Experimental Study On Microstructure And Deformation Characteristics Of Compacted Loess Under Dry And Wet Cycles

The landform of Yan’an City is mainly loess hills and gullies,which is limited by the terrain conditions.During the urban construction,it becomes increasingly prominent for the shortage of land and resource problems.Therefore,the implementation of the development strategy of"expanding from the middle to the outside and building a city on the mountain"has become the only way.However,it will take place the problems of geotechnical engineering related to the stability of high loess filling site,slope protection and the impact of water environment.In Yan’an city,the climate is complex and changeable,the ground water level presents rise and fall consistently,which makes the compacted loess in this area in a repeated dry-wet environment.Aiming at the influence of drying and wetting cycles,this paper designs a self-designed simulation device for loess wetting process,tests the soil water characteristic curve of compacted loess under drying and wetting cycles,establishes the Loess swelling shrinkage deformation model related to suction,and through making the experiment about scanning electron microscope and nuclear magnetic resonance,to analyze the change law of soil pore structure under drying wetting cycle,and explains the deformation mechanism of compacted loess.The main research contents and achievements are as follows:(1)Three cycles of wetting and drying shrinkage of compacted loess were carried out by using a self-designed model device of loess wetting and drying process,and based on the monitoring results of soil moisture and water potential sensors,the SWCC curves of compacted loess in the process of wetting and drying were tested,and the variation rules of SWCC in the process of wetting and drying cycles,dry density and wetting and drying shrinkage were discussed The hysteresis characteristic of soil water characteristic curve under dry wet cycle is analyzed.(2)Scanning electron microscope and nuclear magnetic resonance(NMR)tests were carried out on the soil samples before and after three cycles of dry wet cycling,and the change law of soil pore structure before and after dry wet cycles was analyzed qualitatively and quantitatively.The results show that the small pores and macro pores of compacted loess with dry density of 1.45 g/cm~3 and 1.65 g/cm~3 increase,while the micro pores and mesopores decrease,and the overall pore size increases.However,for compacted loess with dry density of 1.55 g/cm~3,the micro pores and small pores increase,the number of mesopores decreases,and the number of macropores remains unchanged.(3)According to the real-time monitoring data of expansion and contraction deformation of soil,combined with SWCC curve,analyzing the relationship between soil swelling shrinkage strain and suction,and the relevant deformation model is established.The results show that in the process of humidification and dehumidification,the law of suction changing with time is basically the same,and it has a corresponding relationship with the expansion and contraction strain of soil.In the process of humidification,the soil presents the phenomenon of continuous expansion.The larger the dry density is,the higher the expansion strain is.In the process of dehumidification,the volume of soil shrinks.Under the same suction,with the increase of dry density,the greater the shrinkage deformation of soil is.(4)High pressure consolidation test test were carried out on the soil samples before and after the dry wet cycle and combined with soil column compression test.The influence of dry wet cycle on the compression characteristics of soil was discussed.The compression deformation mechanism of soil was analyzed according to the changes of suction and volumetric water content with load in the compression process of soil column.