# Roadway Backfill Mining with Super-High-Water Material to Protect Surface Buildings: A Case Study
###### Abstract  
As coal resources trapped under surface buildings in the Wangtaipu coal mine area impede the efficient mining of coal seams and constrain the sustainable development of coal mines, a super-high-water roadway backfill mining technique for preventing building damage was adopted. According to the control principle and theoretical calculations, an engineering design was established including the reasonable width of segment coal pillars in the working face and technical parameter design. The monitoring results after the implementation of the scheme showed that building deformation was controlled within grade I, protecting the surface buildings. A reasonable roadway design parameter was achieved, with a surface subsidence of 27 mm and horizontal deformation of 0.3 mm/m, indicating that it can ensure the safety of surface buildings from the State Bureau of Coal Industry. The practical trial can provide a reference for the extraction of coal resources under similar conditions and is vital for the sustainable development of the mining industry and economic growth. Keywords:roadway backfill mining; surface subsidence; overburden movement; environment protection +
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This is because the remaining coal pillar can usually support the overburden load, allowing slight surface movement and deformation. Although the strip mining or room mining method has reduced surface subsidence or damage to a certain extent, the mining rate is low due to the coal pillars. Because the harmonic mining is limited by many factors, such as construction organization, geological conditions, and subsidence factor, it is difficult to realize. Meanwhile, grouting injection technology has been successfully applied to reduce surface subsidence and building damage in populated areas. [PERSON] [16] studied shear behaviour of the slurry through field and laboratory testing, and derived formulas for minimum pump pressure and local pressure, and the results are consistent with each other. [PERSON] and [PERSON] [17] used numerical modeling to study the evolution of horizontal fractures at bed separations during longwall mining. By analyzing the influence of filling ratio and maximum subsidence on the grouting effect, [PERSON] and [PERSON] [18] obtained the supporting width that was formed at the center of the gob. Subsequently, the stiffness and distribution of the grouting mass were investigated through field borehole, and the mechanism of grouting to restrain the overburden deformation was revealed [19]. Considering the long-term stability of overburden, the effect of harmonic mining and grouting injection on overburden damage reduction is poor. In fact, the essential problem of \"under-three\" mining is controlling the movement and deformation of the overlying strata. By contrast, backfill mining has been widely used and is becoming an effective technique for reducing the degree of overburden failure in the green mining technical system, and has been developed to solve the global problems described above, especially for \"under-three\" mining [20; 21; 22; 23]. In essence, backfill mining reduces the overburden movement by replacing the coal with backfill material. Meanwhile, its basic principle states that the maximum limited thickness should be controlled according to the specifications for \"under-three\" mining, so as to guarantee the mining influence is within the allowable range. However, according to the study conducted by [PERSON] and [PERSON] [24], heavy metal pollution caused by mining waste in the backfill mining area has resulted in groundwater pollution lasting for decades, with an increase in pollution intensity with respect to time. This is also noted by Chinese scholars [21; 25]. Moreover, backfill material contains toxic metals and inorganic salts that will dissolve and spread in water, which can pose the risk of secondary pollution to the groundwater environment [26; 27].