| As a type of mining method,fully mechanized top coal caving is intended mainly to carry out the mining of extra thick coal seams in China.Due to the considerable mining height,face length,and area of overlying strata as affected by the fully mechanized top coal caving mining of extra thick coal seams,as well as the significant instability and collapse height of overlying strata,the overlying rock structure of large space stope is formed(which is referred to as large space stope for short).The ground pressure of large space stope is complicated by the conditions for occurrence of coal measures strata,such as Tangjiahui,Bojianghaizi,Menkeqing and other coal mines in the west.Usually,the thickness of fine sandstone and conglomerate layers in the mining area strata ranges meters to tens of meters.During the mining process of the fully mechanized top coal caving working face,the large-span thick roof overhanging structure is formed.With a high bearing capacity,the thick roof of stope is resistant to collapse.The self-mining bearing capacity leads to the large-scale accumulation of energy,and the dynamic load caused by breaking and instability can reach a significant level,which has a substantial impact on the surrounding coal and rock masses as well as the support of the working face during mining.In some cases,it even induces dynamic pressure disasters.At present,there is a serious problem facing large space stope,which is the mining instability of thick roof,such as the mechanical causes of instability of thick roof in stope and high ground pressure,the identification of key strata of strong dynamic pressure behavior of overburden in stope,the impact of strong dynamic load behavior of thick roof in stope and other key issues.Therefore,it is imperative to conduct scientific research and find solution to these problems,which is crucial to ensuring the safe and efficient mining of extra thick coal seams and the high-quality development of the industry.In this paper,such research methods as engineering investigation,physical model test,numerical simulation,mechanical theory analysis and engineering practice are adopted in combination to explore the mechanism of how the mining dynamic load transfer and energy accumulation-dispersion of thick roof evolve in large space stope.Considering the engineering background of thick rock occurrence conditions,physical model test and mechanical simulation analysis are conducted to reveal the characteristics of instability and energy accumulation-dispersion evolution of thick roof in large space stope.Besides,the mechanical model is established for the mining bearing structure of thick roof in stope,the mechanical equation is deduced for the mining bearing and energy accumulation evolution of thick roof in stope,and the law of mechanical influence caused by the mining bearing state and energy accumulation evolution of thick roof is determined.According to the key layer theory and the derived mechanical equation of the mining bearing state and energy accumulation evolution of thick roof in stope,a recurrence model is constructed for the mining energy accumulation-dispersion evolution of thick roof in large space stope.Besides,the criteria applied to identify the key layer of strong dynamic pressure manifestation of the mining instability of thick roof in large space stope is established,and an analysis is conducted to reveal the causes for the mining instability evolution and strong dynamic pressure manifestation of thick roof in large space stope.The principle of function conversion and the law of dynamic load propagation are applied to obtain the dynamic load response equation of hydraulic support in working face caused by mining instability of thick roof,and to reveal the dynamic load response characteristics of hydraulic support on the working face of thick roof.Finally,the engineering example is provided to study the countermeasures taken to control the impact of high dynamic pressure on the working face of thick roof,and to evaluate the effect of rock pressure control through engineering applications.The main conclusions of this study are as follows:(1)Physical experiments and numerical simulation are conducted to reveal that the overburden of stope is characterized by the gradual evolution of instability from bottom to top with the increase in mining distance of the working faces.During instability evolution of the overburden,the thick fine sand strata of each layer are treated as the bearing boundary,while the thick fine sand strata roof and the overlying weak rock strata comprise a "composite mining bearing structure".In addition,the coal seam has a large mining height,and overburden mining instability has a significant impact.This results in large space stope shape of normal trapezoidal fracture trace.(2)Due to the large thickness and relatively high integrity of thick roof in stope,there are large amounts of mining energy accumulating at the end and middle of the overhanging area of the roof structure under the context of mining load bearing.At the moment of breaking-induced instability,the energy accumulating on thick roof due to mining is converted into instability kinetic energy together with the work done by the overlying transfer load.This imposes a significant dynamic load on the surrounding coal and rock masses.(3)As for the impact caused by the state of mining load bearing and the energy accumulation evolution of thick roof,the overlying load and overhanging span of the roof play a significant role in making the impact more significant.This contributes significantly to the severe dynamic pressure disaster occurring on the thick roof,the thickness and elastic modulus of which play a certain role in improving the bending stiffness of thick roof structure,thereby extending the limit overhang span of thick roof.Both of them and the mechanical strength of rock stratum are the key influencing factors for the formation of strong dynamic pressure of thick roof.(4)In respect of the dynamic load response of hydraulic support on the working face of thick roof,the overlying load and overhanging span of thick roof in stope play a direct role in aggravating the dynamic load response of hydraulic support at the working face,with the influence reaching the most significant extent.By increasing the limit overhang span,the thickness and elastic modulus of thick roof contribute indirectly to aggravating the dynamic load response of the hydraulic support on the working face.Moreover,when the intensity of impact caused by the same dynamic load is unchanging,the dynamic load response of mining instability on the low thick roof is relatively stronger than on the high thick roof,and its dynamic load response to the hydraulic support on the working face is relatively stronger,because it is affected by the attenuation resulting from the transmission of dynamic load energy.(5)During the first mining-induced break,the dynamic resistance caused by the work done by the overlying load on the thick roof is relatively strong.When the mining cycle breaks,the dynamic resistance caused by the release of cumulative energy of thick roof is relatively strong.Compared with the dynamic load response of the hydraulic support with the initial breaking and periodic breaking,the dynamic load response of thick roof in stope to the hydraulic support with the initial breaking is stronger.(6)The overhanging span is a significant contributor to the unstable dynamic pressure of thick roof in stope.Therefore,an effective technical solution to imposing control on the dynamic pressure sustained by thick roof is to adopt the presplitting roof cutting technology for actively changing the bearing structure and the overhanging limit span of thick roof in stope.Through presplitting blasting technology,the bearing structure of mining energy accumulation of thick roof in the field can be changed to reduce its impact on the dynamic pressure of hydraulic support.Taking into account the on-site conditions,the presplitting blasting roof cutting technology intended for the thick roof in stope is developed and applied to control the impact of strong dynamic pressure within the safe bearing range of the hydraulic support.This is effective in solving the potential safety hazards posed by the impact caused by a strong dynamic pressure of the hydraulic support at the working face when the thick roof is destabilized.Figures [109] Tables [6] References [187]... |
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