Search Results (7)

Fracture expansion in rock masses can be observed by monitoring the break of contacts between the bounding particles via the discrete element method. The latter’s realization in this study via the PFC^2D program tracked the evolution process of the zonal disintegration in an exemplary roadway-surrounding rock affected by mining. Besides, the damage evolution pattern in a high-stress soft rock roadway was simulated by the FLAC^2D program using a strain-softening plastic model, revealing the effects of rock mass strength, stress state, and anchor support on the zonal disintegration of the roadway. Numerical simulation results show that in a roadway with high-level stress, the obvious fractures spread from the roadway surface to the depth of the surrounding rock along a series of geometric planes and cut the surrounding rock into rock mass blocks. Under high crustal stress, conjugate shear fractures occur near the roadway surfaces and form a closed-loop fractured zone after intersecting the conjugate fracture faces. The closed fractured zone becomes a free face, from which conjugate shear fractures develop, forming new closed fractured zones in the deep surrounding rock. By repeatedly generating the closed fracture zones, a fracture network appears in the roadway-surrounding rock. The development of zonal disintegration of roadway-surrounding rock mainly depends on the rock mass strength and its stress state. Zonal disintegration only occurs when the crustal stress of the roadway-surrounding rock exceeds its strength. When the horizontal stress is low and the vertical stress exceeds the rock mass strength, zonal disintegration only occurs on two sides of the roadway. When the vertical stress is low and the horizontal stress exceeds the rock’s mass strength, it only appears on the roof and floor. When the values of cohesion, internal friction angle, and tensile strength are reduced in the same proportion, cohesion has the greatest impact on the expansion of the zonal disintegration zone, followed by the internal friction angle, while the tensile strength effect is the least. In anchor-supported roadways undergoing zonal disintegration processes, the intact zone blocks slide relatively along the fracture surface during the process of loosening and deformation of the surrounding rock, making the anchor rods susceptible to tensile, shear, and bending actions. Full article

The coal walls in a caving face with a tall mining height are prone to rib spalling, which leads to the phased cessation of the mining of the working face, causes heavy losses, and endangers the safety of underground workers. In order to prevent serious rib spalling accidents of coal walls in fully mechanized caving faces with a large mining height and to improve the prediction of and ability to control rib spalling, a load-bearing mechanical model of the roof–coal wall–support system was established based on the moment-balance relationship. The expressions for the deformation and stress distribution in a coal wall were calculated. Then, the influences of key factors on the horizontal displacement of the coal wall were investigated. A numerical simulation model of the working face was established, and an orthogonal test design was introduced. On this basis, the influences of four factors: cutting height, breaking position of the main roof, support strength, and sidewall protecting force of the support on the horizontal displacement and volume of a plastic zone of coal wall, were analyzed. Moreover, their order of importance was ranked on the basis of sensitivity. Based on the engineering conditions and production practices in the Cuncaota II Coal Mine, key parameters for controlling and measures for preventing the rib spalling of the coal wall are proposed to guide practical actions. Full article

The mining-induced ground response (MIGR) has a critical impact on safety management, the mining plan, and entry support. A clear understanding of the characteristics is the foundation of the MIGRs scientific control. This study is the result of the MIGRs development of the non-pillar mining panel with gob-side entry by roof cutting (GSERC). Comprehensive research of the in situ measurements, numerical simulation, and theoretical analysis to determine the ground response characteristics, including mining panel and GSERC, were implemented. The results indicate that the MIGR presents the characteristic of asymmetric development and that the ground response near the non-roof cutting side is more significant than that near the roof cutting side. The development stage of the entry convergence of GESRC can be divided into seven stages; the primary rapid development stage should be paid more attention to in the support process. The entry convergence rapidly increases to 275 mm, 380 mm, 410 mm, and 525 mm, respectively, for the roof cutting rib to the virgin coal rib, the roof near the virgin coal side, the roof of the middle section, and the roof near the cutting side. The hydraulic support end cycle resistance at the roof cutting side and the middle section of the mining panel with the value of more than 30.8 MPa is greater than that at the non-roof cutting side with the value of less than 26 MPa, which presents the asymmetric feature. The numerical simulation results regarding vertical stress development, vertical displacement, and horizontal displacement also presents the asymmetric feature. The MIGR division is divided into five divisions. Division II (the middle section of the panel) and division IV (the entry range near the roof cutting side) should be paid more attention to in the panel mining process. The results of this study can provide technical guidance and theoretical reference for similar engineering practices. Full article

In this paper, roof-cutting technology of directional energy-cumulative presplitting blasting is taken as the research object. Through the numerical simulation software Ansys/Ls-dyna3D, the process of energy-cumulative blasting and non-cumulative blasting is simulated and analyzed by using the ALE algorithm. Moreover, the evolution processes of tensile strain energy, detonation stress wave of explosives, stress state of rock mass, and rock crack damage cumulative are compared in two conditions. In the energy cumulative state, the detonation wave acts centrally on the hole wall in the energy cumulative direction to form an initial crack, and then under the action of the jet of energy cumulative, the crack continues to propagate until it runs through. In the non-energy cumulative state, the crack propagates uniformly around the hole wall, forming irregular short cracks. The simulation is verified by the field test, and the law of crack propagation is the same with the simulation. Therefore, directional energy-concentrated presplitting blasting has good practicability in a roof presplitting operation. Full article

According to the development requirements of green mining of coal resources, it is imperative to improve the extraction rate of coal and the application of safe and efficient mining technology. Pre-splitting and roof cutting technology is widely used in reducing residual coal pillars and safe pressure relief mining, which has become the crucial technology for pillar-free mining methods. Therefore, it is essential to review and discuss the research hotspots, cutting-edge methods, principles of action, and application areas of the development of this technology. Above all, the research data on pre-splitting and roof-cutting development in the past ten years are summarized and outlined. The research’s hot spots are pressure relief technology and gob-side entry retaining technology. Then, the functional forms of pre-splitting and roof cutting technology are discussed and compared, including explosive blasting (directional energy gathering blasting, liquid explosive blasting, and composite blasting), hydraulic fracturing, liquid CO₂ gas fracturing, and mechanized roof cutting (chain arm saw machine and directional cutting roof rig). Through the analysis of field application cases, the application field is divided into three major areas: non-coal pillar mining (gob-side entry driving with narrow coal pillar, gob-side entry retaining with the filling body, completely gob-side entry retaining, and “N00” construction method), pressure relief at working face (thick and hard main roof cracking and end area hard roof cracking), and pressure relief at roadway (gob-side roadway pressure relief and blasting pressure relief technology for roadways). By detailing the process of each application technology one by one, the principle and mode of pre-splitting in each technology are expounded. Finally, the development prospects of pre-splitting and roof cutting in new technical methods, deep pressure relief mining, intelligent unmanned mining, and green and efficient mining are prospected, providing references for similar projects. Full article

According to the movement characteristics of the surrounding rock of the gob-side entry retaining by roof cutting (GERRC), the structural evolution mechanism and stability control countermeasures are studied in this paper. Taking the roof cutting and bulking as the core point, the movement process of the surrounding rock of GERRC is divided into five stages: roof cutting, dynamic pressure support, gangue bulking, double arch roadway support, and roadway formation. Combined with mechanical analysis, the roof pressure of the mechanical model of the short-arm beam formed by roof cutting is calculated, and the roof breaking criterion is obtained, which provides a basis for roof control design and reasonable support strength calculation. A structural model of double-arch roadway protection under the action of hydrostatic roof cutting and gangue bulking was established, and the mechanism of roadway formation stability was studied. The gob-side bulking gangue and short-arm beam are formed by roof cutting, and the solid coal seam forms the immediate balance arch, the bulking gangue in goaf, articulated transfer rock beam, and solid coal seam forms the main balance arch. The two together are called double balance arch. With the support of the double-balanced arch on the overlying rock strata, the roadway below is located in the low-stress zone, which ensures the stability of the roadway surrounding rock. Through mechanical analysis and field observation, the law of bulking and deformation and mechanical characteristics of gob-side gangue in the above structural model are analyzed, and the reasonableness of the structural model of the double-arch guard lane is verified through numerical simulation of the cut top into lanes. and the reasonableness of the structural model of double-arch protection in GERRC is verified by using numerical simulation. In this paper, a mechanical double-arch model has been developed that allows us to understand the mechanism of stress transfer of roof cutting roadway from the perspective of a dynamic balance of roadway surrounding rock and overlying rock strata, helping us to specify efficient support countermeasures. Full article

This paper presents the results of a survey that was undertaken to examine homeowners’ FireSmart mitigation practices and investigate existing incentives and barriers to uptake of FireSmart Canada’s recommended wildfire mitigation activities in the Urban Service Area of Fort McMurray Alberta. Single-family residential property owners, the large majority of whom were affected by the Horse River wildfire, were invited to participate in an online survey. A total of 496 surveys were completed, with a response rate of 38%. We found that most of the participants generally perceive a low to moderate wildfire risk to their properties: they felt there was a low chance of a catastrophic fire happening soon and/or ‘enough’ had already been done to reduce the immediate risk. Although about half of the participants searched for information about FireSmart, having information or knowledge of FireSmart did not translate into substantial adoption of recommended mitigation actions. Survey participants generally preferred and implemented more of the low-cost, low effort mitigation measures such as cutting grasses and cleaning debris, likely for reasons other than wildfire risk reduction. With regard to structural measures, we found asphalt shingles and vinyl siding were present on the majority of homes; although this was not a choice but was provided by the builder or on the home when it was purchased. Very few respondents were willing to replace their siding or roof––the cost was the single biggest factor. In addition, we identified several other factors as negatively influencing homeowners’ mitigation actions, including the tendency to shift responsibility to the municipal government and social pressure such as neighbors not being as proactive in completing FireSmart mitigation measures. Recommendations that may help promote positive wildfire mitigation behaviors are discussed. Full article