| Expansive soil is a kind of special clay,which will appear repeated expansion and contraction and produce obvious cracks under the environment of dry and wet alternation.Therefore,the expansive soil must be improved to improve its engineering properties.Coal gangue is a kind of solid waste associated with the process of coal production.Long term stacking of coal gangue will cause pollution to crops and trees.Using coal gangue to improve expansive soil can not only solve the disasters caused by expansive soil,but also reduce environmental pollution.The reasons of shrinkage and cracks of expansive soil are closely related to the changes of micro pores.Therefore,it is necessary to study the changes of cracks and micro pores caused by drying shrinkage of expansive soils.In this thesis,by adding 0%,3%,5%,7%,9%,11%,13% coal gangue powder into expansive soil and making ring cutter specimens,dry-wet cyclic tests were carried out on each specimen to study the shrinkage of soil samples under dry-wet cyclic condition,the optimum content of coal gangue powder is determined by shrinkage test results.The crack development law of the improved expansive soil with the best content of plain expansive soil and coal gangue powder improved expansive soil in the drying shrinkage process was studied,and the roughness of soil sample surface and crack surface was detected by laser confocal 3D microscope,and the influence of roughness on crack development was analyzed.The micro pore parameters were obtained by mercury injection test,and the influence of micro change on macro shrinkage of soil was analyzed.Finally,the relationships among micro-pores,cracks and shrinkage of soil samples are studied,and the conclusions are as follows:(1)Through the shrinkage test,it is found that the coal gangue powder can reduce the shrinkage of expansive soil,and the shrinkage of plain expansive soil and improved expansive soil can be divided into three stages with different times of dry and wet cycles,it is divided into fast shrinkage stage,slow shrinkage stage and stable shrinkage stage.The optimum content of coal gangue powder is 9%.(2)With the increase of time,the crack area ratio,the total length and the average crack width of the plain expansive soil and the 9% coal gangue improved expansive soil increased rapidly at first and then stabilized gradually,coal gangue powder can delay the crack germination time of expansive soil effectively,and the crack increases with the increase of temperature.The surface and crack surface coarseness of plain expansive soil and 9% coal gangue powder modified expansive soil were detected by confocal laser 3D microscope,the roughness of soil sample is smaller than that of crack surface,and the roughness of improved expansive soil is larger than that of plain expansive soil as a whole,and the larger the roughness is,the smaller the surface roughness is.(3)The porosity,average pore diameter and total pore volume of expansive soil improved by 9% coal gangue powder are obviously smaller than that of plain expansive soil,and the pore types of plain expansive soil and 9% coal gangue powder improved expansive soil are mainly gel pore.With the increase of dry-wet cycle times,the ratio of big pore of plain expansive soil increases,while the ratio of fine capillary pore of expansive soil improved by 9% coal gangue increases,but the ratio of big pore decreases obviously.(4)Through fitting the parameters of shrinkage,fissure and micro-pore of soil sample,it is found that the relationship between micro-pore and shrinkage is a quadratic function of one variable,and the correlation coefficient is over 0.90,showing a negative correlation.The correlation coefficient of the micro-porosity,the area ratio of the fissure and the shrinkage of the soil sample is more than 0.95,which shows a strong correlation. |
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