State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing, 100083, China

    Qingquan Liu

  3. School of Civil, Mining & Environmental Engineering, University of Wollongong, Wollongong, NSW, 2522, Australia

    Qingquan Liu

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  1. Qingquan Liu
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  2. Yuanping Cheng
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  3. Kan Jin
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Correspondence to Yuanping Cheng.

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Liu, Q., Cheng, Y., Jin, K. et al. Effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis. Environ Earth Sci 76, 173 (2017). https://doi.org/10.1007/s12665-017-6509-9

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