Dynamic Behaviors And The Evolution Of Particle Breakage For Coal Gangue Subgrade Filler Under Cyclic Loading

Highway construction often stretches hundreds to tens of thousands of kilometers,and the demand for filling materials is huge.To make rational use of resources and save cost,the subgrade filler should be selected from local materials.The predatory exploitation of quarry will have a great impact on the surrounding environment and ecology.Coal gangue is a kind of waste produced in the process of coal mine construction and coal production,and its discharge and storage are large.Using coal gangue as subgrade filler can bring great economic,social and environmental benefits.Coal gangue used as subgrade filler has accumulated more construction experience,but the research on dynamic performance under cyclic traffic load is still far behind the engineering practice.Therefore,in this dissertation,the coal gangue in Xiangtan area of Hunan Province is taken as the research object,and the dynamic characteristics and particle breakage under cyclic load are studied in depth.The research conclusions have important theoretical and practical significance for understanding the long-term service performance of coal gangue subgrade under cyclic load and guiding the design and construction of coal gangue subgrade.The main research contents and conclusions are as follows.(1)The road performance of coal gangue was studied.The microstructure and mineral composition of coal gangue were obtained.The surface of coal gangue is rough,and there are many layered and flaky particles.Moreover,the particles are connected by cementation,and the contact gaps at the joints are obvious,and there are many internal pores.The main mineral composition of coal gangue is quartz and calcite.The applicability of the existing grading equation for grading design was analyzed,and the sample grading was designed using the fractal model grading equation.The value limit of well grading(when 1.8867 ? D ? 2.6309,the grading is uniform)was obtained.The gradation effect of the compaction characteristics for coal gangue subgrade filler was studied,and the gradation has a great influence on the compaction characteristics of coal gangue subgrade filler.With the increase of fine particle content,the maximum dry density first increases and then decreases,and then the optimal gradation range of coal gangue subgrade filler(i.e.,2.04 ? D ? 2.55)was obtained.(2)The research on static property is the basis of the dynamic property.Considering the influence of the particle size distribution of subgrade filler,the compaction degree of subgrade filling and the depth of subgrade landfill on the strength and deformation,a large-scale triaxial test of coal gangue subgrade filler was carried out.The results show that,with the increase of axial strain,the deviatoric stress and volume strain increase gradually,and the growth rate gradually slows down.The curve of deviatoric stress and strain shows strain hardening type.The specimen has shear shrinkage,but it does not show dilatancy.With the increase of fractal dimension,the deviatoric stress first increases and then decreases,and the best gradation range of coal gangue subgrade filler obtained by large-scale triaxial test and vibration compaction test is basically the same.Therefore,considering the high cost of large-scale triaxial test,the best gradation can be obtained directly by vibration compaction test.The greater deviatoric stress occurs with the greater compactness and confining pressure.Both gradation and compactness have significant effects on cohesion,but have little effect on internal friction angle.(3)Based on the existing results and the actual environment of subgrade,the dynamic parameters were selected for large-scale dynamic triaxial test,and the dynamic stress-strain relationship of gangue subgrade filler under cyclic load is deeply studied.The backbone curve of coal gangue samples does not show a well hyperbolic variation,thus the modified HardinDrnevich model can be used to describe the backbone curve of coal gangue samples.The relationship between the maximum dynamic elastic modulus and confining pressure was studied,and the calculation model of backbone curve of coal gangue subgrade filler considering confining pressure effect was established.Under the cyclic loading,the hysteresis loop of coal gangue subgrade filling is not smooth,and its shape is approximately oval.With the increase of dynamic strain and confining pressure,a series of varies occur in the shape characteristics of hysteresis loop,and the shape characteristics of hysteresis loop of coal gangue subgrade filler were analyzed quantitatively by using the index of hysteresis loop shape characteristics(i.e.,the unclosed degree,inclination of long axis,the area and the fullness).The evolution of the cumulative deformation of the gangue subgrade filler under cyclic load was studied,and the cumulative deformation curve can be divided into the fast growth stage,the slow growth stage and the basic stable stage.All of confining pressure,particle size distribution,compactness and axial dynamic stress amplitude have significant effects on the cumulative deformation.Based on the results of large-scale dynamic triaxial tests,a prediction model of cumulative deformation of coal gangue subgrade filler was proposed.This model can be generally applied to the prediction of different types of cumulative deformation curves(stable/ destructive /critical).The correctness and applicability of this model were verified by combining with the existing results,and the physical meaning of the model parameters was analyzed.(4)The evolution of particle breakage of coal gangue subgrade filler under static shear and cyclic loading was studied.Based on the logistic regression model,the particle breakage evolution model was established,and the correctness of the model was verified.The advantages of the model were compared and analyzed,and the mathematical and physical meanings of the model parameters were analyzed.The formula of relative breakage rate was derived to quantify the breakage degree of coal gangue particles.In the process of sample preparation,the effect of artificial vibration on the particle breakage of coal gangue subgrade filler cannot be ignored,while the effect of isobaric consolidation process on the particle breakage of sample is small.The compactness,gradation and confining pressure have significant effects on particle breakage of coal gangue samples.The greater particle breakage occurs with the greater compactness and confining pressure and the smaller fractal dimension.Increasing the content of fine particles in gradation design will help to reduce the breakage of coarse particles.(5)The variation of roundness induced by particle breakage has a significant impact on the macro and micro mechanical properties of coal gangue subgrade filler.Based on the shape of coal gangue particles before and after the large-scale dynamic triaxial test,the real shape of coal gangue particles was obtained through the self-developed two-dimensional image acquisition platform.The shape of coal gangue particles was analyzed quantitatively by using the particle shape index.The influence of the roundness variation caused by particle breakage on the macroscopic properties of coal gangue subgrade filler was simulated by using the real particle shape via the discrete element.The results show that the coal gangue particles gradually become round under cyclic loading,and the change of roundness of edges and corners caused by particle breakage has a significant impact on the macro and micro mechanical properties of coal gangue subgrade filler.With the increase of roundness,the peak strength of the sample increases slightly,while the residual strength decreases significantly,and the peak dilatancy angle increases.The smaller roundness happens with the higher mean coordination number and sliding rate and particle contact moment.