## 4 Key Parameters of The Coalface on Ground Damage
### Analyses and Discussions  
The damage grade distribution shown in Figure 6 and the results of the variance analysis in Tables 8 and 9 and Figures 8 and 9 based on both the building and timberland damage assessments indicate that:  
(1) For different ground objects, the key parameters of the coalface on the ground damage are distinct. Many studies [4; 7; 8; 17; 19; 21; 27; 28; 29; 41; 53] have discussed the differences of mining-subsidence influences on different surface objects. The sensitive subsidence factors, damage grading method and the damage mechanisms are interested topics. This paper is more like a reverse study, after the assessment of the ground damage, the assessed results are used to back analysis of the so-called key parameters of the coalface. For buildings, the length in the dip direction and the thickness of the coalface are the key parameters; for timberland, only the thickness of the coalface is the key parameter. (2) When assessing the building damage, three ground subsidence deformations (slope, curvature and horizontal deformation) are considered. Higher grade damage occurs along the coalface edges. The influence of the coalface length in the dip direction (factor A) on the extent of global ground damage (the DEI) can be represented by the \(-1.54\times 10^{-5}(A-371.17)^{2}\) in Equation (11), which appears as a quadratic curve, as shown in Figure 8b. As the coalface length in the dip direction increases, the DEI increases first before reaching its extreme value at a dip length of 371.17 m and then decreases. From the simulation experiments, when the length in the dip direction reaches 350 m, the excavation zone is considered to be critical. Therefore, the above rule can be summarized as follows. When the excavation is sub-critical, longer coalface lengths in the dip direction aggravate the global ground damage (increased DEI) due to the increased subsidence deformation. After the excavation becomes critical, longer coalface lengths in the dip direction increase the subsidence range, but the subsidence deformation does not increase (according to general subsidence theories [54]). Therefore, the area with low damage grade(s) enlarges around the center of the subsidence range, which reduces the global damage extent (decreased DEI). In the Shendong coalfield, the mining and geological conditions are simple and favorable to design successive coalfaces in the dip direction to ensure the excavation inevitably becomes critical (or super-critical) after several coalfaces have been mined. The increased coalface length in the dip direction can accelerate the process of sub-critical to super-critical excavation, i.e., it is advantageous for ground protection. The simple conditions in the Shendong coalfield may cause the length in the dip direction of one super-large coalface to reach (or be close to) the demand value of critical excavation, which causes the appearance of an extreme DEI. This case should be avoided, so it is better not to set the dip length of the first mining coalface at around 350 m.  
The influence of the coalface thickness (factor C) on the extent of global ground damage (the DEI) can be represented as the \(-8.98\times 10^{-3}(B-11.09)^{2}\) in Equation (11), which is quadratic and similar to the length in the dip direction, as shown in Figure 8c. The experimental range of the thickness is 2-10 m and is located at the left side of the curve where it monotonically increases. Therefore, it is determined that as the thickness of the coalface increases, the global ground damage aggravates (increased DEI) due to the increased subsidence deformations. However, when the thickness of the coalface is close to a certain value (11.09 m in this case, which is the peak of the curve), the rate of increase of the DEI slows. As the experimental thickness ranges from 2 to 10 m, the right side of the curve (where the thickness >11.09 m) is not supported by the experimental data and cannot be considered as the influencing characteristics on the DEI.