Analysis On The Steady Seepage Of Unsaturated Soil And The Optimization Of Slope Stability Numerical Calculation Method

The gully reclamation project is a key channel management project in Yan 'an city,shaanxi province.For the stability of the excavated slopes in the land improvement project in the hilly-gully region of the Loess Plateau and base on the field investigation,physical testing of loess,steady-state seepage test of unsaturated soil,and geotechnical numerical analysis software Flac 3D are used to optimize finite element strength reduction method to calculate the stability safety factor of the slope,which can provide technical support for pratical construction.The main research results and conclusions of this paper are as follows:(1)This study focused on steady seepage of unsaturated loess,and the new TFB-1 type unsaturated seepage equipment was used in the test and the matric suction of unsaturated loess was measured under different water contents.The results show that the matric suction increased with decreasing water content.This was mainly owing to the low water content,the occurrence of water in the soil between soil particles at contact points.These points can provide a smaller radius of curvature for the shrink film,allowing it to withstand larger pore gas pressure.The transmission of pore water pressure is hence lower;thus,the lower the soil water content,the lower the matric suction.The relationship between the matric suction and volumetric water content of unsaturated loess was best fitted by the Gardner model.(2)The steady-state seepage of unsaturated soil was realized by the new permeameter.The results of the study of infiltration in unsaturated soil showed that the seepage velocity and seepage gradient were positively correlated with a good linear correlation,indicating that the law of unsaturated loess was in accordance with Darcy's law.And the steady-state permeability coefficient of unsaturated soil is measured.The results show that the lower the water content,the higher the matric suction.Water in the soil due to the effect of surface tension only occurs in small pores or soil particles at the junction.The effective seepage area of the sample was thus reduced,and the percolation channel at the interconnections of the small pores was longer,which increased the length of the seepage path.Therefore,the permeability coefficient of unsaturated soil was smaller,as was its relative permeability coefficient.(3)The failure of soil slopes is mainly caused by the increase of soil water content due to rainfall infiltration or irrigation which leads to the decrease of strength and the increase of soilbulk density of soil.The method of calculating the safety factor of slope stability based on the strength reduction of humidification condition,which can not only reflect the law that cohesion and internal friction angle decrease with the increase of water content,but also consider the unfavorable influence of soil bulk density on the stability of slope,improve and optimize the theory of traditional strength reduction method.Its calculation method is more in line with the inherent mechanism of slope instability and has a clear physical and mechanical significance.(4)The new method is used to simulate practical projects by the numerical calculation software Flac 3D.The new method can not only the calculated the value of coefficients of safety are all 3.72~5.93% less than that obtained by traditional strength reduction method.but also get the water content of the soil when the slope reaches the critical failure.In this way,the slope failure can be prevented and predicted by monitoring the water content of slope soil.Therefore,the new method has more scientific,reasonable and safe,and more application value.