Research On The Permeability Function Of Unsaturated Loess

Chinese Loess Plateau, where landslides and avalanche occur at a high incidence, is widely distributed. Rainfall or irrigation infiltration is one of the main factors for developing loess landslide. The groundwater levels are often very deep in loess area, and the migration of moisture in the soil belongs to the unsaturated seepage. So the unsaturated seepage theory is neccessary when we study the slope stability in the condition of rainfall or irrigation and the migration of moisture. The unsaturated permeability coefficient, the basic prarameter of the theory of unsaturated seepage, is a function of water content(matric suction). Its measured methods is different from satured permeability coefficient.The unsaturated permeability function of loess was determined by the instantaneous profile method in laboratory. The sample, taken from southern bank of Jinghe River, Shaanxi province, China, is typical Malan loess. The loess was crushed to powder and seized into a glass column of 200 cm in height and 25 cm in diameter. A quantity of water was slowly sprinkled on the top at the begining of test. Then the water content at definite depths spaced at 5cm along water flow direction was measured by the probes entomb in the column. In addition, another remolded loess specimen which has the same dry density with the loess column was used to measure the soil water characteristic curve by tensionmeter. The data were fitted by three theoretical equations. The data of the water content profiles at definite time intervals and the soil water characteristic curve were used for calculation of the unsaturated permeability function. For better application, the result was fitted using the Van Genuchten function to obtain a continuous permeability function. Moreover, the soil water characteristic curve method was used to estimate the unsaturated permeability coefficient. Compare this result with the test data, we found that the difference between the two methods have close relationship with soil water characteristic curve.According to the laboratory penetration test, SEEP/W numerical model is set up. The unsaturated permeability curve was choosed as the parameters of the numerical model. The numerical model boundary conditions are same as penetration test as to simulate the migration of moisture in column. The simulation results proved that transient profile method is a effective method to measure the unsaturated permeability coefficient. Change the boundary of the numerical model, we found some points in unsaturated seepage area would quickly reach equilibrium, and the upper water will continue to migrating down, until the whole tends to reach a equilibrium and the pore water pressure of points will remain a stable value.In this paper, the investigation of moisture migration in unsaturated soil permeability coefficient and the theory of unsaturated soil in the loess research and engineering application provides the basis data. Research on application of the theory of unsaturated seepage flow rainfall type landslide mechanism is of great significance.