| Gob-side entry retaining(GER)by roof cutting and pressure release,an emerging non-pillar longwall mining technology,is an important safeguard for achieving sustainable and scientific coal mining.This technology is particularly favorable for increasing coal recovery rates,reducing roadway drivage ratios and reducing gas dynamic phenomena.In this thesis,emphasizing particularly on structure optimization and stability control of entry surroundings,comprehensive research methods of theoretical analysis,numerical simulation,laboratory experiment and field test were adopted to explore moving laws and stabilizing mechanism of the entry surroundings.Finally,a series of controlling techniques that are used for stabilizing the entry surroundings in thick coal seam GER by roof cutting and pressure release were developed and tested.The main results are as follows:(1)Based on the principle of GER by roof cutting,the evolutionary process and mechanical mechanism of the entry surrounding structures were analyzed.Mechanical models for the entry roof in the advanced pressure region in front of the mining panel,the dynamic and stable pressure regions behind the mining panel were respectively established.Using the models,the stress distributions and deformation characteristics of the entry roof were obtained.It was found that the most unstable area of the entire entry retaining process was the dynamic pressure region behind the mining panel.Improving the lowest steady-state coefficient and reducing the unstable distance of the dynamic pressure region were quite important for stabilizing the retained entry.Based on the theoretical analysis results,the overall entry control ideas(i.e.optimizing the entry surrounding structure by cutting and fully utilizing the bearing capacity of the entry surroundings)were put forward.(2)The stress distribution and cracking mechanism around the holes induced by the blasting were studied by establishing cumulative blasting mechanical models.It is found that the entry roof is affected by both the in-situ stress field and blasting stress field.The stress-state of a roof point is related to many factors,such as the blasting loads,burial depth,splitting angle and cumulative direction.Under the action of cumulative blasting,the roof rock is affected by penetration of the entry flow,impact of the blast wave,and quasi-static pressure of the detonation gas,which causes the cracks’ development and propagation range to be different.In a cumulative blasting mode,the crack initiation and propagation pressures are larger than those in a free blasting mode,which thus causes the growth radius of the cracks to be longer than that of a free blasting mode.(3)Two simulation methods by applying stress wave and using ALE arithmetic were adopted to explore crack propagation and evolution laws and the damage controlling effects.It was found that the cracks propagated in random directions and the entry roof was easily fractured into blocks in an uncontrolled blasting mode,which inevitably affected the stability of the entry roof.In contrast,when the blasting was controlled using an energy-accumulated device,the blasting energy was accumulated and a series of directional cracks were formed in desired directions.The damage area of the entry roof was significantly reduced in the controlled blasting mode and the effective stress doubled at the energy-accumulated direction.Field test indicated that the proposed energy-accumulated device could effectively direct the blasting energy.As the explosive charge increased,the splitting cracks initiated and developed in desired directions and the cracking rate of the hole surface reached almost 100 percent.Both the simulation and field test verified the effectiveness and reliability of the proposed energy-accumulated device used in GER by roof cutting and pressure release.(4)Taking the caving morphology of the gob roof,the stress distribution around the retained entry,the caving step length of the main roof,the caving speed of the gob roof,the maximal diagonal size of the gangue body,the filling ratio of the gangue rib and the bulking stability of the gangues as key representing factors,a representation system was developed to explore the macro effects of roof splitting on caving,compacting and stabilizing laws of the entry surroundings.Numerical modelling showed that the pre-splitting fracture could effectively prevent part stress from transferring to the entry roof.The gob roof fractured and caved at the fracturing position,and the entry could be effectively retained.However,the caving morphology and bearing capacity of the gangues varied under different splitting conditions,which thus caused the deformation of the entry roof to be different.The roof splitting height mainly affected the bulking and filling ratio of the gangues.A rational roof splitting height should cause the gob room to be filled with gangues and the gangues to effectively bear the upper main roof.The roof splitting strength mainly affected the contacting and restricting degrees between the gob roof and entry roof.A satisfactory splitting strength could help reduce the influence of gob roof caving or compacting on the entry roof,and improve the stability of the retained entry.The roof splitting angle mainly affected the bearing capacity of the gangues on the short cantilever beam.When the splitting line was perpendicular to the roof(i.e.the splitting angle was 0°),the amount of the roof subsidence was the largest.As the splitting angle increased,the amount of the roof subsidence decreased.However,when the splitting angle was too large,the oversize short cantilever beam was unfavourable for the stability of the retained entry.Field monitoring showed that enhancing fracturing effects could promote gob roof caving and enlarge bulking volume of gangues,and thus increase periodic caving span of the main roof,improve filling ratio of the gangue rib as well as reduce unstable range of the entry.Besides,the gangue size decreased and the caving speed accelerated with enhancement of roof fracturing,because it could change the cohesive force between the gob roof and entry roof.Roof fracturing height also had remarkable effects on gangue caving.The fracturing height had a positive relation with the periodic caving span and the filling ratio of the gangue rib.When the fracturing height increased,the unstable range of the retained entry and the lagging room between gangues and the hydraulic supports decreased.However,the fracturing angle had a little influence on the selected mine pressure parameters,except the caving speed.(5)The evolution laws of the roof structure were investigated based on the principle of gob-side entry retaining formed by roof cutting and pressure release.The stress and displacement transfer mechanisms of the surrounding rocks around a thick-seam fast-extracted mining face were investigated.Correspondingly,an active stability control approach for the entry surroundings was proposed by building a rock-steady structure using the upper strata of the main roof,bulking gangues at the gob and cutting cantilever of the entry roof.In terms of entry roof control,the bulking gangues were designed as bearing bodies at the gob area.The constant resistance and large deformation anchor cables,gangue prevention structures and pier-beam unit supports were designed as entry-in support bodies.In terms of gangue rib control,a multi-level control approach was proposed to address the instability problem of the gangue rib in thick coal seams.The severe impact area was within 6 m from the active mining panel,and an impact prevention device was designed to dissipate the impact energy.Variation of the lateral pressure was more gentle within the compression area of the gangue rib,and a sliding-type individual support structure characterized by pressure release was designed and tested.Laboratory tests showed that the sliding-type structure could effectively coordinate with the roof deformation,thus massively improving its recycling utilization ratio.To further improve the monolithic stability of the gangue rib,a wave-type anchor bolt with multi-resistances was invented.The above proposed control structures comprised a scientific control system that aimed to improve the stability of the entry roof and gangue rib in thick coal seam GER by roof cutting.These technologies were finally tested and verified at a thick coal seam mining face in Ningtiaota coal mine,and the entry retaining effects were satisfactory. |
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