## 6 Conclusions  
In this paper, the sandwiched coal-rock specimen was proposed to investigate the stability of overlapped residual coal pillars and the surrounding rock strata in ultra-close multiple seam mining. A series of uniaxial compression tests were designed and conducted for different kinds of sandwiched coal-rock specimens. The progressive failure characteristics and mechanisms of sandwiched coal-rock specimens were investigated. The following conclusions can be drawn:  
(1) The roof strata, upper pillar, interburden strata, lower pillar, and floor strata form a whole bearing system in ultra-close multiple seam mining. The sandwiched coal-rock specimen was firstly proposed to study the stability of overlapped coal pillars and the surrounding rock strata. (2) The mechanical behavior of the sandwiched coal-rock specimen is mainly divided into four stages during the progressive failure process: Nonlinear growth stage of the bearing capacity, linear growth stage of the bearing capacity, yielding growth stage of the bearing capacity, and weakening stage of the bearing capacity. The response of the electrical resistivity and the evolution of AE energy were in good agreement with the mechanical behaviors at different stages, which can be used to reveal the global failure characteristics of sandwiched coal-rock specimens. (3) The distribution of AE events and the development of local strain can provide further insight into the local failure characteristics of coal elements or rock elements in sandwiched system. AE events are more likely to generate in coal elements, which can propagate across the coal-rock interfaces and induce the damage of rock elements in a certain area. Similarly, the unbalanced deformation characteristics of coal elements and rock elements were apparently revealed during the progressive failure process of sandwiched coal-rock specimens. The growth rate of local strain on coal elements was much faster than that on rock elements in the initial loading process. A certain recovery of the local strain was observed on the rock elements after the peak stress. (4) The progressive failure of sandwiched coal-rock specimen was closely associated with the interactions between the coal elements and rock elements. Initial failure usually appeared in the coal elements. In this process, the recovery of the elastic deformation and the output of the strain energy were observed in the rock elements, which can accelerate the rupture of coal elements. In turn, the dynamic fracture energy generated in the rupture process of coal elements can propagate into the rock elements and induce damage of rock elements in a certain area. Consequently, sudden collapse of the sandwiched coal-rock specimen occurred. (5) The experimental results are helpful for maintaining the long-term stability of sandwiched coal-rock systems in ultra-close multiple seam mining. Overlapped coal pillars should firstly be designed with proper sizes. The backfilling method should be implemented to share the overburdening weight and generate a lateral constraint for the residual coal pillars. Once coal pillars begin to rupture, the pre-cracking technique should be applied to mitigate the propagation of stored strain energy in the surrounding rock strata. Moreover, the interburden rock strata can receive the dynamic fracture energy from the upper coal pillars and lower coal pillars. The stability of interburden rock strata should be given more attention in ultra -close multiple seam mining. [PERSON] carried out the experiments, analyzed the results, conducted the theoretical explanations as well as wrote the manuscript. [PERSON] conceived and designed the framework. [PERSON] assisted to conduct the experiments. [PERSON] assisted to analyzed the AE data. [PERSON] provided some help about the response mechanism of electrical resistivity. [PERSON] contributed in language polish.