Research On The Laws Of Water And Salt Transport In Loess Unsaturated Seepage

Unsaturated loess widely spreads in northwest,north and northeast areas of China.Different from the saturated soil assumed under classical soil mechanics,its physical and mechanical properties and deformation features would change with the variation of moisture content.Permeability characteristic is one of the most significant characteristics of unsaturated soil,and the relative research would not only broaden and supplement the unsaturated soil mechanics theory,but also play an important role in guiding the engineering construction and the disaster prevention and control.A one-dimensional loess pillar is designed to simulate the long-time seepage test.Probes are buried in the loess pillar at different depths to collect the data.The results would be used to analyze the change rules of water and salt movement in the loess pillar.After the seepage simulation test is over,soil samples from different depths would be collect and used for soluble salt analysis and SEM observation,which would contribute to further investigation about the interaction between unsaturated permeability characteristic and salt and microstructure.The main conclusions are shown as below:(1)Water movement rules are quite different in dry and wet loess.In dry loess,the water will move down the pillar as saturated wetting front.But in wet loess,it moves as unsaturated seepage.(2)Dry density of loess has great influence on the unsaturated seepage behavior.The bigger the dry density is,the smaller the porosity ratio is.The limited seepage path would impede the water movement,and the narrow pores would enhance the influence from micro acting force.The test results show that when the dry density is greater than 1.4 g/cm~3,its influence on the water and salt movement is evident.(3)During the seepage test,the salt in loess would move with the water.The salt movement rules are controlled by moisture content and dry density.During the process,different salt ions show different movement and redistribution rules.The soil within the top 10 cm range suffers great eluviations and salinity losses.Most salt ions would gather at the impervious bed.Redistributed salts have reaction on the unsaturated seepage characteristics of loess.(4)During the seepage test,particle morphology,pore morphology and skeleton structure of loess at different depths would suffer different changes.With the increase of depth,the contact mode of soil particles changes from loose accumulation mosaic to dense accumulation cementation.So the density of soil increases gradually.Meanwhile,the pores in soil become smaller,and the porosity network become more complicated.These all result in the gradual decrease of the seepage capacity of loess.