| China’s energy characteristics of rich coal,poor oil and little gas promote the development of coal gasification technology,in the process of coal gasification will inevitably produce a lot of gasification residue,the annual production of gasification slag in 2019 was more than 33 million tons,and the stock increased year by year,but the utilization rate is only 8%;At present,the main treatment methods of coal gasification slag are accumulation and landfill,which not only wastes land resources greatly but also has serious potential environmental pollution.There have been many researches on the utilization of coal gasification slag as resource,but there are few reports on the utilization of coal gasification slag as soil improvement material.Loess is widely distributed in China.and Ningxia region is a concentrated area of loess distribution.Loess has the characteristics of large porosity,high compressibility and collapsibility,so it is not easy to be directly used as the foundation bearing layer.Abundant and significant research results have been achieved in the improvement of loess.However,there are no reports on the improvement of loess by coal gasification slag,which accounts for a large proportion of coal-based solid waste,so it is urgent to carry out relevant research.In order to respond to the national goal of "double carbon" and solve the problem of lack of subgrade filling materials in both seasonal freezing area and loess area,coal gasification slag was used to improve loess as subgrade filling.In this paper,the mechanical properties,deformation properties and freeze-thaw resistance of coal gasification slag modified loess were studied through physical and mechanical tests,microscopic tests,unconfined compressive strength tests,direct shear tests,consolidation tests,collapsibility tests and freeze-thaw cycle tests.The freeze-thaw characteristics of loess subgrade improved by coal gasification slag were numerically simulated by COMSOL software,and the changes of internal temperature field,water field and displacement field were studied.After analyzing the experimental and numerical simulation results,the following main conclusions are drawn:(1)The physical and mechanical tests on the loess samples show that the natural water content is 4.9%,the wet density is 1.66g/cm3,the average dry density is 1.58g/cm3,the specific gravity is 2.78,the plasticity index is 15.04%,the non-uniformity coefficient is 7.49,the optimal water content is 17.3%,and the maximum dry density is 1.75g/cm3.The compression coefficient was 0.33MPa-1.The corresponding compressive strength was 319kpa and the corresponding cohesion was 28.9kpa when the curing period was 7d.(2)The unconfined compressive strength test and direct shear test showed that the compressive strength and shear strength of the coal gasification slag-modified loess increased first and then decreased with the increase of the coal gasification slag-modified loess.The compressive strength and cohesion of the coal gasification slag-modified loess increased by 56%and 94.3%compared with the remolded loess at the curing age of 28d and 6%,respectively.(3)According to the consolidation test,the compression coefficient of the improved loess is 0.11MPa-1,which is 66.7%lower than that of the remolded loess.The collapsibility of the loess was improved by mixing coal gasification slag and fly ash.After standard curing,the collapsibility coefficient of the improved loess was 0.0085.(4)Under the conditions of 0,1,3,5,7,9 and 11 freeze-thaw cycles,the effects of freeze-thaw cycles on the compressive strength and shear strength of coal gasification slag-improved loess were studied through freeze-thaw cycle tests.The results showed that under the conditions of optimal water content of 17%and 5 freeze-thaw cycles,The loss rates of unconfined compressive strength and cohesion strength of the improved loess are 21.9%and 1 1.3%,respectively.(5)Considering the strength characteristics,compressibility,collapsibility and freeze-thaw resistance,the optimal content of coal gasification slag modified loess is 6%.(6)Based on COMSOL software,mechanical test results and existing research data the water-thermal coupling analysis of coal gasification slag improved loess subgrade is carried out It is concluded that the temperature field,water field and displacement field at the center of subgrade,shoulder and slope are different at different time and depth.With the decrease of temperature,the freezing period of shoulder and slope center is earlier than that of subgrade center.After frost heaving deformation,the frost heaving amount of the road shoulder is greater than the center of the subgrade,resulting in a concave top surface of the subgrade. |
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