## 4 Experimental Results
### Local Strain Development  
The local stain development for different kinds of sandwiched coal-rock specimens is presented in Figure 11a-d. To gain a deeper understanding of the local strain, the time-stress curves of sandwiched specimens are also shown by the black curves. A positive value of local strain means the lateral strain measured by the longitudinal gauge, while a negative value of local strain means the axial strain measured by the horizontal gauge. The local strain of the coal element (50-100 mm) and rock element (0-50 mm) in the C-R specimen at different loading times is shown in Figure 11a. The axial strain of the coal element and rock element increased at stage I and II. As a whole, the growing rate of the coal element was more rapid than that of the rock element. It is obvious that the axial strain value of the rock element was smaller than that of the coal element. The lateral strain of the coal element was basically close to that of the rock element during the initial loading process. A significant difference between the lateral strains of the coal element and rock element gradually appeared at the later period of stage II. The lateral strain of the coal element was almost twice that of rock element at the end of stage II. When the axial stress yielded at stage III, the growth rate of the local strain was enhanced for the coal element. When the peak stress of the C-R specimen was observed, the axial strain of the coal element and rock element was \(-11.94\times 10^{-3}\) and \(-3.34\times 10^{-3}\), respectively. The lateral strain of the coal element and rock element was +6.78\(\times\)\(10^{-3}\) and +2.98\(\times\)\(10^{-3}\), respectively. At the beginning of stage IV, the axial strain of the coal element accelerated. Then, it tended to decrease from 800 s. The lateral strain of the coal element increased apparently throughout stage IV. For the rock element, both the axial strain and lateral strain started to decrease after the peak stress. The declining rate was almost opposite to the growth rate at stage II and stage III. It suggested that the rock element caused a partial recovery of strain at stage IV, which was induced by the collapse of the coal element. A similar evolution tendency of the local strain was presented for the R-C sandwiched specimen, which is shown in Figure 11b. The axial strain of the coal element (0-50 mm) was always larger than that of the rock element (50-100 mm) from stage I to stage III. Continuous growth of the axial strain was observed on the coal element after peak stress. With the sudden decrease of the bearing stress, a sharp decrease of the axial strain also appeared on the coal element. When the axial stress evolved into stage IV, the axial strain of the rock element tended to decrease slightly. Moreover, the lateral strains of the coal element and rock element were nearly synchronous at stage I and the initial period of stage II. After, the coal element generated a larger lateral strain than the rock element. The growth of the lateral strain continued throughout stage III and stage IV for the coal element. A turning point of the lateral strain was captured on the rock element as soon as the appearance of the peak stress. Then, a recovery of the lateral strain appeared on the rock element. For the R-C-R specimen, the axial strain of the coal element (33.33-66.67 mm) was obviously larger than that of the rock element (0-33.33 mm and 66.67-100 mm) from stage I to stage III (see Figure 11c). The lateral strain of the coal element and rock elements was nearly coincident at stage I. The difference was gradually highlighted when the axial stress turned into stage II. The lateral strain of the coal  
Figure 11: Evolution of the axial stress and local strain for sandwiched coal-rock specimens at different loading times. (**a**) C-R; (**b**) R-C; (**c**) R-C-R; (**d**) R-C-R-C-R.  
element was significantly more than that of the rock elements at stage II and stage III.