Research On The Distribution Characteristic Of The Negative Skin Friction Of Single Pile In Soft Soil Foundadation Under Surcharge Load

In recent years,with the development of tall buildings quickly,the pile foundation have been widely used due to its high capacity、small settlement and better stability in the engineering.But in the actual project,the pile negative skin friction have an impact on pile bearing characteristics deeply under some special conditions, for example,when a pile through the thick loose fill,collapsible loess,soil and other under-consolidated, or the soft soil around of pile,the ground near a pile withstand a greater long-term partial load.The negaive skin friction may occur during the construction process or before the use of upper structure or during use.For example,negative skin friction could cause damage of the pile strength or destruction of the pile bearing stratum for end-bearing piles;negative skin friction could cause additional settlement,make the settlement of the upper structure increased or uneven for friction piles.The influence of the loading sequence of different working conditions are more evident under the large area load in soft soil,but also very necessary for further study.Therefore, this paper make a discussion and analysis on the distribution characteristics of pile negative skin friction and the distribution of the neutral position systematically under the laege area loading condition(considering the loading sequence of different working conditions) by using the indoor model test and the finite element numerical simulation method.(1) First,the paper made a summary about research status for the negative skin friction at home and abroad,summarized the research result of pile negative skin friction in theoretical research,field test,model test and numerical simulation aspects; Based on the summary of negative skin friction,generalized the theoretical calculation method of negative skin friction and introduced the determination method of neutral position and the calculation method of pull-down load,came up with the primarily research consents for this paper.(2) Designed the model test program, used the tubes as model pile which was long 80 cm,outer diameter is 3cm, inner diameter is 2.6cm.Determined the mechanical parameters of foundation backfill through a series of basic soil test, divided the test conditions into 12 groups through considering the two moisture content of the ground fill(20% and 25%),two lying layer(silty clay and coarse sand) and three loading mode(only surcharge loading, top of the pile load was applied after applying surcharge loading, top of the pile load was applied after surcharge loading which was applied after top of the pile load),analyzed the layered soil settlement,pile settlement,the change regular of the neutral position,the axial force,drag load and skin friction and other aspects.(3) Model tests results showed: largest under surcharge loads, the distribution of neutral point position was the deepest, the negative skin friction was the smallest under top of the pile load was applied after applying surcharge loading; the negative skin friction zone were larger and the distribution of neutral point position was deeper under lying layer of coarse sand; the neutral position slightly darker under moisture content equal to 25%.Through the resule of the comparative analysis of neutral point position which were determined by the pile-soil relative displacement and the positions of peak axial force respectively,this paper presents the range of the neutral point which was in 0.51-0.60 l under silty clay and was in 0.60-0.74 l under coarse sand.(4) Based on Midas/GTS finite element software, relied on a bridge engineering during operations under ground surchcrge loading,determined the ultimate bearing capacity of single pile firstly.Then calculated and analyzied the variation of the pile settlement, the neutral position, axial force, pile side friction and other aspects under three loading conditions(only surcharge loading, top of the pile load was applied after applying surcharge loading, top of the pile load was applied after surcharge loading which was applied after top of the pile load).The numerical results obtained: the neutral point position was in 0.53-0.63 l under the surcharge loading,which is similar with the test results;The loading order had a greater impact on the pile settlement, distribution of axial force and negative skin friction. the bearing capacity of pile was larger under top of the pile load was applied after surcharge loading which was applied after top of the pile load.