Evaluation of land damage degree of miningsubsidence area with high groundwater level

Abstract: The evaluation of land damage degree is an important basis for the formulation of land reclamation plan and the establishment of compensation standard for farmers in coal mining subsidence area. Mining subsidence area with high groundwater level has the characteristics of flat terrain, lower phreatic water depth, more recoverable coal seams, large thickness and big surface subsidence coefficient which lead to severe surface subsidence and seeper. Land is seriously destroyed. Thus, this paper takes Dongtan coal mine as an example to evaluate land damage degree of subsidence area with high groundwater level that will be damaged by coal mining from 2014 to 2018. The underground water level more easily rises above the surface elevation after mining, which leads to surface seeper in coal mining area with high groundwater level and when the perennial seeper is formed, there will be total crop failure for farmland. In Dongtan coal mine, the maximum buried depth of groundwater is 4 m. When the surface subsidence depth is more than 4 m, there will be perennial seeper, and therefore, the area with sinking depth of more than 4 m is designated as severely damaged area by the limit condition method. Based on a systematic review of existing research, the evaluation index system of other areas with sinking depth of no more than 4 m is constructed and evaluation units are divided with proper method. The weights of the indices are determined by the method of relationship of the order based on centralized decision-making of fuzzy opinions. Damage degrees are evaluated by the extension method. The final evaluation results show that the total damaged area of land is 1 675 hm2, of which the slightly damaged area is 968.2 hm2, the moderately damaged area is 379.53 hm2, and the severely damaged area is 327.27 hm2. The comparison with the simple empirical method of determining damage degree by means of surface seeper shows that the empirical method often underestimates the level of land damage in the mining area with high groundwater level. Areas where differences occur are concentrated in the middle and lower slopes of subsidence basins. Tilt deformation causes the slope of the land to become larger, and increases soil erosion; the area is under tensile force, and soil compaction is reduced, which is not conducive to the growth of crops, so the region near middle slopes can also be moderately damaged area. For the subsidence basin in downhill region, squeezed effect improves the land compaction, which is not conducive to the absorption of water and fertilizer for crops, and the area is prone to salinization and swamping, which is not conducive to the growth of crops. Although there is no perennial water area, the extent of the damage can be severe. Therefore, when determining damage degree of the land in coal mining subsidence area with high groundwater level, the condition of surface seeper should not be deemed as the only standard. The method proposed in this paper provides a reference for this type of subsidence area: The perennial water area is designated as severe damage area by the limit condition method, and comprehensive evaluation index system should be constructed to evaluate damage degree of land in other areas. The study provides theoretical support for the development of land reclamation plan and the establishment of compensation standards of farmers in Dongtan coal mine, and provides a reference for the evaluation of land damage degree in the same type of coal mining subsidence areas with high groundwater level.