Research On Seepage Of Unsaturated Loess Foundation And Mechanical Characteristics Of Pile-soil Interface

Loess is a typical unsaturated soil.The collapsibility,seepage characteristics,and contact characteristics of soil and structure,as well as the engineering problems caused by loess have always been one of the important topics in the field of soil mechanics research.Although the development of soil mechanics seepage theory and consolidation theory which are based on the effective stress principle(K.Terzaghi)has been relatively complete,there is still a certain gap when solving the problem of unsaturated soil,hence failing to solve such problems as characteristics and laws of various special soils and engineering application.Through theoretical and experimental research,an in-depth discussion on both the seepage characteristics of loess(saturated and unsaturated)and the influence of seepage on the mechanical characteristics of the interface between soil and structure has been conducted in this paper.And the main work is as follows:(1)The experimental research on physical and mechanical properties of loess has been carried out.The Q₃ Malan loess indoor routine test of Chaijiatai in Qilihe District of Lanzhou was carried out.The physical and mechanical parameters of the samples in the research area were tested,and the physical and mechanical properties of the rock and soil were obtained.The soil-water characteristic curve of undisturbed loess under dry and wet cycles was determined by using the GCTS soil-water characteristic curve instrument,and the Van Genuchten model was selected for fitting after comparison.(2)Experiments and theoretical studies on the interaction between soil and structure have been carried out.The Shear Trac-?large-scale direct shear tester produced by the Geocomp Corporation of America was applied to carry out the direct shear test of the contact surface of collapsible loess and concrete structure,and the different moisture content(four different moisture contents were designed in this paper),different roughnesses(smooth surface and rough surface of the concrete)and the characteristics of the pile-soil interface under different normal stresses(50k Pa,150k Pa,250k Pa and 350k Pa,respectively)were studied.As can be seen from the test results that water and force will damage the primary structure strength of the soil and affect the generation of secondary strength after the failure.With the increase of water content,the shear strength of the soil on the contact surface will decrease,and the thin film formed on the contact surface will reduce friction.The shear deformation mechanism and mechanical characteristics were studied through shear test,and the damage form of the contact surface was studied through numerical simulation.(3)Experiment and research on the pile-soil contact characteristics in the loess area have been conducted.Through on-the-spot immersion test,the bearing capacity characteristics of the loess foundation pile foundation under natural conditions was studied.The friction resistance and settlement characteristics of the pile foundation under the combined action of intermittent rainfall(humidification and dehumidification)and load,as well as under extreme weather conditions(continuous heavy rainfall)and load were studied.The reasonable range of the influence of collapsibility on the pile foundation in loess area was determined.The generation and development of the negative friction resistance of the pile foundation caused by the collapsibility of loess were also studied.(4)Theoretical research on the seepage of unsaturated loess and the friction resistance of the pile-soil interface has been implemented.According to the principle of conservation of mass and Darcy's law,the distribution of soil water content and the change of wetting front during the infiltration of unsaturated loess foundations(rainfall)were deduced,and the theoretical results were verified.Starting from the contact mechanism of soil and structure and considering the influence of the collapsibility of the soil around the pile,an analytical solution of the load transfer differential equation of a single pile considering the collapsibility of loess was deduced based on the principle and concept of the load transfer method.The load transfer analysis method of a single pile was provided and its rationality was verified.(5)Numerical simulation research on the seepage characteristics of unsaturated loess has been conducted.Combining the test parameters and using the interaction of climate-unsaturated soil,a calculation model considering evaporation was established to simulate the infiltration process of water in the unsaturated loess base in the case of rainfall infiltration and the change of the water field over the elapse of time.As can be shown from numerical simulation results that the seepage path will become longer as the infiltration depth increases,and the friction and pore resistance of the soil will gradually reduce the total fluid seepage head(matrix suction and position head),and the seepage speed will be slower.The volumetric water content of the soil increases slowly with the growth of wetting front.And soil will collapse when the volumetric water content reaches its maximum value.The pores decrease and the volumetric water content decreases after soil collapse.(6)Numerical simulation has been adopted to study the infiltration process of unsaturated loess foundation water under the condition of rainfall-evaporation and the change of water field with time.It can be seen from the simulation that rainfall-evaporation,whose range of influence is mainly within 10m of the surface,has no effect on the pore water pressure of deep soil.Rainfall and evaporation will lead to the hysteretic effect of soil-water characteristic curve,and the change of pore water pressure caused by rainfall infiltration has a hysteretic effect.With the increase of time,the matric suction effect gradually increases,and evaporation will become the main factor controlling the change of pore water pressure of soil.(7)Numerical simulation of the interaction between loess foundation and piles under extreme climatic conditions(intermittent rainfall and continuous rainfall)and under load has been conducted.Loess soaking is a humidizing and deforming process in which soil mass becomes wet to a certain moisture content or saturation.Soil humidizing or dehumiding may affect the pile side friction resistance,hence,the effect of soil seepage on the mechanical properties of the soil-structure interface was studied.