## 5 Discussions
### Implications for Ultra-Close Multiple Seam Mining  
The progressive failure characteristics of sandwiched coal-rock specimens are a reflection of the catastrophic collapse of the roof strata-upper pillar-interburden-lower pillar-floor strata system in the ultra-close multiple seam mining. The experimental results are helpful for predicting the long-term stability of overlapped residual coal pillars and the surrounding strata. Some valuable implications can be concluded from this study to guide the safety of ultra-close multiple seam mining, which are presented as follows:  
(1) Coal pillars, possessing lower UCS and E values, are damaged much earlier in the sandwiched coal-rock systems. In ultra-close multiple seam mining, overlapped coal pillars should firstly be designed with a proper size to support overburdened strata. Furthermore, the stability of residual coal pillars gradually deteriorates due to the coupled effects of environmental weathering, mining-induced stress, and other unfavorable factors. Additional measures should also be implemented to maintain the long-term stability of residual coal pillars. Backfilling along the coal pillars is one of the potential methods [37]. The backfilling body can not only share the overburdened strata's weight with the coal pillars, but can also generate a lateral constraint for the coal pillars. Therefore, the backfilling method along the coal pillars is proposed to enhance the overall stability of the sandwiched coal-rock system in ultra-close multiple seam mining.  
(2) Surrounding rock strata, with higher UCS and E values, can promote the failure of coal pillars. It provides a further explanation of the mechanisms of coal pillar rupture. In ultra-close multiple seam mining, the stored strain energy in the surrounding rock strata begin to be released when the bearing load of the coal pillar weakens. It can spread from the surrounding rock strata into the coal pillars and accelerate further rupturing of the coal pillars. The more the released strain energy, the more serious the damage to the coal pillar. Therefore, the strain energy of the surrounding rock strata should be eliminated or transferred with related measurements. The pre-cracking technique is an alternative technology to mitigate the propagation of stored strain energy in the surrounding rock strata [38]. Then, the failure of coal pillars can be delayed, and the overall stability of the sandwiched coal-rock system can be maintained for a long time.  
(3) Dynamic fracture energy can propagate from the residual coal pillars into the surrounding rock strata. The induced damage then appears in the surrounding rock strata. A certain distance of damage occurs when the dynamic fracture energy spreads into the roof strata or floor strata. That is, the rupture of residual coal pillars can lead to failure of the surrounding rock strata. For the interburden rock strata, it can receive the dynamic fracture energy from the upper coal pillar and lower coal pillar. Therefore, the damage may distribute throughout the interburden rock strata when the total maximum distances of the dynamic fracture energy are more than the heights of the interburden rock strata. In this situation, the stability of interburden strata should be paid much more attention in ultra-close multiple seam mining.  
Figure 12: Interactions between the coal elements and the rock elements in the sandwiched coal-rock system.