The exploitation and utilization of coal resources are not only prone to causing water table lowering, but also produce a large amount of CO₂ and coal-based solid waste. A scientific concept that employs the CO₂ and solid wastes to develop filling bodies and inject them into the mined-out area, to sequestrate CO₂ and mitigate the overburden migration and thus preserve the overlying aquifer, is proposed. Continuous extraction and continuous backfill (CECB) mining was selected as the mining method to meet the aforementioned objectives. Additionally, carbon dioxide mineralized filling body (CMFB) under ambient temperature and pressure was developed, with fly ash as aggregate, and CO₂ gas, silicate additives and cement as accessories. The uniaxial compressive strength (UCS) and tensile strength of CMFB with various curing times and fly ash contents were tested indoors. A physical analogue simulation and FLAC^3D numerical calculation were then successively implemented on the premise of determining a similar material ratio of CMFB in analogue simulation and calibrating the parameters of the CMFB in numerical simulation. The deformation of aquifuge and water level lowering while using CECB and CMFB with various proportion of fly ash were obtained. When using the CMFB with 75% fly ash content and 28 d curing time, the maximum values of vertical displacement, horizontal displacement, inclination, horizontal deformation and curvature of aquiclude were 26 mm, 6.5 mm, 0.12 mm/m, 0.08 mm/m and 0.0015 mm/m², respectively, and the water table decreased 0.47 m. The results show that the CMFB with 75% fly ash is the most appropriate ratio to realize water preservation mining, CO₂ sequestration and harmless treatment of solid wastes, contributing to the green and sustainable development of coal areas.

Coal mining under the aquifer in Northwest China has brought a series of ecological problems, such as the decline of groundwater levels and the death of surface vegetation. The study of the impermeability of rock strata between coal seams and the overlying aquifers is of great significance to solve these problems and realize water-preserving coal mining (WPCM). Based on mining-induced overburden damage and permeability deterioration, the concept of the “three seepage zones” of overburden is proposed, namely the pipe flow zone, water seepage zone and nominal water-seepage-resistance zone (NWSRZ). Meanwhile, the concept of water-seepage-resistance strata (WSRS) is put forward from the aspects of initial permeability, structural strength, swelling and the stratigraphic structure of the overlying strata. AHP-fuzzy comprehensive evaluation (AHPF) is employed to construct a model to evaluate the water-blocking capacity (WBC) of the WSRS. The model includes three secondary factors and nine tertiary indicators, and the weights and membership functions of the indicators are determined. Subsequently, the model is generalized and applied to the Yu-Shen mining area. The membership degrees are spatially visualized by means of thematic maps. The comprehensive evaluation values Φ of the WBCWSRS of 400 boreholes in the mining area under backfill mining, narrow strip mining, slice mining and longwall cave mining are calculated. Then, the Kriging method is employed to plot the zoning maps of Φ under four different mining methods. In view of different grades of WBCWSRS, three corresponding countermeasures, i.e., mining methods optimization, curtain grouting and underground reservoir construction, are put forward. The fluid–solid coupling embedded in FALC^3D software is employed to establish a numerical calculation model to simulate the water table fluctuation of the underground aquifer under the four mining methods, and the reliability of the model is verified indirectly. In this paper, mathematical theory is combined with WPCM to develop an evaluation model of WBCWSRS, which provides a reference for the coordinated development of coal extraction and water resource preservation in arid and semi-arid mining areas.

Longwall backfill mining effectively mitigates the height of water-conductive fractured zone (HWCFZ), preventing it from reaching the overlying aquifer and thus preserving the groundwater. However, it has the disadvantages of insufficient filling time and space as well as the mutual constraints between filling and mining. A novel continuous extraction and partial backfill (CEPB) water-preserving mining method was therefore proposed. The analytic hierarchy process (AHP) method was employed to identify the factors affecting the HWCFZ of CEPB, and five main factors, namely, the hard-rock lithology ratio, mining height and depth, and the width of the Wongawilli and protective block, were determined based on the weight distribution. UDEC software was used to establish a numerical model to simulate the HWCFZ under five factors. By using a multiple linear regression analysis of the numerical simulation results, a model for predicting the HWCFZ was established. It was applied in a colliery of the Yu-Shen mining area, and the HWCFZ was 57.7 m, 9% higher than that of borehole television logging of 53.1 m from the field measurement, indicating its rationality. Subsequently, the model was generalized and applied to the whole mining area, and the thematic map of the HWCFZ and the protective zone thickness of CEPB and longwall caving mining were obtained. The criterion for water-preserving mining based on the equivalent permeability coefficient of the protective zone is then proposed, which can provide guidance for the mining parameters optimization of the CEPB.

The consumption of coal resources has caused an increase in CO₂ emissions. A scientific concept that can realize CO₂ sequestration, the harmless treatment of solid wastes, and coal extraction under buildings, railways, and water bodies (BRW) is proposed. First, a novel CO₂ mineralized filling body (CMFB) is developed by employing CO₂ gas, fly ash, silicate additives, and cement. It is then injected into the mined-out mining roadways (MRs) of the continuous extracting and continuous backfill (CECB) mining method to ameliorate the overburden migration and thus extract the coal body under the BRW. The AHP-fuzzy comprehensive evaluation method was employed to construct a prediction model for the suitability of this concept. Subsequently, the evaluation model is generalized and applied to the Yu-Shen mining area. Each indicator affecting adaptability is plotted on a thematic map, and the corresponding membership degree is determined. The aptness for 400 boreholes distributed in the entire area was determined and a zoning map which divides the whole area into good, moderate, slightly poor, and extremely poor suitability was drawn. This paper puts forward a mathematical model for predicting the suitability of using CECB and CMFB to sequestrate CO₂. Research results can provide references for determining the site of CO₂ sequestration under the premise of maximizing the economic and ecological benefits, which is conducive to constructing ecological, green, and sustainable coal mines.

To address the issues of corrosion weakening of solid-waste-based backfill material caused by mine water, a novel hydrophobic solid waste backfill (HSBF) material was developed using polydimethylsiloxane (PDMS) and a silane coupling agent (SCA) as hydrophobic modification additives, and NaOH (SH) and sodium silicate (SS) as alkali activators. Fly ash and slag were chosen as the primary raw solid waste materials. The rheological properties of the hydrophobic-treated backfill slurries were measured, and the resulting physicochemical properties were compared with the unmodified reference group. This study reveals that the fresh HSBF slurry follows a Modified Bingham (M-B) model with shear-thinning characteristics. The addition of PDMS causes an increase in the water contact angle of the hardened HSBF material with F8S2 to up to 134.9°, indicating high hydrophobicity. Morphological observations indicated that PDMS mainly attaches to the inorganic particles’ surface through the bridging action of SCA for the hydrophobic modification of the backfill material. The overall strength of the HSBF materials was further ensured via fly ash–slag ratio optimization, and was found to be enhanced up to 98% by increasing slag content from 20% to 50%. This is mainly attributed to the hydration of slag, forming C-S(A)-H gel, which contributes to the increased strength. The novel HSBF material enables the elimination of cement in mine backfilling applications, demonstrating good economic benefits. Its excellent mechanical and hydrophobic properties can not only prevent overburden displacement in goaf areas, but can also mitigate water resource loss from overlying strata and simultaneously reduce the safety risks associated with long-term mine water deterioration.

This research introduces a novel dual CO₂ storage (DCS) approach by simultaneously storing CO₂ gas in abandoned mines and securing it within mineralized backfill. For this method, CO₂ mineralized backfill materials (CMBM) are pumped into CO₂ mineralized storage segments (CMSSs) to support the roof while gaseous CO₂ is injected into gaseous CO₂ storage segments (GCSSs) to maximize storage amounts. This study focuses on the Yu-Shen coal area in Yulin City, Shaanxi Province, China. A three-level evaluation model was constructed to predict DCS feasibility based on the analytic hierarchy process (AHP) and fuzzy comprehensive assessment method. The model was generalized and applied to the whole coal area. Each indicator affecting adaptability is plotted on a thematic map to determine the corresponding membership degree. The aptness for 400 boreholes distributed in the entire area was derived and a zoning map which divides the whole area into different suitability was drawn. This paper puts forward a mathematical model for predicting DCS suitability. The findings establish an engineering paradigm that simultaneously addresses CO₂ sequestration, industrial waste recycling, and ecological water table preservation. The research results can provide references for determining the site of DCS, contributing to the generalization of DCS in a larger range.