The dynamic compressive strength of the coal specimens was significantly weakened by boreholes under high strain-rate loading; the dynamic compressive strength and the dynamic modulus of elasticity of coal rock showed a decreasing trend, with increasing numbers of boreholes and a rising and decreasing trend with increasing borehole spacing; the number and spacing of boreholes appeared to be design parameters that could weaken coal–rock material under high strain-rate loading; during the loading of coal and rock, initial cracks appeared and expanded in the tensile stress zone of the borehole side, while secondary cracks, which appeared perpendicular to the main crack, expanded and connected, destroying the specimen.
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Quantitative Study of the Weakening Effect of Drilling on the Physical and Mechanical Properties of Coal-Rock Materials.
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The elastic modulus showed a decreasing trend, wherein the ability of the specimen to resist deformation decreased as the borehole spacing increased, and both the compressive strength and the elastic modulus of the coal specimens showed a trend of first increasing and then decreasing, showing that the boreholes may exert a certain deterioration and modification effect on the mechanical properties of the coal specimens. 3.3.
Under the same impact, the reflected energy ratios of coal specimens with different numbers of boreholes show an increasing trend with increasing numbers of boreholes, while the transmitted energy ratios show a decreasing trend, and the overall trend of the dissipated energy ratio is not obvious ( Figure 14 ).