| With the rapid development of the west,a large number of urban buildings,industrial buildings and road bridges have been built on the collapsible loess foundation in northwest China.In the large-thickness loess area,the foundations of buildings and structures are mostly pile foundations.Due to the special collapsibility of collapsible loess,side friction resistance is generated on the pile foundation,which causes uneven settlement,tilt,and even cracking of various buildings.Therefore,it is necessary and urgent to carry out the research on the side friction resistance of pile foundation in the loess area with large thickness.Based on the pile foundation engineering on a collapsible loess site in Lanzhou,this paper uses the load transfer method to study the mechanical characteristics of the pile body.Aiming at the problem of pile foundation stress on collapsible loess foundation,an indoor compression test,theoretical analysis and numerical simulation were carried out.The main work and research achievements are as follows:(1)According to the method of evaluating the collapsibility of loess by indoor compression test,a large number of collapsible indoor uniaxial compression tests were carried out on collapsible loess soil samples of a project in Lanzhou.Analyze the influence level of the five factors of natural density,dry density,water content,porosity ratio and compression modulus on the collapsibility of loess.Establish a regression model of the porosity ratio,natural water content,and compression modulus to the collapsibility coefficient.According to the standard method,Calculation of the variation trend of the collapsible amount of loess around the pile with the depth.The exponential function is used to represent the displacement curve of the collapsible loess.(2)Based on the load transfer method,establish a single pile force model considering the loess collapse around the pile.In this model,an exponential function is used to represent the displacement curve of the loess collapsing around the pile.Three load conditions are considered: pile top loading,loess collapsing around the pile,simultaneous loading at the pile top and loess collapsing around the pile.The threefold transfer function is used for the pile end and the pile side.The soil around the pile is divided into three stages: fully elastic stage,semi-elastic and semi-plastic stage,and fully plastic stage.There are two geological conditions: a single homogeneous soil layer and multiple layers of soil.Based on the above conditions,the analytical solutions of the displacement,the axial force and the negative friction resistance of the pile body is derived.Finally,combined with engineering examples for calculation and analysis,and using PLAXIS 3D finite element software to verify the calculation model.(3)With the collapsible loess field in a project in Lanzhou as the background,using the PLAXIS 3D finite element software,the distribution laws of the soil displacement around the pile,the axial force of the pile body and the side friction resistance of the pile body were simulated and analyzed.Considering the integrity of pile body and the interaction between pile and soil,the embedded beam element model and solid pile element model are used to establish the three-dimensional finite element model of pile body in collapsible loess field.In the model,the small strain model is used to simulate the soil.It is used to simulate the deformation of soil by applying uniform load on the top of the soil around the pile.There are two geological conditions: single homogeneous soil layer and multi-layer soil layer.By applying loads to the top of the pile and the top of the soil around the pile,the displacement of the soil around the pile,the axial force of the pile body,and the lateral friction of the pile body are calculated and compared.Under the condition of gradually loading,the results of different pile models are compared and analyzed,and the rule of load transfer of pile body is obtained. |
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