Research On Bearing Characteristics Of Negative Skin Friction Of Pile Foundation Under Surcharge Load

Since the scholars proposed the concept of negative friction resistance of pile foundation in the last century,it has attracted extensive attention and research,especially in the later stage of filling and loading,which is likely to cause large pile soil settlement.There is a big security risk.However,the existing literature has relatively little research on pile groups and multiple pile types using test methods,and the research on the factors affecting negative friction resistance is not comprehensive,and the calculation theory does not consider the effect of stacking.Therefore,this paper uses indoor model test,finite element numerical simulation and theoretical calculation to fully understand the negative friction resistance of pile foundation.The main research work can be summarized as follows:(1)The indoor model tests of single pile,circular 5 pile and rectangular 3 × 3 group pile are carried out.Seven kinds of test schemes are established by changing the level of balanced surcharge load and the size of pile spacing.The analysis of negative friction characteristics is carried out from the angles of pile-soil settlement,axial force of pile body,neutral point position,tensile force under pile body and group pile effect,and the bearing characteristics of different pile types are compared Sex.For the same pile type,the increase of pile spacing and pile loading level will cause the increase of pile-soil settlement and pile shaft axial force,and the increase of downward tension will cause the neutral point to move down,and the pile group effect coefficient will increase,when the pile spacing reaches 6D,the pile group effect will basically dissipate,among which the change of pile loading level has the greatest impact on the negative friction resistance characteristics;for different pile types,the pile settlement and pile shaft force will increase the buried depth of the neutral point shows single pile >circular 5 pile > rectangular 3 × 3 pile,in which the position of the sexual point is basically in the range of 0.77 L-0.93 L;there is also a size relationship between the angle pile > side pile >central pile in the axial force,down tension,group pile effect coefficient and the buried depth of the neutral point,in which the group pile effect coefficient of the angle pile can reach more than 0.9,which is more similar to the mechanical characteristics of the single pile.(2)Based on the indoor model test as the basic parameters,the finite element model is established,and the research on the influence factors of negative friction resistance is extended by setting different pile end bearing layer,stacking load range,construction loading sequence and the properties of pile-soil contact surface.With the increase of elastic modulus of bearing layer and contact coefficient of pile-soil,the corresponding axial force of pile increases,the settlement of pile top decreases,and the neutral point moves down;the loading range is greater than 1.5 l × 1.5 In the case of L dimension,it can be equivalent to full load design;the axial force under the condition of pile load first and soil load second is larger than that under the condition of pile load first and soil load second,the neutral point is closer to the bottom of the pile end,and the downward tension caused by negative friction is larger,and the soil load and pile load are in the middle when they are loaded at the same time.(3)Relying on the load transfer method reflecting the displacement relationship of the pile and soil,the Box Lucas function model is used to regression fit to determine the transfer function of the relationship between the resistance of the pile foundation and the relative displacement of the pile and soil,and the Boussinesq solution is introduced to propose the pile side considering the balanced stacking size and range The frictional resistance calculation theory uses effective difference method and iterative calculation to solve,and the reliability of the method is verified by engineering examples.The maximum calculated value is 12.7%larger than the experimental value.