Study On Moistening Deformation Characteristics Of Large Thickness Self-weight Collapsible Loess And Negative Skin Friction For Pile

The pile foundation is a foundation treatment method commonly used in large-thickness collapsible loess area.The negative skin friction of pile will be caused with the occurrence of moistening deformation,which is the focus point in the design of pile foundation in loess area.The probability of large thickness loess fully moistening to saturation is relatively low,it will increase the project cost and the difficulty of construction according this assumption.The actual situation is that the soil layer reaches different moistening levels along the depth.Therefore,this paper studied the moistening deformation characteristics of loess by laboratory experiments,and gived the calculation method of moistening deformation.By studying the mechanical characteristics of pile-soil interface,the calculation method of negative skin friction considering the relative displacement of pile-soil was given.The main contents of the paper are as follows:(1)The K0 oedometer was used to study the variation law of moistening deformation and the coefficient of earth pressure at rest in the moistening process by the moistening test of undisturbed loess.The results show that the coefficient of moistening deformation has a power function relationship with the level of moistening,the proportion of moistening deformation to total deformation is parabolic with the vertical load,when the vertical load is equal to the overlying self-weight load,the proportion is the largest;During the moistening process,the coefficient of earth pressure at rest increases linearly with the moistening level,when the vertical load is close to the overlying self-weight load,the coefficient of earth pressure at rest increases the most,up to 0.22.The expression for calculating the moistening deformation modulus from the compression modulus,poisson's ratio and the moistening deformation coefficient was proposed,which can comprehensively consider the difference of the moistening deformation coefficient and the coefficient of earth pressure at rest in different moistening degress.The calculated results were close to the measured values of the two situ immersing tests in Xinjiang and Ningxia.(2)The calculation methods of moistening deformation were given from two aspects.On the one hand,the double-oedometer method was adopted,taking stress as variable and humidity as parametric variable,the Cam-clay model was modified to make it suitable for the undisturbed loess,then the water content was introduced into the model according to the variation law of model parameters with the water content,so that it can describe the stress-strain relationship of undisturbed loess under different water contents;On the other hand,taking humidity as variable and stress as parametric variable,according to the distribution shape of the test points on the stress ratio-moistening level coordinate,the linear moistening yield surface is adopted,and the mixed hardening law is determined by the hardening law along the moistening failure line and the moistening level coordinate axis respectively,according to the associated flow rule and the corresponding relationship between the moistening volumetric strain and the moistening shear strain during the moistening process,the incremental expression of the moistening volumetric strain and the moistening shear strain are derived,so,the moistening deformation constitutive model of the undisturbed loess is established.By comparing with the experimental data,the results calculated by the two methods can be consistent with the experimental data.(3)Through the direct shear test of loess-concrete interface,the influence of water content,roughness,dry density and structure on the mechanical properties of the contact surface during the shearing process was quantitatively analyzed;The expressions of initial shear modulus and ultimate shear strength were simplified based on the hyperbolic model of the contact surface,and the water content was introduced into the model to establish a modified hyperbolic curve of unsaturated loess-concrete interface,which can describe the pile-soil load transfer characteristics under different water contents..(4)A set of negative skin friction testing equipment was designed.The negative skin friction of the pile during the moistening process was measured used the equipment.The results show that the negative skin friction increased with the increase of the moistening level.At lower moistening levels(less than 0.3),the growth rate of negative skin friction is slow and keep a small value.In the near saturation phase(0.8-1),the negative skin friction also increases slowly and tends to a stable value.Between them,the negative skin friction values corresponding to different moistening levels differ greatly.(5)The expression of the moistening deformation depth limit and the reduction method of overlying weight under the influence of the boundary were proposed and the calculation method of pile-soil relative displacement caused by moistening deformation under the influence of boundary was given;The method for calculating negative skin friction of pile foundation by using pile-soil relative displacement and load transfer function was given based on displacement coordination method.(6)Based on the shear displacement method,a new load transfer function was proposed based on the assumption that the relationship between the distance from the pile axis and the vertical deformation of the soil around the pile is hyperbolic.The load transfer function takes into account the variation of the shear modulus of the soil with the shear strain and the shear strength of the soil around the pile,which can well reflect the nonlinearity of pile-soil load transfer.The negative skin friction of the pile obtained by the load transfer function is close to the measured value of the field test.