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A Theoretical and Experimental Study of Stress–Strain, Creep and Failure Mechanisms of Intact Coal

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Abstract

Deep mining and drilling activities typically trigger significant deformation of coal/rock roadway and even coal/rock dynamic hazards, which has attracted considerable attention. Hence, in this article, the mechanics and failure properties of intact coal are examined through triaxial compression tests and creep tests. First of all, triaxial compression experiments demonstrated that the complete stress–strain curves of intact coal are characterised by Class-I behaviour; and the post-peak modulus is negative. Second, with increasing confining pressure, peak strength and residual strength rise linearly, whereas elasticity modulus and peak strain increase as a power function form. Thirdly, compared with conventional triaxial load, tiered cyclic load can stiffen the strength of intact coal. Moreover, the data of creep tests are successfully fitted to the Burgers model. Maxwell elasticity modulus is significantly smaller than other creep parameters; Maxwell viscosity coefficient shows a positive correlation with deviatoric stress. Contrarily, Kelvin elasticity modulus and Kelvin viscosity coefficient drop with the rise in deviatoric stress. Additionally, with increasing deviatoric stress, single-step creep strain and average creep strain rate, proposed in this study, increase in a power function form. Finally, failure characteristics of intact coal under triaxial stress could be classified into four types (mainly shear failure and multiple shear fractures). The failure-plane angle generally linearly increases with the rise in confining pressure applied.

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Abbreviations

\(B\) :

The ratio of circumferential strain to axial strain

c :

Cohesion

D :

Cylinder diameter

\(E\) :

Elasticity modulus

\(E_{\text{K}}\) :

Kelvin modulus

\(E_{\text{M}}\) :

Maxwell modulus

\(k_{i}\) :

Post-peak modulus

L :

Cylinder length

\(q\) :

Axial deviatoric stress

t :

Time

\(\beta\) :

Failure-plane angle

\(\dot{\varepsilon }_{\text{c}}\) :

Average creep rate

\(\varepsilon_{1}\) :

Axial strain

\(\varepsilon_{\text{creep}} \left( t \right)\) :

Axial strain evaluated a certain time instant t

\(\varepsilon_{3}\) :

Circumferential strain

\(\varepsilon_{\text{p}}\) :

Peak strain

\(\varepsilon_{\text{c}}\) :

Single-step creep strain

\(\eta_{\text{K}}\) :

Kelvin viscosity

\(\eta_{\text{M}}\) :

Maxwell viscosity

\(\mu\) :

Poisson’s ratio

\(\sigma_{n}\) :

Effective normal stress imposed on the failure plane

\(\Delta \sigma_{\text{s}}\) :

Intensity difference

\(\sigma_{2}\) :

Intermediate principal stress

\(\sigma_{1}\) :

Major principal stress (axial stress)

\(\sigma_{3}\) :

Minor principal stress (confining pressure)

\(\sigma_{\text{p}}\) :

Peak strength

\(\sigma_{\text{r}}\) :

Residual strength

\(\varphi\) :

Internal friction angle

CTL:

Conventional triaxial load

SEM:

Scanning electron microscope

TCL:

Tiered cyclic load

TCS:

Triaxial compression strength

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Acknowledgements

This work was supported by the Natural Science Foundation of Bei**g Municipality (Grant No. 8192036); the National Key Research and Development Program of China (Grant No. 2018YFC0808301); the Youth Foundation of Social Science and Humanity, Ministry of Education of China (Grant No. 19YJCZH087); the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) (Grant No. WS2018B04); and the Fundamental Research Foundation for the Central Universities (Grant No. 2009QZ09). The authors also acknowledge Prof. Jian** Wei, Mr **aolong Chen, and Mr Suye Jia for their support during the experimental process. Additionally, the first author would like to express sincere appreciation for the scholarship provided by the China Scholarship Council (CSC No. 201906430024) and University of Wollongong. The authors are grateful for the anonymous reviewers’ insightful comments and thank Mr Kevin Marston for proof-reading the manuscript.

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Authors and Affiliations

  1. School of Emergency Management and Safety Engineering, China University of Mining and Technology (Bei**g), Bei**g, 100083, China

    Liang Zhang & **angchun Li

  2. School of Civil, Mining and Environmental Engineering, University of Wollongong, Wollongong, NSW, 2522, Australia

    Liang Zhang & Ting Ren

  3. School of Safety Engineering, China University of Mining and Technology, Xuzhou, 221116, Jiangsu, China

    Ting Ren

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  1. Liang Zhang
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  2. **angchun Li
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  3. Ting Ren
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Correspondence to **angchun Li.

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Zhang, L., Li, X. & Ren, T. A Theoretical and Experimental Study of Stress–Strain, Creep and Failure Mechanisms of Intact Coal. Rock Mech Rock Eng 53, 5641–5658 (2020). https://doi.org/10.1007/s00603-020-02235-8

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  • DOI: https://doi.org/10.1007/s00603-020-02235-8

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