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Backfill coal mining technology has drawn widespread attention due to its benefits of “controlling surface deformation and subsidence, reducing mining-induced disturbance in the stope, and recycling solid mine wastes”. However, the backfill coal mining technology is still progressing slowly in China. The geological environment of China’s mining areas is complex and highly diversified, and backfill coal mining is expected to fulfill different goals in a wide range of engineering scenarios. These facts explain the poor reproducibility of backfill coal mining projects. This study reviews the existing backfill coal mining systems in China. Based on findings from a survey of engineering cases, we summarize five types of new backfill coal mining methods classified by deployment style; namely, borehole grouting backfill, roadway backfill, borehole–roadway backfill, in situ backfill, and roadway-in-situ backfill. A total of 15 backfill coal mining methods falling into the above five categories are described. An engineering design workflow for backfill coal mining consisting of five steps is proposed; namely, identifying the targets of backfill, analyzing the feasibility of deploying the backfill system, comparing the engineering quantities of different engineering schemes, estimating the economic efficiency of backfill, and backfill performance tracking and monitoring. Real cases of backfill engineering design are analyzed to inform the fast and reasonable design of backfill strategy for specific working faces in certain coal mines.

Underground backfilling stands out as a crucial technological strategy for the eco-friendly and effective management of solid waste in mining operations. However, existing backfilling techniques have led to increased production processes at the working face, resulting in a reduction in coal extraction efficiency. Addressing the temporal and spatial interference between mine solid waste backfilling and coal mining is essential. To overcome this challenge, this study introduces a novel post-mining spatial gangue slurry backfilling method. Radar detection was employed to ascertain the typical characteristics of the subsequent space collapse roof shape. Stress monitoring and compaction experiments were conducted to establish the relationship between stress and the bulking coefficient of the overlying rock mass, identifying subsequent spatial void structure characteristics. The development of a CO 2 mineralized coal-based solid waste filling material, utilizing conventional low-calcium fly ash under normal temperature and pressure conditions, was presented. This paper provides a comprehensive understanding of the post-mining spatial gangue slurry backfilling method, outlines the spatial layout approach for the corresponding system, and analyzes research challenges associated with gangue slurry backfilling materials and the technology of slurry injection borehole layout. The research aims to innovate an efficient underground disposal model for gangue, contributing to the refinement of the technical system for the comprehensive disposal and utilization of gangue.

Coal fire is a global catastrophe. Xinjiang suffers the most severe coal fire in China and even in the world. Coal firefighting work has been being conducted for decades in Xinjiang. In this paper, coal fire detection, extinguishing, and monitoring approaches that were derived from coal firefighting experience are introduced in detail by taking the Fifth Fire Area (FFA) of the Heshituoluogai coal fire for instance. We first introduce the geology and fire situation in the FFA. Before developing efficient strategies to extinguish it, magnetic and self-potential methods are adopted to delineate the extent of the fire. A composite index is proposed to better indicate the fire. The comprehensive coal firefighting method is illustrated in detail, which consists of surface cooling, excavation and leveling, borehole drilling, borehole water injection and grouting, and loess backfill. The subsequent temperature and CO monitoring records show that the fire is extinguished successfully without burnback. The methodology presented here provides guidance and reference for putting out other coal fires around the world.

Thermal properties of grouting materials for borehole heat exchangers (BHE) are currently analysed with varying measurement methods and analysis procedures, resulting in difficulties when comparing values of different studies. This study therefore provides the first comprehensive investigation of different analysis procedures by systematically comparing the influence of the measurement method and the sample preparation on the determination of the thermal conductivity and the volumetric heat capacity. Seven dissimilar grouting materials with varying water–solid ratios (W/S) and compositions are analysed. The thermal conductivities of the materials range between 0.9 and 1.8 W m−1 K−1 (transient plane source method, TPS). The volumetric heat capacities range between 3.01 and 3.63 MJ m−3 K−1 (differential scanning calorimetry, DSC). From the findings of this study, a standardised analysis of grouting materials is provided which suggests mixing of the grouting material at a high mixing speed and sample curing under water for 28 days at room temperature. The benefits of calculating the volumetric heat capacities of grouting materials from the specific heat capacities of dry samples measured with the DSC, the water content and the bulk density are demonstrated. Furthermore, an estimation procedure of volumetric heat capacity from the W/S and suspension density with an uncertainty of smaller ± 5% is provided. Finally, this study contributes to consistency and comparability between existing and future studies on the thermal properties of grouting materials.

The thermal properties of grouting materials characterise the heat transfer around borehole heat exchangers (BHE). However, these properties are typically determined in the laboratory. Thus, this study aims to assess the properties of grouting materials in the field. Two BHE grouted with two different grouting materials within unsaturated loess and limestone were excavated up to a depth of 15 m. Collected field samples show higher thermal conductivities by 13% ( W / S  = 0.3) and 35% ( W / S  = 0.8) than laboratory samples of the same material. These differences in thermal properties are mainly related to the filtration of the grouting suspension. In addition, with a short-time enhanced thermal response test (ETRT), 17% lower in-situ thermal conductivities are determined than in comparison with the field samples. The deviations are attributed to the geometry of the borehole, the trajectory of the BHE pipes and the heating cable. Thereby, this study shows the limitations when transferring laboratory-derived properties to a field site and emphasises the importance of considering site conditions, such as geology and hydrogeology.

Subsoil foundation study was carried out in Owo Local Government of Ondo State, Southwestern Nigeria with an aim to assess the soil geophysical and geotechnical properties for design of shallow foundation structural support. The geophysical method which consists electrical resistivity [involving vertical electrical sounding (VES) and combined profiling field techniques] and very low frequency electromagnetic. The geotechnical method involved field cone penetrometer test and laboratory analysis of eight soil samples, collected at a depth between 1 and 3 m. These were complimented by hydrogeological measurements (static water level and hydraulic head determinations) and borehole drilling. The result showed that the upper 3 m is competent to support structural foundation elements with an allowable bearing capacity ranging from 291 to 293 kN/m 2 (resistivity values of 59–569 Ω-m) with low settlement values of 0.72–0.75 mm. These bearing pressures are considered appropriate for use in design of bases, strip or raft foundations. The AASHTO classification of A-2-4/A-5 constitutes about 62.9%. The average static water level (5.6 m) will not or seriously affect footing structures even in wet season. However, the result of the borehole drilling and VES/Resistivity structures revealed the presence of clayey weathered layer at shallow depth less than 5 m in some places, therefore relatively high consolidation settlement is expected in those places upon loading. The depth to basement is 35 m, this overburden thickness would assist in evenly distribution of the foundation/structural load to the basement rock. All the determined geotechnical parameters of the subsoil fall within the specification recommended for foundation material by Federal Ministry Works and Housing of Nigeria. The quartz-schist and schistose quartzite are the most prominent rocks observed in the field. The Schist tends to slide/split along their planes of schistocity while the quartzite show significant fractures and joints which might require grouting/backfilling during construction activity.