Analysis Of Pile Negative Friction Caused By Surcharge Loading

In pile foundation engineering there are instances of negative friction due to thesurcharge loading. Negative friction is applied at the pile as an external load withadversely affect instead of providing support to the pile. Research on negative frictionhas been done by many scholars, but still more research in this area is needed. Thisthesis starts from analysis of standard methods for calculation of negative friction, afterthat factors influencing distribution of negative friction are taken into considerationand then some recommendations about calculating piles with negative frictions aremade.The thesis consists of several parts listed below:（1）A method to estimate the negative friction is proposed. There are manymethods to estimate the negative friction, but still none of those can consider a largenumber of influence factors. In this thesis the friction of single pile caused bysurcharge loading shows a three-step distribution and several control points cancomputed by the regress method. Negative friction can be obtained easily by thismethod and the accuracy is verified by contrast with FE method.（2）New analysis of some problems related to negative friction. Some practicalengineering problems have not been addressed before. Such as the sequence of toploading and surcharge loading; the situation when surcharge loading is reduced afternegative friction generated; the effect of surcharge loading on ultimate bearing capacity,etc. Calculation results show, that the sequence of loading represents plastic loading orelastic unloading on each side of pile-soil interface, when top loading before surchargeloading, friction curve "moves up" with top loading value increasing; when surchargeloading applied before top loading, friction curve "moves right" with top loading valueincreasing. Decreasing surcharge loading reduces negative friction and may have sometensile stress in pile. Surcharge loading increases the pile ultimate bearing capacity ifthe bearing capacity is determined by the soil supporting the pile without consideringthe settlement and the material strength of the pile.（3）Negative friction of the pile group based on a single pile cap. Calculationsshow that the friction is still the same whether the pile cap is on the pile or not, but axial force and settlement of the foundation pile are significantly affected; friction andaxial force are different of foundation piles with different locations, but the depth ofneutral points are all the same; the downdrag force and settlement have positivecorrelation with the number of piles，the downdrag force has positive correlation withspacing, settlement has inverse correlation with spacing（4）Shielding piles. Calculations show that, the shielding piles can reducenegative friction and settlement of pile foundation playing a significant protective role.In addition the shielding effect has a positive correlation with the length, diameter androws of shielding piles, but it has an inverse correlation with distance of shielding pilesbetween foundation piles and spacing of shielding piles.（5）Proposals for the National Standard improvement. The calculation results ofthis thesis are compared with results got using National Standard and FE method. Theweak points of National Standard are discussed and proposals for its improvement aresuggested.（6）Analysis of real project examples with negative friction. Some calculationsof negative friction caused by surcharge loading in the junction of bridge and tunnel ofthe Hong Kong-Zhuhai-Macao Bridge are done. Improvements are proposed to theproject design, which does not meet the settlement requirements.