## 4 Experimental Results
### Local Strain Development
It should be noted that the local strain of the upper and lower rock elements was not consistent after stage II. It was attributed to the heterogeneity of the rock elements. The original defects in the different rock elements were not exactly same. At stage IV, the earlier decreasing of the local strain appeared slightly in the rock elements, which indicates the recovery of the local strain. At this moment, the local strain on the coal element was still increasing. When the loading time was about 725 s, there was a sudden decrease in the local strain for the coal elements. It predicted the rupture of the coal element. Ultimately, a little fluctuation of the local strain was apparent in the coal element. Figure 11d shows the local strain of the coal elements and rock elements at different heights in the R-C-R-C-R specimen. With the increase of the loading time, the local strains of the coal elements and rock elements presented a gradual increasing tendency from stage I to stage III. As shown in Figure 11d, the axial strain and lateral strain of the rock elements (0-20 mm, 40-60 mm, and 80-100 mm) were almost the same from stage I to stage III. Additionally, little differences were also observed for the coal element at the height of 20-40 mm and 60-80 mm. As a whole, the growth rate of the local strain on the coal elements was faster than that of the rock elements. Consequently, the local strains generated on the coal elements were also larger than those of the rock elements. After the peak stress of the R-C-R-C-R specimen, the rebounding of local strains was observed on the rock elements at the height of 0-20 mm and 80-100 mm. However, the rock element (40-60 mm) was an exception. The lateral strain was still increasing at stage IV. Although several chaotic jumps and drops were found, the overall growth of the axial strain was also presented for the rock element (40-60 mm) at stage IV. Coal elements at the height of 20-40 mm and 60-80 mm experienced further damage at stage IV. The growth of the lateral strain was maintained at stage IV. The axial strain of the coal elements was not stable as a whole. Initially, it increased gradually. With the sudden decrease of the bearing stress, a decrease of the axial strain for the coal elements (20-40 mm and 60-80 mm) successively appeared at 1100 s and 1250 s.  
Based on the mechanical behaviors, electrical resistivity responses, AE characteristics, and local strain evolution, the failure characteristics for different kinds of sandwiched coal-rock specimens can be summarized as follows: (1) Four stages were observed during the progressive failure process of sandwiched coal-rock specimens, i.e., stage I: Nonlinear growth stage of the bearing capacity; stage II: Linear growth stage of the bearing capacity; stage III: Yielding growth stage of the bearing capacity; and stage IV: Weakening stage of the bearing capacity. (2) The response of the electrical resistivity and the evolution of AE energy were in good agreement with the mechanical behaviors at different stages, which could be used to reveal the global failure characteristics of sandwiched coal-rock specimens. The distribution of AE events and the development of local strain provide further insight into the local failure characteristics of coal or rock elements in the sandwiched specimens. (3) AE events generated earlier in coal elements, which expanded along with the increase of the loading time. Until the failure of sandwiched coal-rock specimens, almost all areas of coal elements were distributed with AE events. AE events also propagated across the coal-rock interfaces, thus local damage of rock elements was induced. For the C-R, R-C and R-C-R specimens, the AE events were distributed in the local areas of rock elements. For the R-C-R-C-R specimen, the upper and lower rock elements experienced local damage. However, the AE events were scattered throughout the whole interburden rock elements.