## 4 Experimental Results
### Mechanical Behaviors  
During the whole loading process, the stress-time relations for different kinds of sandwiched coal-rock specimens are shown by the dark curves in Figure 6. According to the bearing characteristics, the overall tendency of the mechanical behaviors was basically similar. It can be divided into four stages. Stage I was the nonlinear growth stage of the bearing capacity. Stage II was the linear growth stage of the bearing capacity. Stage III was the yielding growth stage of the bearing capacity. Stage IV was the weakening stage of the bearing capacity. A thorough description of the mechanical behaviors is presented as follows (see Figure 6):  
(1) The axial stress of C-R, R-C, R-C-R, and R-C-R-C-R grew during stage I in an up-concaved form (see Figure 6a-d). Original pores and cracks inside the testing specimen, especially in the coal elements, were enclosed gradually with the rising stress. The compaction deformation was dominant at stage I. As a result, the nonlinear growth of the bearing capacity was presented for different kinds of sandwiched coal-rock specimens.  
(2) As presented in Figure 6, the bearing stress of the sandwiched coal-rock specimen increased linearly during stage II. The sandwiched coal-rock specimens mainly experienced elastic deformation. The duration of stage II was approximately 460 s, 440 s, 300 s, and 360 s for C-R, R-C, R-C-R, and R-C-R-C-R, respectively.  
(3) For different kinds of sandwiched coal-rock specimens, the bearing capacity began to depart from the linear growth. As shown in Figure 6, weakening growth of the bearing stress was monitored for different kinds of sandwiched coal-rock specimens. It indicated the arrival of the yielding growth stage. Plastic deformation was dominant at stage III. It was noted that a little stress fluctuation was observed for C-R and R-C-R-C-R (see Figure 6a,d). However, the overall yielding growth of the bearing load was not influenced. Stage III proceeded until the appearance of peak stress. It was the arrival of the limit bearing capacity for sandwiched coal-rock specimens. As shown in Figure 6a,b, the peak stress was 26.24 MPa and 27.88 MPa, respectively. Apparently, the limit bearing capacity for C-R and R-C was almost equivalent in value. The limit bearing stress of R-C-R, shown in Figure 6c, was approximately 17.15 MPa. It was 12.98 MPa for R-C-R-C-R, as shown in Figure 6d.  
Figure 6: Developments of the axial stress, AE energy, accumulated AE energy, and resistivity for a sandwiched coal-rock system at different loading times. (**a**) C-R; (**b**) R-C; (**c**) R-C-R; (**d**) R-C-R-C-R.  
(4) After the peak stress, the bearing load of the sandwiched coal-rock specimens tended to decrease as a whole. At the initial period of stage IV, the weakening rate of the bearing capacity was relatively small. Subsequently, a large drop of the bearing stress occurred within a short time. It indicated that the sandwiched coal-rock specimens experienced intense damage. The continuity and integrity were seriously destructed. It was worth noting that stage IV for C-R, R-C-R, and R-C-R-C-R was relatively longer than that of R-C. The bearing stress of R-C specimen declined within a very short time during stage IV, which could be explained by the earlier instability of the coal element at the lower position. The bearing capacity of R-C was lost sharply as soon as the coal element failed.