Experimental analysis on permeability law of remolded loess under freezing-thawing condition

XU Jian;WANG Zhangquan;REN Jianwei;YUAN Jun;School of Civil Engineering,Xi′an University of Architecture and Technology;Northwest Electric Power Design Institute Co.,Ltd.of China Power Engineering Consulting Group;State Key Laboratory of Frozen Soil Engineering,Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences;

remolded loess;;freezing and thawing cycle;;SEM test;;microstructure;;permeability coefficient

【Objective】The permeability of remolded loess during freezing-thawing process was investigated to provide reference for design and construction of loess slope,road and dam in seasonal frozen ground.【Method】The impacts of freeze-thaw cycles on remolded loess(Q3)were studied by rapid freezing-thawing test under closed system.The soil samples placed in the F-T chamber were subjected to 0,2,4,5,10,14,17 and 20freezing-thawing times with constant temperature of-20°C for 12 hand 20°C for another 12 h.In order to quantitatively observe and analyze the microstructure of remolded loess under freezing-thawing cycles including particle morphology,connection mode,pore morphology and porous area ratio,the scanning electron microscope(SEM)test was conducted on dry samples with dry density,water content and size of 1.7g/cm~3,18.0%and 1cm×1cm×2cm,respectively.Meanwhile,triaxial specimen with water contents of 15.0%,18.0%,21.0%,28.0% and saturated water content were compacted to targeted dry densities of 1.4,1.5,1.6and 1.7g/cm~3 in five layers.The triaxial permeability test was also conducted with confining pressures of 100,200 300 and 400kPa to analyze the influence of confining pressure,water content,dry density and freezing-thawing times.【Result】Freezing-thawing action destroyed the surface structure strength of soil greatly and transverse and longitudinal crack appeared on the surface.The microstructure of remolded loess changed significantly under freezing-thawing condition.The number of aggregate decreased obviously and the soil cementing property was weakened.Image processing software showed that the void area ratio exponentially increased from 0.76 to 0.79 with the increase of freezing-thawing time.The permeability coefficient of remolded loess exponentially decreased with the increase of confining pressure,and the difference under high confining pressure of 400 kPa was small.The permeability coefficient followed parabola law with the increase of dry density and water content and the permeability coefficient was greater with the dry density and water content of 1.6g/cm~3 and 21.0%,respectively.With the increase of freezing-thawing time,the permeability coefficient exponentially increased by 1-2order of magnitude after 20 freeze-thaw cycles.【Conclusion】Freezing-thawing action can destroy structural strength of loess and enhance permeability of remolded loess.However,the permeability of remolded loess reached a stable state after 20freezing-thawing times.Cracks caused by freezing-thawing action enhanced the permeability.The effect of freezing-thawing on loess permeability was closely related to initial condition of soil,freezing-thawing times,and confining pressure.