Study On The Effect Of Soil Coarse Particle Cracking And Microstr Ucture Under Freeze-thaw Dry And Wet Cycles

In response to the problems of how to accelerate the weathering process of coarse soil particles under external intervention conditions in order to solve the problems of sand mixing,ground strength decline,and land reclamation difficulties in fallow pressed sand lands.Taking the coarse particles(gray and green slate)of the central Ningxia arid zone as the main research object,to simulate the process of freeze-thaw dry and wet cycles,and the two-factor(temperature and salinity)completely randomized test was used to clarify the changes of mass loss rate,water absorption rate,and electrical conductivity and pH of the drench solution of soil coarse particles under the effect of freeze-thaw dry and wet cycles;the fractalization process of soil coarse particles was explored.The microscopic images of soil coarse particles before and after freeze-thaw dry and wet cycles were obtained by using CT scanning technology,and a three-dimensional pore network model was established based on the CT images,and statistical analysis and fractal theory were used to reveal the changes of porosity,fractal dimension and pore throat parameters of soil coarse particles.The main conclusions were obtained as:(1)The effects of different factors on the cracking process of soil coarse particles were clarified.The mass loss rate of soil coarse particles increased with the increase of the number of cycles under all treatments.The variation of soil coarse particle mass loss rate at the same temperature was the smallest in distilled water treatment and the largest in sodium sulfate(Na2SO4)treatment;the largest soil coarse particle mass loss rate was at-20℃ under distilled water treatment,while the largest soil coarse particle mass loss rate was at-5℃ under salt solution treatment.The water uptake rate of soil coarse particles under different aqueous salt solution treatments showed a trend of increasing and then decreasing with the number of cycles.The conductivity values of the drench solutions were increasing with the number of freeze-thaw wet and dry cycles.pH was increasing and then decreasing with the number of cycles,and its pH fluctuated in the range of 7.4～8.1.(2)The effect of freeze-thaw dry and wet cycles on the porosity of soil coarse particles was elucidated.The CT images of soil coarse particles before and after freeze-thaw dry and wet were analyzed.With the increase of the number of freeze-thaw dry and wet cycles,the cross-sectional porosity was increased and then decreased.The freeze-thaw dry and wet damage of soil coarse particles is carried out from the surface to the inside,and the porosity on both sides is larger,while the pore distribution in the middle is more stable.And the bulk porosity is gradually increasing,but the growth is gradually becoming slower.(3)The effects of freeze-thaw dry and wet cycles on the pore fractal dimension of soil coarse particles were elucidated.The fractal dimension of soil coarse grains increased from 1.5 to 1.8 with the increase of freeze-thaw dry and wet cycles,and the fractal dimension of body increased with the increase of cycles,and then decreased,and the fractal dimension of body increased from 2.6 to 2.85.The fractal dimension of body increased with the increase of porosity.(4)A maximum sphere pore network model was constructed to quantitatively analyze the changes of pore throat parameters.The maximum sphere algorithm was used to establish the pore network model,focusing on the changes of soil coarse-grained pore parameters before and after freeze-thaw dry and wet cycles,including pore equivalent radius,pore volume,throat length,throat equivalent radius,coordination number and other parameters.The pore equivalent radius of each sample was in the range of 341～5300μm,and the throat equivalent radius was distributed in the range of 1～25 00 μm;the coordination number of samples was distributed in the range of 1～15.The proportion of small pores(less than 500 μm)to the total number of pores increased from 1%to 8.1%with the increase of the number of freeze-thaw cycles.In summary,under the conditions of freeze-thaw dry and wet cycles,temperature and water-salt action have a certain promotion effect on the cracking process of soil coarse particles;the pore network model quantifies the changes in the microstructure of soil coarse particles after experiencing freeze-thaw dry and wet cycles.The results of the study can provide a theoretical basis for accelerating the process of soil coarse particle loam formation and contribute to the sustainable development and utilization of soil resources in retired sandy areas in arid regions.