## 4 Results and Analysis  
### Physical Analog Model's Results and Analysis  
#### 4.1.1 Movement Characterization of the Overlying Strata  
The working face has a strike length of 190 cm. Therefore, the coal seam was mined 5 cm at each step in the physical model. During the advancement process of the working face, the whole movement progression of the overburden was photographed with a high-performance digital camera for MatchID. The periodic cracks and collapse of the overburden strata experienced six major deformation stages, as shown in Figure 8. However, the deformation stages of the entire simulation are smaller compared to those in cavity mining due to the filling body's support. Figures 8 and 9 show the strata and ground subsidence evolution characteristics of the overlying strata.  
* First deformation stage (advancement of mining to 55 cm)  
The first major crack occurred when the working face advanced to 55 cm and was a horizontal separation fracture about 1 mm in height between the immediate and main roofs (Figure 8a). This indicates the failure of the immediate roof and the beginning of subsidence due to the advancement of mining.  
* Second deformation stage (advancement of mining to 85 cm)  
Due to the sufficiently long advance of the working face at 85 cm, a second major crack occurred, which was a horizontal separation fracture above both the immediate and main roofs (Figure 8b). However, the one above the main roof was much smaller than the one above the immediate roof, which was 2 mm in height at this stage. A small vertical crack appeared as well. This is due to the influence of the first deformation stage of the immediate roof and the high strength and stiffness of the main roof layer of the model, which was still in its start-up stage. The horizontal fracture of the overlying strata above the first deformation stage indicates shear and tensile failure.  
* Third deformation stage (advancement of mining to 95 cm)  
When the working face reached the halfway point of its advancement, at 95 cm, the additional stress caused by mining increased rapidly, and the deformation of the main roof of the model changed from the start-up stage to the rapid deformation stage (Figure 8c). The underlying rock mass of the immediate roof layer of the structure bent and sank due to the pressure the overburden strata exerted on the model. The immediate roof deformation  
Figure 7: Contour map of ground subsidence, in mm, seen with a filling ratio of 80%.  
damage reached about 5 mm in height, and the movement range of the deformation of the model was significantly expanded.
* Forth deformation stage (advancement of mining to 115 cm)  
When the working face advanced to 115 cm, the strata movement curve of the immediate roof gradually changed from an asymmetric scoop shape to an approximately symmetrical parabolic shape (Figure 8d). The immediate roof underwent severe damage. The sinking of the main roof slightly increased; however, it continued to maintain and synchronize the movement of the overburden strata of the model.
* Fifth deformation stage (advancement of mining to 140 cm)  
When the working face advanced to 140 cm, the main roof bent and sank, but without major damage. Several layers above the main roof lost their support and bent slightly as well, causing about a 3 mm dip in height (Figure 8e). However, the separation fracture was slightly smaller due to the efficiency of the filling body's support.
* Sixth deformation stage (advancement of mining to 190 cm)  
When the working face reached the final advancement distance of 190 cm, the immediate roof underwent its final fracture (Figure 8f). However, the immediate roof's damage was smaller compared to that in the middle of the goat. Each layer below the main key layer of the model bent and sank before entering a stable state.  
Figure 8: Structural evolution of the overlying strata as the working face advanced.