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Creep Behaviors of Extremely Soft and Fractured Coal Rock with Different Moisture Contents and Rare Secondary Creep Performance

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Abstract

Water injection technology has been widely used to prevent and reduce coal caving during roadway excavations in extremely soft and loose coal seams. To evaluate the influence of this technology on the long-term stability of such roadways, a self-developed triaxial creep test system was adopted to obtain the creep behaviors of extremely soft coals with various moisture contents (0.95, 2.30, and 3.26%) at a 0.2 MPa confinement, the creep characteristics and failure modes were compared, the formation condition of a rare secondary creep phenomenon was discussed, and a nonlinear creep model was newly established. The findings showed the following. (1) Extremely soft and fractured coal was characterized by significant rheological features, i.e., a large creep strain, a fast creep rate, and susceptibility to accelerated creep failure. The cumulative strain in natural water state and after 328 h was 2.99%. The steady-state creep rate was as high as 4.89 × 10–5/h, which remained unchanged during the entire steady-state creep stage. These features are markedly different from those of other rocks. (2) When the moisture content increased to 2.3%, the creep strain reached 9.19% within 61 h, and the steady-state creep rate was 4.23 × 10–4/h. After the primary creep, there was an obvious secondary creep process, during which the secondary transient creep stage and secondary steady-state creep stage occurred successively. (3) As the moisture content increased from 0.95% to 3.26%, the ability of the coal sample to resist long-term deformation decreased, i.e., the creep duration before specimen failure decreased dramatically and the creep rate increased markedly; in particular, the steady-state creep rate increased in a power function. (4) A certain water content (~ 2.30%) and confining pressure were required for the occurrence of secondary creep process. (5) The proposed nonlinear creep model could describe the full creep stage perfectly, especially the tertiary creep stage in primary creep process and the entire secondary creep process.

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Acknowledgments

This work was financially supported by the Open Fund of State Key Laboratory of Coal Mining and Clean Utilization (2021-CMCU-KF011).

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Authors and Affiliations

  1. State Key Laboratory of Coal Mining and Clean Utilization, Beijing, 100013, China

    Jianwei Yang

  2. Key Laboratory of Rock Mechanics and Geohazards of Zhejiang Province, Shaoxing University, Shaoxing, 312000, Zhejiang, China

    Xingkai Wang & Hao Chen

  3. State Key Laboratory of Coal Resources and Safe Mining, Xuzhou, 221116, Jiangsu, China

    Wenbing Xie

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  1. Jianwei Yang
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  2. Xingkai Wang
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  4. Hao Chen
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Correspondence to Xingkai Wang.

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Yang, J., Wang, X., Xie, W. et al. Creep Behaviors of Extremely Soft and Fractured Coal Rock with Different Moisture Contents and Rare Secondary Creep Performance. Nat Resour Res 32, 357–371 (2023). https://doi.org/10.1007/s11053-022-10140-w

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