Study on the Mechanical Properties and Failure Mechanisms of Coal–Rock Composite Specimens Considering Variations in Weaker Components

To clarify the influence of α (rock–coal height ratio) and λ (rock–coal strength ratio) on the mechanical properties and failure characteristics of coal–rock composite specimens, where the weaker component varies with rock properties, four sets of coal–rock composite specimens with λ values of 0.26, 0.35, 0.59, and 3.81 were subjected to uniaxial compression tests under conditions of α = 1:3, 1:1, and 3:1. The results show that: There are significant differences in the mechanical properties of coal–rock composite specimens compared to individual coal and rock specimens. Both α and λ have significant effects on the mechanical properties and failure modes of coal–rock composite specimens. The variation in uniaxial compressive strength, elastic modulus, and peak strain in coal–rock composite specimens with respect to α is significantly influenced by rock properties. These variation patterns are not entirely identical for different rock properties. For coal–rock composite specimens at different α values, the trends in uniaxial compressive strength, elastic modulus, and peak strain as a function of λ are identical. Both uniaxial compressive strength and elastic modulus exhibit a pattern of increasing rapidly at first and then more slowly with increasing λ, and both can be quantitatively described by exponential functions. Peak strain follows a pattern of rapid decrease, rapid increase, and gradual increase with increasing λ. However, for any given change in λ, the magnitude of the changes in uniaxial compressive strength, elastic modulus, and peak strain is significantly influenced by α. When λ is small or large, the weaker component in the coal–rock composite specimen is the primary source of failure. When λ falls within a certain range, both the strong and weak components undergo relatively complete failure. When λ increases beyond a critical value, only the weaker component fails, while the stronger component remains intact and does not fail. As α increases, the degree of failure in the coal–rock composite specimen gradually decreases.