## 1 Introduction
For example, the vertical subsidence, slope and horizontal deformation have been considered as the sensitive evaluation factors for ground damage [28]; and more complex criteria have also been used, such as a criterion with 11 indexes that include organic material, vegetation coverage, an additional mining-induced slope, etc. [7]. To repair these subsided lands, some reclamation methods have been performed and achieved good results [22; 26; 29]. In the mining area, the building is one of the concerned and well-studied ground objects. Its damage assessment methods are classified into three types [30; 31], including empirical, analytical and numerical methods. The empirical method employs mining subsidence values to assess the building damage extent [32; 33; 34]. It is simple but can only achieve a rough estimate for each single building. The numerical method uses numerical simulation software (based on the finite element method [35; 36] or the distinct element method [37]) to study the damages of individual structures. It can provide detailed mechanical analyses but is time-consuming. The analytical method mainly uses beam systems to model the buildings and then computes the stress and strain within the buildings [38; 39; 40]. Its complexity of the modeling and computation and accuracy of the result are between the aforementioned two methods. Given that the building (also the other ground objects) damage assessment is an intermediary of this study, we do not intend to improve its precision or expand its applicability. Therefore, the empirical method will be employed in this study, as it is simple and has available industry standards. In practice, coal mine companies are more concerned about their profits and prefer to implement land reclamation or provide pecuniary reparation for the mining-damaged land or surface attachments (e.g., buildings). Protective exploitation is not normally under consideration unless there are important surface objects. The present research [41; 42; 43; 44] on land reclamation in the mining area is mainly concentrated on the stage of post-mining. It is not an optimal solution for environment protection, as the damaged environment needs a long time and sometimes is difficult to be recovered. Therefore, reducing the damage extent of the ground surface by optimizing the mining design before excavation is worth studying. At least, it can become a choice when the mining area environment is increasingly concerned. This research aims to identify the key parameters of super-large coalfaces as related to the damage extent of the ground surface. Then these key parameters can be used to optimize the coalface design before underground exploitation to reduce the ground damage. The research process can be mainly divided into three steps:  
(1) Subsidence Simulation  
Considering only field surveying data is insufficient to study the influence of subsidence on the damage. In addition, the permutations and combinations of the coalface parameters are too vast to be accounted for in computer simulations. Therefore, the orthogonal design method is employed to design a series of simulation models to study such ground damage. (2) Damage Assessment  
As the existing ground damage assessment criteria are non-uniform, sometimes complex and aiming at a single ground point, a new damage extent index (DEI) is introduced in this paper. It has the characteristics as follows: (i) can be used to represent the global damage extent of the entire subsidence range; (ii) based on the damage grading of the ground points, which can be assessed by the existing criteria; (iii) quantitative and can be employed to build relationships with the coalface parameters. (3) Key Parameter Determination  
The analysis of variance (ANOVA) method is used to determine the key parameters of the coalface whose variations significantly influence the DEI. The workflow of this study is shown as in Figure 1.  
Figure 1: Flow diagram of this study.