Research On Pile-Soil Interaciton Charcteristics And Settlement Calculation Method Of Composite Foundation Under Embankment Load

Composite foundation, for its advantages in economy and utility, has been widely used in subgrade of high-speed railway and expressway.Results of field tests, laboratory tests and existing theoretical research showed that basement stiffness has an obvious effect on composite foundation's characteristics.Pile-soil interaction characteristics of composite foundation under flexible load were studied by using Mindlin and Boussinesq union iteration method (M&B method for short).Meanwhile,considering that settlement calculation method of composite foundtion used in existing specifications is still composite modulus method, empirical coefficient of settlement calculation's variation and time effect features of settlement after construction were studied based on foundation information, subgrade design parameters and settlement field test data. Our works are as follows:(1) Based on the pile-soil separation modeling principle, Mindlin and Boussinesq union method was used to calculate additional stresses of composite foundation under embankment load, then layerwise summation method to calculate settlements of pile and inter-pile soil. Combining with settlement distribution function between pile and soil,shear strains at interface of pile and soil were obtained. Then based on stress-strain relationship of pile-soil interaction,the value and distribution of skin friction were determined. Keeping pile-soil stress ratio at a certain value, a semi-analytical method was created by introducing iterative algorithm to analyze allocation between skin friction and tip resistance and distribution of skin friction along depth., based on coordination between pile-soil interaction and settlement The semi-analytical method also can be called Mindlin and Boussinesq union iteration method.(2)Using M&B method analyzed an engineering case, and calculated results and field test value were compared,that is comparing pile axis stress and measured pile strain,and stress of inter-pile soil axis and actual strain of soil. Distributions of calculated and measured values are nearly the same,indicating that above semi-analytical method is reasonably practicable.(3)By calculating fundamental example defined by commonly used design parameters, effects of embankment height, stress ratio, pile length, soil modulus, pile modulus and pile spacing were studied on neutral point position, pile axis stress, soil axis stress, skin friction of pile, differential settlement between pile and soil. Results could provide reference for better mastering pile-soil interaction characteristics.(4)Analysis was taken out based on foundation information, subgrade design parameters and settlement field test data of 24 sections in Wuhan-Guangzhou high-speed railway Empirical coefficient of settlement calculation's variation and time effect features of settlement after construction were studied.The conclusions are as follows:under embankment load,end-bearing high-strength CFG pile composite foundation with shallow soft soil has outstanding deformation resistance,and its corresponding empirical coefficient changes between 0.63 and 0.17,which is lower than recommended value in building foundation code by 30 percent;foundation settlement shows a significant time effect features,and ratio between settlement after construction and final settlement decreases with time; that ratio and time before stable of foundation settlement increases obviously with faster embankment filling rate and larger pile spacing.