Research On Long-term Bearing Performance Of Bridge Foundation In Complex Environment

With the construction of a large number of viaducts and the construction of bridges across rivers and seas,the construction of bridges on soft subgrade at home and abroad is becoming more and more common,and the construction conditions of bridge projects are becoming more and more complicated.Many bridge projects in coastal areas are facing the problem of conditions of deep soft soil.Due to the complex environment such as large-area surcharge on the ground or the self-weight consolidation of the unconsolidated soil,the soil around the pile will sink over time,resulting in negative skin friction on the side surface of the pile and additional dragload.The settlement of the pile top also increases.In view of the current problems faced by the calculation of the time-varying effect of the bearing performance of pile foundations under soft soil,this paper is mainly based on the sub-project4 of the National Key Basic Research and Development Plan(973 Program)"Evolution and Long-Term Performance Design of Bearing Behavior of Deep Water Foundations in Complex Environments" and research project of the Zhejiang Provincial Department of Transportation "Research on Long-term Settlement Characteristics of Pile Foundations for Bridges on Deep and Soft Foundations(2017037)",the research status of pile foundation bearing time-varying effects and soft soil creep characteristics is systematically introduced.The research method combines indoor test,theoretical research,field test and numerical calculation.The theoretical and experimental research have been carried out in terms of the time-varying effect of the bearing capacity of the bridge pile under surcharge.A more systematic calculation method of long-term bearing capacity of pile foundation is formed to provide a theoretical basis for the design of pile foundation in soft soil of coastal area.The main work content is summarized as follows:(1)To reveal the mechanism of pile-soil interaction in the process of consolidation and settlement of soft soil under surcharge,firstly,the nonlinear equation of non-darcy one-dimensional consolidation of soil was derived based on non-darcy flow law.It was taken into account that the nonlinear characteristics of the soil that was caused by the change of the pore ratio and permeability coefficient of soil and other physical parameters in consolidation process.The numerical solution of excess pore water pressure was obtained by finite difference method.The calculation model of consolidation settlement of soil surrounding the pile was established.Furthermore,based on the pile-soil load transfer model,the influence of the effective soil stress on the shear strength of the pile-soil interface was considered.The calculation theory of long-term bearing time effect of pile foundation in consolidated soil was proposed.In addition,the iterative solution calculation program was written by using Python language.The time-dependent distribution of dragload,negative skin friction and the neutral surface were obtained.Finally,the calculation results were compared with the existing tests,and the effects of drainage conditions on the dragload distribution and neutral surface position change of friction piles and end-bearing piles were further studied.The results show that Compared with the non-Darcy flow,effective stress of soil and strength of pile-soil interface are overestimated according to the calculation results of Darcy's flow law.The drainage conditions have a great influence on the neutral plane position of the pile foundation.When it is double drainage,the neutral surface of the floating pile moves upward with time,while the end bearing pile is stable near the pile tip.The neutral plane moves down with consolidation when it is single drainage.The calculation results in this paper are similar to the overall variation rule of the centrifuge model test results,and the error is acceptable.The proposed method which calculates the dragload and neutral plane position in this paper can efficiently and accurately predict the long-term bearing response of the pile foundation during the non-Darcy flow consolidation process of the soil around the pile.(2)A group of triaxial drainage shear tests with a confining pressure of 100 to 400 k Pa were carried out on the undisturbed silt soil samples of viaduct's test area in Longgang Town,Wenzhou City.Failure deviatoric stress of the silt soil was obtained under different confining pressures.On this basis,four stress levels were set for the samples under different confining pressures,and 16 sets of triaxial drainage shear creep tests were carried out.The results of the triaxial creep experiment were fitted by the least square method,obtaining the parameters of different empirical creep constitutive models and component creep constitutive models.(3)The predicted strains of models beyond the test time were compared for different empirical models and component models.The results show that both the empirical creep model and the component creep model can describe the creep characteristics of the soil during the test time,but not all models are applicable beyond the test time.Considering the effect of the actual confining pressure and partial stress of soils at different depths on creep deformation,the bidirectional linear interpolation algorithm was used to calculate the creep settlement of the soil around the pile.Combining the load transfer method and the shear displacement method,a calculation method of a single pile bearing's time effect was proposed,it considered the creep characteristics of the soil around the pile.The effects of the surcharge,the thickness of the soil layer on the side of the pile,the compression modulus ratio of the pile end and the side of the pile and the vertical load on the pile top on the bearing capacity of the single pile in the creep soil were studied.(4)Based on the Merchant creep constitutive model,the two-parameter log empirical model and the non-Darcy flow law,a partial differential equation for consolidation creep is established.The difference format is used to obtain the numerical solution of the equation.Combining with the load transfer model,the calculation method for the time-varying effect of single pile bearing in creep-consolidated soil was established.Compared with the traditional consolidation calculation method,it is found that the calculation result of the dragload is not safe when only considering the consolidation effect at the same average consolidation degree.It is extremely necessary to consider the adverse effect of soft soil creep consolidation on the pile foundation.At the same time,the degree of influence of the non-Darcy consolidation parameters and Merchant creep model parameters on the deformation of the pile-soil was analyzed.With the increase of the non-Darcy consolidation index n,the dragload of the pile becomes smaller and the neutral point shallower;the larger the starting pressure gradient,the smaller the dragload,but much smaller than the effect of consolidation parameters n.(5)On the basis of the Yongtaiwen Longgang viaduct project,a long-term full-scale model field test of single piles was carried out under large-area surcharge,and long-term settlement observations of engineering piles were performed in the Longgang No.4 bridge and approach bridge.They were mainly introduced,such as formulation of the test plan,the experimental method and the application of the fiber Bragg grating FBG sensor in the bridge pile foundation project.According to the indoor creep test results,the calculation results and measured results of the time-varying effect of the single pile bearing under surcharge in the creep soil.A comparison was made to verify the rationality of the method.Several time-varying effect calculation methods proposed in this paper were compared and analyzed.It revealled the similarities and differences of the various methods.The results show that under the large-area whitetail,the vertical compression deformation of the shallow soil at the side of piles is the largest.The shallower the soil layer,the stronger time effect of the settlement of soil;the measured dragload of the pile increases with time,and the neutral point movpsalter.There is a phenomenon that the neutral point of the test pile is not unique;There exists the non-uniform settlement of the bridge foundation of the adjacent span,but the additional longitudinal slope of the adjacent bridge pier meets the requirements of the code.