Abstract
To study the damage and destruction behavior of small coal pillars in coal mine roadway driving along gobs under long-term in-situ stress and multiple engineering disturbances, an unconfined compression experiment under a discontinuous cyclic load was designed, with the holding time as a variable. An electro-hydraulic servo rock testing machine was used to impose a discontinuous cyclic load on the coal sample and perform a final uniaxial compressive strength test. The changes in pore number and diameter in the coal under stress were monitored by nuclear magnetic resonance analysis. An increase in holding time in the discontinuous cyclic loading resulted in a significant increase in the number and diameter of pores in the coal sample; the coal porosity continued to increase, and the proportion of pores in the coal changed. The proportion of micropores decreased gradually, whereas the proportion of mesopores and macropores (cracks) increased. The degree of internal specimen damage increased with an increase of holding time, which resulted in a gradual decrease in final uniaxial compressive strength. Therefore, under the action of a long-term stress, to improve the bearing capacity of the coal pillar while avoiding gas and water influx into the working face in the goaf, the coal pillar should be reinforced with multi-layer and multi-grain grouting.
Highlights
The results show that the increase in axial displacement roughly contributed to the permeability reduction, and excessive amplitude of cyclic load posed a limited boost to the permeability enhancement
Both ends were ground flat according to International Society for Rock Mechanics (ISRM) requirements
The samples after being subjected to the force was treated with vacuumsaturated water, and Nuclear magnetic resonance (NMR) analysis equipment was used to collect the final pore and fracture information of the coal sample to obtain the final
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Eberhardt et al [5] analyzed the mechanical characteristics of fracture damage during the uniaxial cyclic loading and unloading of rocks and studied the propagation conditions and fracture criteria of microcracks through experimental results. The long-term continuous force expands the internal fissures of the coal fully, and its mechanical properties and pore characteristics are changed [13], which affects the bearing capacity strength of the coal pillar. Nuclear magnetic resonance (NMR) is a new type of non-destructive testing technology that utilizes quantum magnetism at the atomic scale [14] It has been used extensively in tests and research in the fields of crack identification, pore distribution, rock meso-structure damage, and rock physical and mechanical characteristics under different conditions, and has become an important rock physical test analysis method [15,16]. According to the coal failure test, the damage and strength attenuation mechanism of the coal body were studied
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