Study On Soil Microstructure Response To Strength Deterioration Under Freezing-Thawing Cycle

Seasonal frozen soil region is widely distributed in China,accounting for more than the land area.The freezing-thawing cycle is one of the important reasons for the deterioration of the physical and mechanical properties of the loess in seasonal frozen soil area,such as luring the slope project to collapse,The attenuation of mechanical strength of loess under freezing-thawing is the controlling factor of the stability of the Loess Slope in the seasonally frozen soil area,and the mecrostructural performance of the attenuation of the mechanical strength of the loess is the damage of the soil structure.So it is necessary to study the mechanism.of the microstructure response of the Loess under the effect of freezing-thawing cycle.Based on the Shanxi intact loess as the res,earch object,through indoor tests,different freezing-thawing cycles has been taken on intact loess under natural state,this paper studies the change rule of pore distribution,microstructure and shear strength of the intact loess under different freezing-thwing cycles,and analyzes the relationship beween microstructure and shear strength.The main research results are follows:(1)Through scanning electron microscope test and computer image processing technology,the microstructure of loess under the freeziog-thawing cycles 01,1,3,5,10,15,20,0 is qualitatively analyzed.It is found that the microstructure features of the intact loess are changed from attached types base type,overhead-mosaic into hole,plane-plane contact and bond contactt into sattached type and breccia type,overhead into hole,point-surface contact,plane-plane contact and bond contact.(2)Through the NMR test,the law of loess pore distribution was analyzed,and the relationship between pore distribution and freezing-thakwing cyecles was studied.The result shows that the T2 spectrum has three peaks.According to the pore diameter,it can be divided into micro,small,medium and large pores.With the increase of the number of freezing-thawing cycles,the volume content of micro and small pores decrease first and then increase.The volume content of medium and large pores volume increase first and then decrease,and the change law of void ratio is the same as that of medium and large pore volume.Middle and large volume pores are the main reasons for controlling the change of void ratio.(3)Based on the fractal dimension theory,the change law of the microstructure characteristic parameters of the intact loess particles under different freezing-thawing cycles is quantitatively analyzed,the influence mechanism of the freezing-thawing cycle on the characteristic parameters of the loess microstructure is described.The result shows that with the increase of freezing-thawing cycles,the circularity of the particle shape decreases first and then increases,the complexity of the space structure increases,the orientation of particle increases,and the distribution of homogeneity of particle size increases first and then decreases.(4)Through the unconsolidated undrained triaxial test,the shear strength of intact loess under different freezing-thawing cycles is studied.The result shows that with the increase of freezing-thawing cycles,cohesion decreases first and then increases,and the internal friction angle increases first and then decreases.(5)Based on the the principle of grey correlation method,the correlation between the fractal dimension of the average shape coefficient,the area perimeter method,the probability entropy,the area probability distribution index,the fractal dimension of the particle distribution and the shear strength index are analyzed and evaluated by the correlation model.It is found that under different freezing-thawing cycles the influence weight of the fractal dimension of the area circumference method is the largest.Particle shape has the main contribution to shear strength.