## 3 The Occurrence Mechanism of SCWID
### Verification by Physical Modelling  
Based on the mechanism illustrated in Section 3.1, the presence of the thick UCA plays an important role in the occurrence of the SCWIDs. To further verify the mechanism illustrated above, a physical experiment system was designed to study the role of an unconsolidated confined aquifer in the process of load transfer, as shown in Figure 4a. Two physical models, either with or without UCA, were considered. In this simulation experiment system, the rubber water belt filled with sand was used to simulate the confined aquifer. The head pressure was provided by a certain height of water tank and the flow control valve was used to simulate the recharge permeability characteristics. Simulation material, 50 cm thick, was laid on the upper part of the water belt to simulate parts of the rock strata, and the top of the model was loaded using the lever principle to simulate the weight/pressure (48 kPa) of the upper remaining rock strata as shown in Figure 4b. The top of the simulation system was equipped with a hydrological observation hole, which was filled with water, and the sensor was used to collect the water head height in the hole. In the process of simulation, with the mining of working face, the pressure at the bottom of the water body and the change of water head height in hydrological monitoring hole  
Figure 3: Schematic diagram of influence on different key stratum (KS) breakage on water flowing fracture: (**a**) KS1 breaks without UCA; (**b**) KS2 breaks without UCA; (**c**) two KSs break entirely with UCA. were monitored by pressure sensor. The coal seam was mined from the right side of the model to the left side. Fifty centimeters of protective coal pillar was left at the boundary to eliminate the boundary influence. Compared with the prototype, the model similarity ratios are geometry 1:100, weight 1:1.5, rigidity 1:150, stress 1:150, internal friction angle 1:1, and Poisson's ratio 1:1. Table 2 shows lithology parameters and mechanical parameters in the experiment. In the preparation of the model, refined silver sand was used as aggregate and the plaster was used as cement. Sheet mica was used between layers. Under the UCA, pressure sensors were installed to record the load distribution. The pressure sensor can be seen in Figure 4c. The water supply can be seen in Figure 4d. Figure 4: Simulation experiment system: (**a**) simulation experiment system; (**b**) loading equipment; (**c**) pressure sensor and rubber bag with water; (**d**) water supply. In the simulation experiment, the model started mining from the right side to the left side, leaving a 50-cm-wide boundary pillar on the right side. As shown in the simulation results in Figure 5, without the simulation of UCA water pressure, the coal seam was continuously being mined to the left. When the mining width/distance reached 60 cm, KS1 did not break, and there was no crack in the upper strata of KS1 and KS2. When the mining width reached 73 cm, KS1 was broken. At this time, the strata between KS1 and KS2 also collapsed with the breaking of KS1. At this time, the new fractures were mainly located between KS1 and KS2, and there was no fracture above KS2. The coal seam continued to be mined to the left. When the mining width reached 79 cm, KS2 broke and caused obvious damage to the rock strata at a certain height above KS2. At the same time, the original fracture between KS1 and KS2 developed upward with the breaking of KS2, and a new fracture was formed above KS2. In this experiment, KS1 broke earlier than KS2, and the fracture development degree was obviously affected by the breakage of the KS, indicating a staged development process. Following the experiment described above (the UCA water pressure was not considered), the simulation experiment considering the presence of UCA was performed, and the simulation results are shown in Figure 6. The results show that when the coal seam was mined 53 cm to the left, there was no damage in KS1 and KS2, and there was no fracture in KS1 and its upper strata.