Research On Microstructure And Engineering Characteristics Of Alar Saline Soil In Southern Xinjiang

Xinjiang is a typical inland arid climate,with high temperature in summer,cold in winter and much more evaporation than rainfall.In the process of evaporation,soluble salt in the soil rises with the capillary phenomenon and accumulates in the surface soil layer.Under natural conditions,due to the lack of rainfall,the process of leaching and desalination is very weak,resulting in the widespread accumulation of salt in the soil,forming a large area of saline soil.Based on the long-term field investigation,collection and analysis of the existing research results,the material composition,deformation characteristics,macro and micro structure and small strain change of saline soil in southern Xinjiang were studied by means of physicochemical test,collapse test,compression test,salt expansion test,scanning electron microscope test,nuclear magnetic resonance test and resonance column test.Firstly,the saline soil samples with different buried depth were tested to understand the mineral composition and basic engineering properties of the saline soil in southern Xinjiang.The salt content of saline soil in southern Xinjiang changes obviously with depth,and the salt content gradually decreases with the increase of depth.The surface is usually saline soil or strong saline soil.The cations are mainly Na+,K+,and the anions are mainly SO42-and Cl-.Based on the X-ray diffraction experiment,the X-ray diffraction patterns were analyzed by hijhcore plus and MDI jade software,and the mineral composition content of saline soil was obtained quantitatively.According to SEM images,the microstructure of saline soil was analyzed qualitatively and quantitatively.From the micro point of view,the structural change characteristics of soil under different salt content are analyzed,and quantitative research is carried out at the micro level.The main work and research results are as follows:(1)According to the boundary water content test of different chloride and sulfate contents,the liquid limit and plastic limit of saline soil decreased with the increase of NaCl content.With the increase of Na2SO4 content,the liquid plastic limit increased gradually.The cohesion of C varies greatly with water content,dry density,NaCl and Na2SO4 contents.The internal friction angle of ? changed little with water content and dry density,decreases with the increase of NaCl content,and increases with the increase of Na2SO4 content.(2)The collapsibility experiment of saline soil with different dry density showed that there was a negative correlation between dry density and collapsibility,and the smaller the dry density is,the larger the collapsibility is.When the dry density is 1.3?1.3 5g/cm3,the maximum collapse occurs under the pressure of 25kPa.With the increase of dry density,the pressure of maximum collapse also increases.With the increase of freeze-thaw cycles,salt expansion was cumulative,but the increase of salt expansion rate is decreasing.From the test process curve,the salt expansion rate increased greatly at the beginning,and then decreases gradually.(3)Under different dry density and moisture content of 17%,the shear modulus increases with the increase of dry density.The dry density is 1.45g/cm3,and the shear modulus decreases with the increase of water content.Based on the condition of 18%moisture content,shear modulus first increases and then decreases with the sand content.When the sand content is 10%,the shear modulus reached the peak.According to the determination method of shear modulus proposed by Hardin et al,considering the influence of confining pressure and sand content,the relationship between shear modulus Go and confining pressure and void ratio was established,and the fitting parameters A and n of confining pressure l00kPa and sand content 0?25%were determined.With the increase of water content,the shear modulus decreases gradually.When the moisture content was in the range of 0.08?0.17%,the shear modulus decreases rapidly.When the moisture content was greater than 0.17,the shear modulus decreases slightly.Using the water content as a variable,the Hardin equation was modified,and the modified dynamic constitutive model was G0=AF(e)F(?)(?‘/Pa)n.Using the contents of NaCl and Na2SO4 as variables,Hardin equation was modified again.Using NaCl content as a variable,the modified equation was as follows:Go=AF(e)F(?NaCl)(?'/Pa)n.Using the content of Na2SO4 as the variable,the modified equation was as follows G0=AF(Na2SO4)F(e)+B(?'/Pa)n.(4)The SEM results under different Na2SO4 contents showed that when the Na2SO4 content was low,the soil particles contacted each other and the salt crystals were not enough for effective cementation.With the increase of Na2SO4 content,there were more and more Na2SO4 crystal particles,and the crystal morphology was long columnar.The connection mode between particles changed from sheet frame contact and accumulation contact to cementation contact.With the increase of NaCl content,more and more NaCl particles adhered to the surface of flaky minerals.The arrangement and combination of saline soil with different NaCl content were honeycomb structure and layered structure.(5)Nuclear magnetic resonance(NMR)test showed that under the condition of different sand content,when the pore size distribution was in the range of 0?0.1?m,the number of pores was the most,accounting for 74%?83%,and the content in this pore size range was much larger than that in other pore size ranges.Before and after loading,the first and second peak values are obvious;after loading,the second peak value disappears,and the mesopores and macropores were compressed into small voids.With the increase of the first peak value,the area proportion and the small pore content,99.99%pore size distribution was between 0?0.1? m.(6)The results of SEM quantitative analysis under different Na2SO4 and NaCl contents showed that the percentage of micropores was the largest,followed by small pores,and the percentage of macropores was the lowest.Although the percentage of macropores is the smallest,the percentage of pore area is the largest.The pore area of saline soil is mainly determined by macropores and mesopores.