Study On The Characteristics Of Negative Skin Friction For Pile Groups

Negative skin friction is one of most important and difficult projects on pile foundation design field,which is widely attentioned by engineers.However,the problem of negative skin friction is very complex and there are some issues that have not been well solved in engineering practice.Study on dragload,neutral point,group effect and considering consolidation time of soil,loading sequence effect for vertical or inclined pile groups are taken by using model test,numerical simulation and theoretical calculation methods.Based on limit equilibrium principle,the theoretical calculation formulas for pile group effect coefficient under negative skin friction and uplift capacity of pile groups are built.A new method:pedestal wedge pile which can reducing the influence of negative skin friction is developed,and its mechanical properties are preliminary studied.These researches may be providing helpful reference conclusions for engineering design.Concretely,the main research contents and results of this paper includes:(1) Model test on dragload and neutral point which influenced by the water content of pile surrounding soil and the stiffness of pile end soil are designed.Combined with numerical simulation method,the characteristics of negative skin friction which influenced by loading sequence of pile head load and surface load,friction coefficient of pile-soil interface and length-diameter ratio of pile are discussed.The results show that,in surface loading after pile head load conditions,the dragload and downdrag which caused by negative skin friction are maximum.The dragload of end bearing pile are larger than that of friction end bearing pile under the same surface load.When the water content of pile surrounding soil is close to optimum water content,the position of neutral point is higher than other conditions.(2) Model test and numerical simulation on negative skin friction for vertical single pile and pile groups under pile head load and surface load are designed.Dragload,downdrag and neutral point which are influenced by soil consolidation progresses,loading rate or mode,pile spacing,the positions of pile(central pile,edge pile and corner pile),the numbers of piles and surface load grade are discussed.The results show that,negative skin friction increases rapidly at beginning and then slowly down,finally stabilizes at a constant as soil consolidation progresses.Negative skin friction can be fully developed when the relative displacement of pile-soil reaches to a small value(several mm).Negative skin friction decreases rapidly at beginning and then slowly down,finally stabilizes at a constant as loading rate decrease.Dragload caused by step loading mode is a little smaller than that caused by direct loading modes,and the difference become smaller when surface load increasing.Dragload and downdrag of comer pile are larger than that of edge pile and central pile because of pile group effect.(3) Model test and numerical simulation on negative skin friction for inclined single pile and pile groups under pile head load and surface load are designed,and compares with the results of relevant vertical single pile and pile groups.Dragload,downdrag and neutral point which are influenced by tilt angles and pile spacing are discussed.The results show that,pile group effect is relative not significantly because of the existing of tilt angles.In small tilt angles range,the dragload distributions along pile depth of inclined pile is similar to that of vertical pile,and its values increased slightly with the tilt angles increasing.Downdrag increased significantly with the tilt angles increasing.The lateral displacement is developed significantly under surface loading,and its values increased with the surface load or tilt angles increasing.(4) Based on limit equilibrium principle,assuming an effective influence area around various locations of piles,the theoretical calculation formula for pile group effect coefficient under negative skin friction is built.Various pile parameters such as the arrangement of group piles,pile spacing,length-diameter ratio of piles and soil properties such as density,angle of internal friction and pile-soil interface friction coefficient are considered in the formula.This formula can be calculated easily,and the parameters in the formula can also be obtained easily.Especially,when the effective influence area calculation method is simplified,the formula is more easy to taken.The calculatied values are then compared with model test results carried out by this paper and also with the engineering example results reported on literature.The calculation results are found to be in good agreement with the measured values, which validating the accuracy of the developed calculation formula.(5) A new method:pedestal wedge pile which can improving bearing capacity and reducing the influence of negative skin friction is developed,and national patent(patent number:200810011854.4) is application.The construvtion technology and mechanical mechanism of new pile type are brief introduced.Its mechanical properties of bearing capacity,uplift capacity and negative skin friction are preliminary studied,and the conventional pedestal pile,wedge pile and equal section pile which are in the same concrete volume are comparative analysied.The mechanical properties of pedestal wedge pile which are influenced by modulus ratio of pile end soil and pile surrounding soil,friction coefficient of pile-soil interface,wedge angles,diameter of enlarged pile head and pile modulus are discussed.The results show that,the mechanical properties of pedestal wedge pile have the characteristics of both conventional pedestal pile and wedge pile.When modulus ratio of pile end soil and pile surrounding soil is in middle,it is suitable to chose pedestal wedge pile.It can develop pile shaft and pile top resistance adequately in the same time.In the same surface load,downdrag of pedestal wedge pile is minimum,and dragload are between conventional wedge pile and pedestal pile.It is showed that,pedestal wedge pile is one of effective methods which can reducing the influence of negative skin friction.(6) Based on limit equilibrium principle,assuming a composite failure surface(inverted truncated circular pyramidal and cylindrical surface),the theoretical calculation formula for uplift capacity of single pile and pile groups is built."Maximum minimum principle" is used to determining the coefficients of critical length of pile(L_cr) and coefficient of failure curves (N),which are assumed for the failure surface shape.Then the failure surface and uplift capacity(P_u) are determined.Various pile parameters such as the arrangement of pile groups, pile spacing,pile length,pile diameter and soil properties such as unit weight,angle of internal friction and pile-soil interface friction angle,which influencing the uplift capacity of pile groups directly are considered in the formula.The calculatied values of uplift capacity and failure surface of pile groups are then compared with numerical simulation results and experimental results.The calculation results are found to be in good agreement with numerical simulation and measured values,which validating the accuracy of the developed calculation method.