Mechanical properties and damage constitutive model of coal in coal-rock combined body

Coal and rock mass bear loads together in underground engineering such as coal mining and roadway support, resulting in different mechanical properties from those of pure coal and rock mass. Accurately obtaining the mechanical properties of coal in coal-rock combined body can help to properly understand the deformation and failure characteristics of the underground coal mass. First, an experimental method to obtain the stress and strain of coal in coal-rock combined samples using strain gauges was proposed and validated. Then, tests on thirteen types of coal-rock combined samples were conducted. The results show that the rock experienced a strain recovery and played a loading effect on the failure of coal when coal-rock samples failed. The compressive strength of coal in the sample increased with the rock strength, and decreased with the coal-rock height ratio. Furthermore, two damage constitutive models of coal were established by serially connecting one damage body with one or two Newton body(s), to reveal the influences of rock(s) on the mechanical behavior of coal in coal-rock samples. Finally, test results were fitted using the two models. It was showed that the two models can accurately describe the stress-strain curves of coal in coal-rock samples and reflect the influences of rock(s), coal-rock combination form and coal-rock height ratio. The fitting constants of the two models have clear physical meanings and can be easily obtained, which are expected to be widely used in underground engineering.