Research On The Characteristics And The Control Of Pile Foundation's Negative Friction Subjected To Surcharge Loads

The issue of the negative friction of pile foundation has been widely concerned by domestic and overseas scholars,since the mid-20 th century till now. Some study results have been achieved through theoretical analysis,in-situ experiment,model tests,and numerical analysis method. However,the amount of the research results which taking group piles as research objectives is relatively small,which will result in inadequate theoretical study of negative friction pile group,and there is little mechanism study and outcomes about isolation piles' effect on weakening the characteristics of pile group foundation's negative friction due to surcharge loads,so that it cannot be a guidance of isolation pile's design effort.This paper,on the basis of summarizing the existing studies of piles' negative friction,designs a experiment of single pile's negative friction under the action of surcharge load,and builds ABAQUS finite element model in order to gain a comparative result of the experiment and finite element simulation,then to verify the feasibility of finite element's simulation on pile foundation negative friction resistance properties. The finite element model of pile group under the action of several group surcharge load is established to analyze the difference of single pile's negative friction in different conditions. Besides,this paper makes an analysis on isolation pile's effect on group of piles' negative friction and the change of pile foundation's bearing capacity in the condition with isolation pile and without isolation pile. In view of the current specification is still lack of the setting parameters of the isolation pile,the parameters affecting the control effect of the isolation pile are analyzed. The feasibility and suggestion of setting parameters of isolation piles are given.This paper comes to the conclusion and engineering suggestion as follows:(1) Firstly,because of the group of piles' effect,foundation piles in different position will accept stress in different sizes in different conditions. The external pile foundation's axial force is greater than the internal pile foundation's axial force,but the coefficient of shielding effect on internal pile foundation is greater than the coefficient of shielding effect on external pile foundation. Secondly,in the condition of the same surcharge lode,when the number of pile group foundation is established differently,the more the number of pile foundation are set,the greater drag load of the whole pile groups will be,however,the average value of piles' drag load reduces.That is to say the coefficient of negative friction's group of piles effect is anti-depend on the number of pile group foundation,the number of soil settlement between piles will reduce significantly.Thirdly,when the number of pile group foundation remains unchanged,the coefficient of negative friction's pile group effect has a positive correlation with the degree of surcharge load and the space between piles. The more space of piles,the more drag load will be accepted by each pile,and the coefficient of shielding effect is relatively reduced. However,the coefficient of negative friction's pile group effect has a inverse correlation with the load on pile top. That is to say,the greater load were set on the top of pile,the more little the drag load of each pile would be,and the coefficient of shielding effect will increase. Finally,when the surcharge load remains unchanged,group of piles' internal pile foundation with pile cap will accept smaller drag load and the external pile foundation will accept more drag load than without pile cap under the regulating effect of pile cap. However,pile cap has a little influence on the whole drag load of pile group.(2) When group of piles with isolation piles,the average drag load will reduce because of the sheltering effect,and the soil settlement of pile cap with isolation pile is lighter than that without isolation pile,which suggests that isolation pile can decrease the vertical deformation of pile foundation and can improve its stability. It has protective effect on pile foundation. The interval between isolation pile and external pile foundation,the length of isolation pile and the length of side pile cross section have a big impact on group of piles' protective effect. Besides,the three mentioned above has a significant impact on the coefficient's change of external pile foundation's sheltering effect,whereas,it has a smaller impact on internal pile foundation's sheltering effect coefficient. The longer the distance between isolation pile and external pile foundation of group of piles,the more inadequate of the sheltering surcharge load's effect. What's more,the longer the length of the isolation pile and the longer the length of side pile cross section is,the better the effect of surcharge will be. Isolation pile can insulate the negative friction under the action of surcharge load,and ensure the pile foundation's bearing capacity.(3) The following recommendations based on finite element simulation results.1?Make sure that the length of isolation pile is similar to the length of the protected pile foundation,so that isolation pile has enough embedded depth,which can ensure that sheltering surcharge load effect will give play.2?In order to facilitate the construction,sectional dimension of isolation piles should be the same as the sectional dimension of protected pile.3?It is consider isolation piles' construction feasibility that the distance between isolation piles and the protected pile should be as small as possible.4?The effect of load isolation pile can play a role for isolation areas of the design which is less than 9 times length of protected pile's cross section from the outside pile of pile group.