###### Abstract
[11] analyzed the seepage characteristics of a gas-bearing coal-rock body subjected to different loadings. [PERSON] et al. [12] simulated the hydraulic fracture propagation of a combined coal and artificial roof/floor. [PERSON] et al. [13] numerically carried out lateral pressure unloading numerical tests of composite coal-rock models with PFC2D to study energy evolution and burst behavior. Both the experimental and numerical results showed that the failure characteristics of a combined coal and rock body were significantly different from those of a pure coal or rock specimen, which illustrates that residual coal pillars and the surrounding rock strata need to be studied as a whole. In view of this theoretical analysis, [PERSON] et al. [10] proposed a new nonlinear constitutive model with the concept of natural volumetric strain to describe the deformation of a combined rock and coal body. [PERSON] et al. [14] established an equivalent homogeneous model of a coal-rock body based on the strain energy equivalency principle. The proposed compression-shear model could be used to reflect the strength behavior of a coal-rock body containing the structural plane. [PERSON] et al. [15] developed a damage constitutive model of coal with a cascade system of a damaged body and Newton body to investigate the stress-strain curves of coal in the combined coal-rock specimen. The accuracy of these constitutive models was also verified by the results of laboratory tests or numerical simulations. The failure mechanism of the roof-pillar-floor system was explored to some extent. The influencing factors associated with the mechanical properties of a combined coal and rock specimen were further investigated by many scholars. [PERSON] et al. [5] found that the strength and deformation of a combined coal and rock body were strongly dependent on the loading rate. [PERSON] et al. [12] believed that the peak strain of a composite rock-coal-rock sample decreased with an increase of the rock strength. [PERSON] et al. [16] explored the effect of the interface angle on the failure characteristics of combined coal and rock specimens. The coupled effects of the height ratio and loading rate on the failure modes of composite samples were also examined by using granular dynamic models [17]. The factors in these researches were diverse, and contained not only the internal material parameters of a combined coal and rock body, but also involved the external loading parameters of a combined coal and rock body. The instability characteristics and mechanisms of a combined coal and rock body in specific geological conditions were well reflected by these valuable researches. In the existing researches, the combined rock-coal specimen represents the roof-pillar system. The combined coal-rock specimen represents the pillar-floor system. Additionally, the rock-coal-rockspecimen is a reflection of the overall roof-pillar-floor system. The failure characteristics of the rock-coal specimen, coal-rock specimen, and the rock-coal-rock specimen offer a deeper insight into the occurrence mechanisms of dynamic disasters in single seam mining. However, it is difficult to use the obtained results to directly guide the safety of ultra-close multiple seam mining. Generally, ultra-close multiple seam mining is widely conducted in China [18] and other famous coal-mining countries [19; 20]. Various multi-seam mining configurations are defined based on the mining methods at the upper and lower levels. Multi-seam pillar mining is one of the main configurations in practice [21]. In this situation, the overlapped residual coal pillars are designed at different mining levels [22]. The roof strata, upper pillar, interburden strata, lower pillar, and the floor strata, shown in Figure 1, form a whole bearing system in the underground space. That is, different kinds of sandwiched coal-rock systems exist in ultra-close multiple seam mining. It includes not only the C-R, R-C, and R-C-R sandwiched system, but also the R-C-R-C-R sandwiched system.