## 6 Discussion  
### Comparative Analysis of Physical Modelling Result and Numerical Simulation Result  
Figure 14 illustrates the process of mining induced strata movement obtained by numerical simulation, which is basically consistent with the physical modelling test result. The main deformation phenomenon reappeared in numerical simulation, though the internal displacement of the rock mass is not obtained by physical modelling, the displacement obtained by numerical simulation is consistent with the displacement vector graph in the physical modelling test and can support the phenomenon of strata movement obtained by physical modelling. Displacement monitoring points are set in ten strata from the first strata directly above the mined area to the slope surface (layer 7, 8, 10, 12, 14 and 16 illustrated in Figure 7), and the horizontal spacing of these monitoring points is 15 cm. Figure 15 shows the vertical displacement of the strata directly above the mined-out area, and the displacement value is built up with the mining. The maximum displacement will always appear in the strata that are directly above the mined area, which indicates that mining-induced unloading triggered the strata movement. All the displacement curves of the four mining steps tell us that a significant increase in displacement began to appear near the boundary between the mined area and surrounding rock, which indicates that due to the existence of steep joints, the rock mass deformation transferred mainly upward to the slope surface. The final well-shaped displacement curve also indicates a well-shaped rock mass subsidence. In this study, the mining-induced strata movements in the Yanqianshan iron mine can be summarised as follows. I. The main deformation modes were: (a) the strata separated from the flat joint surface due to the losing of support and the overburden pressure provided by the strata, (b) the arch was formed by the collapsed rock mass, and (c) the subsidence pit was formed by downward strata movement along the steep joint surface and ground cracks formed by extension of steep joints. II. Transfer of the strata movements. The strata movement transferred gradually from the roof of the mining area to the ground surface. The mining depth or the depth of overlying strata were of significant influence on the depth of subsidence pit formed. Both the displacement vector in Figure 12b and the subsidence curve in Figure 15d showed that the mining-induced deformation in the rock mass  
Figure 13: Front view of the model slope after four stages of mining. Subsidence pit was formed above the mining area. that has a realistic joint system (Figure 5) was well-shaped, and the boundary of the subsidence pit was controlled by the steep joints. III. Surface deformation. Ground surface cracks formed by the extension of steep joints experienced near the slope shoulder, the subsidence pit at slope surface resulted in tensile force in horizontal direction, the steep joint extended under the tensile force and the cracks formed in a certain depth from the slope surface to the internal. The results from the numerical simulation and physical modelling were consistent with each other. However, several issues need to be highlighted. First, the strata movement process at the macro level was simulated in the numerical modelling, while the strata movement at the micro level was considered in the physical modelling. Therefore, the phenomenon investigated and obtained by the two methods are not completely consistent, especially in Figures 10a and 14c. Second, the simulation of the mining process between two modelling methods was also different. The mining process in the physical modelling was achieved by removing the sandbag, yet little sand was left in the sandbag. No such issue occurs in the numerical simulation. In addition, the physical modelling can reflect the rock joint influence on the mining induced strata movement in detail and reproduce the whole process of slope failure microscopically.