Study on the effect of layered structure of overlying strata on soil damage in mining areas in the loess covered region of northern Shaanxi

Ecological protection in coal mining areas is a major problem that needs to be solved urgently for high-quality development in the Yellow River Basin. Scientific understanding of the response law of the overlying strata of the coal seam to the surface transmission of coal mining damage is a major demand for realizing the full life cycle protection of the ecological environment in the coal mining area. Taking the coal mining area in the loess-covered area of northern Shaanxi as the research object, the field geological survey, sampling test, numerical simulation, laboratory experiment and theoretical calculation method are comprehensively used to study the ‘response-transmission’ characteristics of the layered structure such as the number of overburden layers, layer thickness, lithology, spatial position of thick and hard rock layers and coal seams on underground coal mining. The influence of coal seam overburden structure on the stress field of mining overburden under the same mining conditions is studied. The damage characteristics of soil physical, chemical, microbial and erosion characteristics under different coal seam overburden structure and the mechanism of soil physical, chemical and microbial characteristics change on erosion characteristics in coal mining area were revealed.The results show that: ① Under the same or similar mining conditions, the different results of surface ecological damage in coal mining subsidence areas mainly depend on the stress-strain response, bottom-up transmission and soil damage characteristics of the overlying rock (soil) layer of coal seam to underground coal mining activities. Under the coupling of overburden structure and mining stress transfer, the mining damage characteristics of vertical strong chain plastic deformation zone (VCP), horizontal strong chain elastic-plastic deformation zone (HCEP) and horizontal strong chain elastic deformation zone (HCE) are formed from bottom to top. Among them, the interbedded structure and characteristics of the lower part of the coal seam overburden have an important influence on the interface height of the plastic deformation zone. The overlying rock (soil) layer and the surface soil (soil) layer of the coal seam are the response carriers of underground coal mining activities and their damage. The number of strata, thickness, lithology, spatial position of thick and hard strata and coal seam and surface deformation are important geological factors affecting the types and characteristics of soil damage. ② The position of thick sandstone in the overlying strata is the first main controlling factor of the development height of the underground ‘VCP-HCEP’ interface and the ‘HCEP-HCE’ interface and the degree of surface deformation, which determines the development position of the ‘two interfaces’ and the degree of surface deformation. High thick sandstone has a significant inhibitory effect on the development of the ‘two interfaces’ and surface deformation. As the second main controlling factor, the sand layer coefficient is positively correlated with the development of ‘two interfaces’ and surface deformation under the condition of established thick sandstone position and sand layer coefficient less than 73%. ③ Based on the two main controlling factors, four basic types of overburden structure were divided. From the perspective of the coupling of ‘basic type of underground overburden structure + degree of surface morphological change + surface mining damage form + soil quality characteristic index’, it was found that the type of ‘low thick sandstone + sand layer coefficient greater than 73%’ had the most significant effect on surface soil damage. The damage of soil physical, chemical and microbial characteristic indexes on subsidence slope was concentrated on the top and middle of slope, with a decrease of 11.81% ～ 42.01% (p < 0.05). Mining ground fissures are the main cause of soil damage on subsidence slope. ④ The amplification effect of ‘low thick sandstone + sand layer coefficient greater than 73%’ type on soil erodibility K-value on slope surface was the most obvious, with an average increase of 26.38% (p < 0.05), and the average increase of soil erodibility K-value at the top of slope was the largest, which was 37.98% (p < 0.05). The average increase of soil erodibility K-value in the fracture area of the same part of the subsidence slope was 92.28% higher than that in the non-fracture area (p < 0.05). The erodibility K-value of loess subsidence slope was significantly positively correlated with soil porosity (p < 0.01), and significantly negatively correlated with soil moisture content, clay mass fraction, organic matter mass fraction, available nutrient mass fraction, soil microbial quantity and enzyme activity (p < 0.01).Therefore, the overburden structure type area of ‘low thick sandstone + sand layer coefficient greater than 73%’ is the key area of ecological damage in the loess gully coal mining area of northern Shaanxi. Artificial measures such as filling and subsidence reduction and soil improvement should be taken to protect and repair. For the overburden structure type area of ‘low thick sandstone + sand layer coefficient less than 73%’ and ‘high thick sandstone + sand layer coefficient greater than 73%’, artificial assisted ecological restoration can be adopted. For the overburden structure type area with ‘high thick sandstone + sand layer coefficient less than 73%’, the natural restoration method can be adopted for ecological restoration.