## 5 Discussions
### Progressive Failure Mechanism of the Sandwiched Coal-Rock System
#### 5.2.2 Induced Damage Mechanism of Rock Elements  
Previous studies showed that coal elements are usually the main rupture damage elements, while rock elements are rarely damaged [14; 17]. This is because the UCS of rock elements in these literatures was significantly more than that of coal elements. In some cases, the UCS of rock elements was even three times more the strength of the coal elements [18]. The current research found that not only were the coal elements ruptured, but the rock elements were also damaged in the sandwiched coal-rock system. The following section explains the induced damage mechanism of rock elements in detail. Dynamic fracture energy was generated along with the development of macro-ruptures in the coal elements [34]. It could propagate from the coal elements into the rock elements within a short time. Then, a certain degree of damage appeared in the rock elements. Due to the higherUCS and E, the propagation speed of the dynamic fracture energy weakened gradually in the rock elements. Correspondingly, the damage degree at different heights of the rock elements was not same. The closer to the coal-rock interface, the more serious the damage was [35]. In contrast, the induced damage in the rock elements was relatively weak in the area far from the coal-rock interface. These associated descriptions could provide an explanation for the damage mechanism of rock elements in the sandwiched coal-rock system. It could also be illustrated by the distribution of AE events in Section 4.3.2. It is worth noting that whether the rock element is completely damaged also depends on its own height [36]. When the maximum propagation distance of the dynamic fracture energy exceeded the height of the rock element, the damage was distributed throughout the rock elements. Contrarily, the damage was scattered just at the partial area of the rock elements. For the C-R or R-C specimen, the height of the rock element (50 mm) was obviously more than the maximum propagation distance of the dynamic fracture energy. That is, the transferred dynamic fracture energy was not enough to destroy the entire rock element. The induced damage in the rock elements was mostly distributed near the coal-rock interface. As shown in Figures 7 and 8, the damage in the rock element was gathered primarily at a height of 30-50 mm in the C-R specimen, while it was distributed mainly at a height of 50-70 mm in the R-C specimen. For the R-C-R specimen, the dynamic fracture energy could not only spread into the upper rock element (66.67-100 mm), but could also propagate into the lower rock element (0-33.33 mm). The maximum propagation distance of the dynamic fracture energy was less than the height of the rock element (33.33 mm). Therefore, the damage of the rock elements was relatively small. The AE events in the rock elements were mainly distributed near the upper and lower coal-rock interfaces (see Figure 9). For the R-C-R-C-R specimen, the dynamic fracture energy generated by the coal element (20-40 mm) expanded into the rock elements at the heights of 0-20 mm and 40-60 mm. The dynamic fracture energy generated by the coal element (60-80 mm) developed into the rock elements at the heights of 40-60 mm and 80-100 mm. That is, the interburden rock element (40-60 mm) received the dynamic fracture energy from the upper and lower coal elements. In this situation, the height of the sandwiched rock elements was just 20 mm. It was less than the total propagation distances of the upper and lower dynamic fracture energy. Therefore, the damage was distributed all over the interburden rock elements (40-60 mm). As shown in Figure 10, massive AE events were also observed at the height of 40-60 mm in the R-C-R-R-R specimen. For rock elements at the heights of 0-20 mm and 80-100 mm, the received dynamic fracture energy was only generated from the adjacent coal element, which was relatively smaller in value. Therefore, the rock elements at the heights of 0-20 mm and 80-100 mm were not scattered fully within the induced damage.