| The contradiction between the extensive distribution of coal mining subsidence areas and the shortage of land for ground engineering construction has become increasingly prominent in China.The use of land in coal mining subsidence areas for engineering construction is an effective way to alleviate this issue.It has become an effective measure to ease the shortage of land for construction in mining areas,improve the capacity of land and resources,advance the construction of new urbanization,and promote the green transformation of coal mining cities.Based on the construction of surface engineering in longwall goaf of coal mines,the mechanical properties,deformation,and failure characteristics of broken rocks with different particle sizes and lithologies in longwall goaf were systematically studied using theoretical analysis,similarity simulation,numerical simulation,laboratory mechanical tests,threedimensional reconstruction technology,and field monitoring method;the research results are as follows:(1)Based on the numerical and physical simulation tests for the structural characteristics of overburden in longwall goaf,the trapezoidal and periodic forward movement characteristics of overburden failure after mining were analyzed,and the layer by layer transmission of mining induced fractures with overburden failure was studied.The overburden structure was clearly formed at the steep part of the subsidence curve.Therefore,the overburden structure on both sides of the goaf and the compaction characteristics of the broken rock mass in the caved zone were determined to be the main sources of residual surface deformation.(2)The main influencing factors of residual deformation in longwall goaf were analyzed,and the overburden was divided into original rock fracture,tensile fracture,structural void,and void compaction areas based on the distribution characteristics of residual void in longwall goaf.The regional boundary and zoning method were given,and it was suggested that the overburden structural void area produced the maximum residual deformation in the longwall goaf.(3)The instantaneous mechanical compaction characteristics of the broken rocks in the longwall goaf caving zone were tested.The instantaneous compression and deformation processes of the broken rocks in the caving zone were divided into three stages: fast compaction,slow compaction,and stable consolidation.The stress-strain,deformation modulus,porosity,and quality change characteristics of the broken rocks in the longwall goaf caving zone with different block sizes,lithologies,and different combination lithology proportions were obtained,as was the influence law on the bearing and deformation characteristics of the broken rock mass.Based on the fractal theory,the particle size of the compressed rock block was screened and studied,and the relationship between fractal dimension D and the particle size,strength,and the proportion of different combination lithologies was obtained.(4)Based on the acoustic emission(AE)monitoring results,the increase and change of energy and ring count were used to reflect the complexity and evolution of the internal structure reorganization of the broken rock mass.The evolution of AE count,cumulative count,AE energy and cumulative energy with time,and their corresponding relationships with the three deformation stages of single and combined lithology rock masses in caved zone were determined.(5)A lateral compaction device that can realize computerized tomography scanning for broken rock has been designed and developed.Based on the watershed algorithm of topological theory,the digital three-dimensional structure model and pore space model of the broken composite lithology rock sample in the longwall goaf caving zone for different stress compression conditions have been reconstructed.The spatiotemporal evolution characteristics,such as pore structure,pore distribution,block movement,and crushing,have been transparently and quantitatively characterized in the bearing process of the broken rock in the longwall goaf caving zone.The deformation mechanism of the broken rock in the caved zone during the bearing and compression was revealed from microscopic observations.Based on the compaction characteristics of broken rocks in the caving zone,the overburden structure characteristics of longwall goaf were analyzed,and the residual deformation mechanism based on the compaction characteristics of broken rocks in the goaf was obtained.(6)The creep and compression characteristics of broken rocks in different single and combined lithological caved zones were tested by staged loading tests,and the influence characteristics of different stress levels and strengths on the creep and compaction characteristics of broken rocks in the caved zones were obtained.Based on the creep compaction test of the broken rock in the caved zone,the constitutive model of the creep strain time of the broken rock in the caved zone was established using the kelvin volume creep model.(7)Based on the mechanism of residual deformation,a prediction method of residual surface movement and deformation in longwall goaf was proposed and constructed based on the creep characteristics of broken rock in caved zone,and the subsidence prediction analysis is conducted for the study area.RTK and D-In SAR monitoring methods were used to monitor the surface subsidence deformation in the study area,and the correctness of the void partition of overburden in goaf,the source of surface residual deformation,and the prediction model of surface residual movement and deformation were verified. |
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