Study On Instability Mechanism And Treatment Technology Of Old Foaf Under Construction Loads

Central and eastern China is a densely populated and highly urbanized area,where there are prominent contradictions between urban construction land and arable land protection.As an important coal base of China,it has many goafs,and it is effective to restore coal mine subsidence land into construction land through scientific reclamation.Thus,it is of great theoretical and practical significance for environmental governance of Chinese mines,land development or utilization in mines,major infrastructure construction,construction and planning of small towns by exploring instability mechanisms and control techniques of old goafs.Aiming at large-scale old longwall goafs,this paper analyzes and examines distribution and evolution laws of fracture fields,evolution laws of stress shell structures,residual deformation laws,stability evaluation and grouting reinforcement techniques of load-bearing old longwall goafs from the perspective of their instability mechanisms and control techniques by theoretical analysis,similar material simulation,laboratory experiment and field measurement.The major conclusions are reached as follows:(1)Spatial morphology of overburden strata is analyzed for large-scale old longwall goafs,graphically divided into "(?)-shaped" and“?-shaped" structure type based on relationships between residual cavities and coal pillars.In view if key strata are fractured,they are vertically classified into "elastic foundation beam with both ends located at the coal or rock pillar and the vacant middle" and "voussoir beam".The decision for classifying large-scale old longwall goafs for separate analysis is made by considering whether there are“?-shaped”overlaying strata or not in the horizontal direction.According to their different types of vertical overburden structures,the old longwall goafs are identified to have "beam" and "arch" structured overlaying strata.(2)In light of existing problem that poor bulking effect exist in similar simulation materials,a method is proposed for immediate roof simulation.Distribution laws of fracture fields in the old longwall goafs are quantitatively explored based on mined out area residential voidage,and void distribution is discovered to be markedly divisional.Besides,residual deformation laws are studied for load-bearing old longwall goafs.The results suggest that both load value and position impact residual deformation.At last,a method is put forward for subsurface division of goafs based on value of residual deformation.(3)Under loads,the residues of stable settling areas mainly settle because crushed stones are compacted in the caving zones,so compressive deformation and fractal features of crushed stones with different particle size,axial stress and lithologic characters are explored under distinct occurrence conditions,so as to figure out how different factors impact compressive deformation and fractal features.(4)Two different mechanical structural models are conceptually designed for roof rock beams of different old longwall goafs.Based on catastrophe theory,two corresponding cusp catastrophe models are built for old longwall goafs with instable roof rock beams.Meanwhile,criteria for catastrophe and instability of roof rock beams are derived for two types of old longwall goafs.(5)As main load-bearing structures,stress arch-shells of old longwall goafs bear all weight of strata to the subsurface and loads on ground surface.Under the load of building structures the breakup and instability of top rock beams result in instability of macro stress arch-shells on the roof,so that they become incapable of bearing loads,relatively great uneven subsidence is detected on the surface and threats are posed to security of buildings or structures on the surface of this area.Therefore,grouting reinforcement technologies are put forward for macro stress arch-shells of the old longwall goafs,and its rationality is verified by the measured date.(6)In view of the problems existing in grouting filling system and technology of goaf and distribution law of void field in old longwall goaf,a set of quick grouting system and technology of goaf is designe,adding set of aggregate and control equipment of slurry surface are developed.The scientific of quick grouting system and technology is proved by grouting effect.