Study On Side Friction Behavior Of Pipe Piles In Deep And Collapsible Loess Layer Under Subgrade Loading

Large areas of deep loess are distributed in the northwestern part of China.Due to the characteristics of the loess' s water collapsibility,the construction of expressways,railways and other projects in such areas require high requirements for deformation control.Therefore,the selection of appropriate engineering treatment methods is crucial important.The pipe pile has the characteristics of high bearing capacity and convenient construction,and thus has become a basic form commonly used in engineering.Due to the relative displacement of the pile and the surrounding soil after the deep loess layer collapsing,the pile body has negative frictional resistance,which increases the pile body load and reduces the bearing capacity of the pile.It has a direct impact on the normal use of the project,so it is deep and wet.Based on the engineering background of highway roadbed widening in deep loess area,the influence of collapsibility of loess stratum on roadbed stability is eliminated by pipe piles penetrating deep loess stratum supporting in bearing stratum.On the basis of summarizing negative friction resistance of pile foundation at home and abroad,centrifugal model test of collapsibility process of deep loess foundation is carried out by using 60 gt centrifuge of Chang'an University,and with the help of Abaqus.The finite element software is used to simulate the test conditions,and the results are compared with the test results.The variation rule and influencing factors of negative friction of pipe pile under flexible load are analyzed.The main research contents and conclusions are as follows:(1)The model test of widening the subgrade of the pipe pile foundation in deep loess area was designed by the modified TLJ-3 geotechnical centrifuge.The form of the “sparse pile foundation” piles were selected according to the project purpose(the pile spacing was set to 5 times respectively pile diameter,6 times pile diameter,7 times pile diameter),and using the upgraded centrifuge water immersion equipment to realize the collapsibility of the loess foundation,and the fiber grating and laser displacement meter are used to monitor the deep loess roadbed before and after the soft soil under the subgrade load.The variation law of lateral friction and axial force of pipe pile and the influence of loess collapsibility on roadbed settlement;(2)When using the pile-soil piles in the centrifugal test,compare the treatment effects of different spacing piles.The test results show that the piles spacing is different from the negative frictional resistance and settlement when the piles are 5 times larger than the piles and 6 times.Small,and 7 times pile diameter piles have negative changes in negative friction and subgrade settlement,and the treatment effect is worse.The comparison of test results and consideration of actual engineering factors is the best combination of 6 times pile diameter piles..(3)Using Abaqus finite element software to establish a model to simulate the centrifugal test conditions.The results of centrifugal test show that the variation of the side frictional resistance of the pile under the same working condition is consistent.Considering the limitations of indoor centrifugal test on the simulateable working conditions,the influence of embankment load,pile stiffness,bearing layer elastic modulus and loess collapsing thickNess on pile side frictional resistance was further analyzed by finite software.(4)Through the centrifugal model test and numerical analysis,on this basis,further analyze and summarize the influencing factors of the negative friction resistance and the determination method of the neutral point position,and propose to reduce the negative skin friction resistance of the pile in the deep collapsible loess area specific measure.