Experimental Study on Gangue Backfilling Materials Improved by Soda Residue and Field Measurement of Surface Subsidence

In coal mining, the problems of massive discharge of solid waste, environmental pollution, and surface subsidence disaster are urgent to be solved. Based on this engineering background, the feasibility of using solid waste soda residue to improve gangue cemented backfilling material was discussed, and the surface subsidence of the test working face was measured in this study. Besides, the influence of soda residue on the performance of gangue cemented backfilling materials was analyzed through laboratory tests. The experimental results show that 1) as the content of soda residue increases within the range of 0–12%, the slump of the soda residue gangue backfilling material (SRGBM) slurry gradually increases, and the bleeding rate increases. The early strength and later strength of SRGBM increase first and then decrease with the increase in soda residue content. 2) The optimal ratio of the soda residue cemented backfilling material is soda residue: fly ash: lime: cement: gangue = 6%: 34%: 10%: 2.5%: 47.5%. Compared with the reference group, the slump of the material is increased by 12.7%, the bleeding rate is only 3.8%, and the early strength and later strength are increased by 449 and 187%, respectively. 3) The addition of soda residue promotes the hydration reaction of the slurry system of soda residue cemented materials. The coexistence of C-S-H gel and N-A-S-H gel reduces the connectivity of pores and improves the strength of the material. 4) The maximum surface subsidence of the test working face is only 245 mm, and the surface subsidence control effect is good. Therefore, the preparation of SRGBM with soda residue can achieve energy saving and emission reduction, with significant technical, economic, and social benefits, and has good promotion and application value.

Movement Laws of Overlying Strata above a Fully Mechanized Coal Mining Face Backfilled with Gangue: A Case Study in Jiulishan Coal Mine in Henan Province, China

The aim of this study is to obtain movement laws of overlying strata above a fully mechanized coal mining face backfilled with gangue and solve the problem of surface subsidence during coal mining. This study was carried out based on gangue backfilling mining of Jiulishan Coal Mine (Jiaozuo City, Henan Province, China) from the perspectives of deformation of backfilled gangue under compaction, surrounding rock of a stope, and activities of key strata. The method combining with rock mechanics, viscoelastic mechanics, control theory of rock mass under mining, and numerical simulation was used based on physical and mechanical characteristics of backfilled gangue. On this basis, the research analyzed the temporal-spatial relationships of activities of surrounding rock of the stope, compressive deformation of backfilling body, failure depth of the floor, deformation characteristics of the main roof with laws of surface subsidence. The movement characteristics of overlying strata above the fully mechanized coal mining face backfilled with gangue and the traditional fully mechanized mining face were compared. It is found that, under the same conditions of overlying strata, movement laws of overlying strata are mainly determined by the mining height of coal seams and the heights of a caving zone and a fracture zone are nearly linearly correlated with the mining height. Through analysis based on thin-plate theory and key stratum theory, the location of the main roof of the fully mechanized coal mining face backfilled with gangue in coal seams first bending and sinking due to load of overlying strata was ascertained. Then, it was determined that there are two key strata and the main roof belongs to the inferior key stratum. By using the established mechanical model for the main roof of the fully mechanized coal mining face backfilled with gangue and the calculation formula for the maximum deflection of the main roof, this research presented the conditions for breaking of the main roof. In addition, based on the theoretical analysis, it is concluded that the main roof of the fully mechanized coal mining face backfilled with gangue does not break, but bends. The numerical simulation results demonstrate that, with the continuous increase of strength of backfilled gangue, the stress concentration degree of surrounding rock reduces constantly, so does its decrease amplitude. Moreover, the compressive deformation of backfilling, failure depth of the floor, and bending and subsidence of the main roof continuously decrease and tend to be stable. The mechanical properties of backfilling materials determine effects of gangue backfilling in controlling surface subsidence. Gangue backfilling can effectively control movement of overlying strata and surface subsidence tends to be stable with the increase of elastic modulus of gangue.

Mechanical analysis of basic roof fracture mechanism and feature in coal mining with partial gangue backfilling

Coal mining with partial gangue backfilling (CMPGB) method has the advantages of both high filling efficiency and excellent workface capacity, which breaks through the technical bottleneck of full-section backfilling mining. In order to reveal the fracture mechanism and characteristics in CMPGB workface, this paper presents a comparative analysis of the filling ratio of different filling patterns in CMPGB. A local composite elastic foundation mechanical model of basic roof in CMPGB was established using thin elastic plate theory. Then, Galerkin’s semi-analytic solution process was designed according to local composite elastic foundation characteristics. A deflection equation of the basic roof was derived, and a critical condition of the basic roof breakage was given. Based on engineering calculation example of Ji15-31010 workface of Pingdingshan No. 12 Coal Mine, the following conclusions are drawn. (1) At the basic roof of caving section, tensile-shear failure occurred in workface, cutoff position, and transition section, while compressive-shear failure occurred in the central part of the goaf. The basic roof showed a typical local “C–X” failure characteristic. (2) The value of first caving span decreased from 32.7 to 31.4 m as the elastic foundation coefficient of backfilling body increased from 70 × 106 to 120 × 106 N/m3, with a decreasing amplitude of only 4.1%. The increase of density of backfilling body only changed the support structure of backfilling section and had an insignificant effect on the first caving span. (3) The value of the first caving span decreased from 59.1 to 21.68 m as the length of caving section increased from 40 to 140 m, indicating that the first caving span was mainly influenced by the length of caving section. The measured value of the first caving span of Ji15-31010 CMPGB workface was 29.8 m, which was close to the theoretical value of mechanical model.

An Innovative Method for Placement of Gangue Backfilling Material in Steep Underground Coal Mines

Using gangue backfilling in underground coal mining not only controls the roof deformation in the gob area but also reduces the amount of mining waste rock. However, due to the limitations of the complicated engineering conditions, backfilling mining in the steep coal seam is not widely applied. In this study, a long-distance backfilling technology with a scraper winch for a steep coal seam was proposed and applied in a flexible shield supporting working face in Datai Mine, Beijing. Aiming at the problem of the decreasing backfilling ratio in field practice, numerical simulation was carried out to research the moving law of gangue in the goaf. The gangue mainly experienced four stages: gangue landslide stage, small-scale subsidence stage, funnel-shaped subsidence stage, and large-scale subsidence stage. The moving area of the gangue could be divided into five areas including a motionless area, a landslide area, a subsidence area, a funnel-shaped subsidence area, and a to-be-backfilled area. With the increase of the inclined length of the working face, the moving time of the gangue increased gradually. Based on the simulation results, the scheme of backfilling and mining in Datai Mine was optimized, for which the inclined length of the working face was shortened, and a higher backfilling ratio was obtained.

Surface Subsidence Control Mechanism and Effect Evaluation of Gangue-Backfilling Mining: A Case Study in China

Comprehensive mechanization solid backfilling mining is a new technology developed in China for coal mining and surface subsidence control. Based on a gangue-backfilling project in the Yangzhuang Coal Mine, the characteristics of underlying strata and surface deformation were studied by similar-material simulation method. When the ratio of the sponge to foam was 1 : 3, the mixture can simulate well the deformation characteristics of gangues in the similar-material model. On this basis, the movement and deformation characteristics of the overlying strata caused by gangue-backfilling mining were studied. The findings indicate that compared with caving mining, the expansion coefficient of overlying strata, the interlayer fracture, and the subsidence value were smaller in backfilling mining, with the integral overlying strata subsidence occurring. Meanwhile, the reduction ratio of surface subsidence after backfilling mining was more than 85%, verified by the subsidence-monitoring results. The research outcomes in this paper have significance for coal resource exploitation of similar mines around the world.

Acoustic Emission and Ultrasonic Characteristics in the Failure Process of Cemented Waste Concrete-Coal Gangue Backfilling (CWCGB) under Uniaxial Loading

The acoustic emission (AE) characteristics, change law of the ultrasonic velocity, and internal failure mode of cemented waste concrete-coal gangue backfilling (CWCGB) with 600 days of curing time were studied under uniaxial loading conditions. Waste concrete particles of 5 mm acting as fine aggregates substituted for 30% and 50% fine coal gangue in the cemented coal gangue backfilling (CGB). AE was used to test the ring count and changing rule of the accumulated energy, locate the event for positioning, and calculate the number of events. The average ultrasonic wave velocity was measured via an ultrasonic detector. The characteristics of the microfractures were observed via a scanning electron microscope. The results showed that the specimens with 30% and 50% waste concrete replacement rates underwent ultrasonic wave velocity stabilization and a rapid decline stage under uniaxial compression; for the former case, the decline started earlier. The AE ring count attained peaks at the pore compression stage, yield stress point, stress peak value, and residual stress stage with no added waste concrete and 30% and 50% waste concrete substitution rates. The value and consequent frequency of the ringing count peak and cumulative energy slope increased with increasing waste concrete substitution rate. A microcrack was observed at the interfacial transition zone between the cement paste and gangue owing to the alkali-aggregate reaction effect. However, a better bonding performance was exhibited by the waste concrete particles and paste.