Study On Bearing Characteristics Of Pile Foundation Under Surcharge Load In Coastal Reclamation Area

With the development of economic construction in the domestic eastern coastal areas,the shortage of land resources has become an important factor restricting urban development.Then developing reclamation to expand living space has become a main way to solve the problem of land shortage.In terms of the behaviour of poor soil strength and thick soft soil layer in the coastal reclamation area,the later reclamation may easily induce uneven settlement and lateral deformation of surrounding soil,causing settlement,cracking,and serious offset of adjacent bridges and building foundations,then it would bring great safety problems to the serviceability of bridges and buildings.However,there is currently a lack of research on the bearing characteristics of pile foundation of bridge and building in reclamation areas affected by surcharge load,and there is a lack of systematic calculation methods and design theories.The existing design codes have been difficult to ensure safe economic design of pile foundation under the influence of surcharge load in reclamation areas.It makes the safe economic design and protection of pile foundation under surcharge load become a vital issue,which restricts the successful development of coastal reclamation projects.Therefore,there is an urgent need to systematically and thoroughly study the impact of surcharge load on adjacent pile foundation in coastal reclamation areas.This research is mainly derived from the project of Zhejiang Provincial Department of Transportation “Study on the influence and control measures of bridge pile foundation in soft clay area of reclamation and excavation”(No.2014H10),“Study on long-term settlement characteristics of bridge pile foundation on deep and thick soft embankment”(No.8500001375).This study focuses on theoretical derivation and experimental research.After a large number of literature investigations and numerical analysis,the study on bearing characteristics of pile foundation under surcharge load in coastal reclamation area has been systematically carried out.The main work and conclusions of this study are as follows:(1)Based on the consolidation and drainage creep test of coastal soft clay,the traditional component models(e.g.Merchant model and Burgers model)and different empirical models are used to describe the consolidation creep characteristics of the coastal soft clay,then it reveals the regularity of soft clay stress-strain and strain-time relationship.Based on the traditional Merchant model,a fractional Merchant model with Abel dashpot is constructed by using Caputo fractional function.Then the fractional Merchant model is used to predict the creep strain-time variation relationship of coastal soft clay,and it is found that the fractional model is more suitable for describing the creep characteristics of coastal soft clay than the traditional creep model.(2)Based on the Boussinesq additional stress calculation theory,the theoretical calculation formulas are derived for the vertical additional stress of different soil depths in the surcharge load area and outside the surcharge load area under rectangular distributed loads and strip distributed loads.Based on the Mesri creep model and Boussinesq additional stress calculation theory,a calculation method for the long-term settlement of soft clay foundations is proposed,and the long-term settlement of the foundation under local surcharge load and embankment strip loading is predicted and analyzed,then the applicability of the proposed settlement calculation method is demonstrated.(3)Based on trilinear load transfer model,negative friction calculation method of pile foundation was established,and the analytical solutions of the pile settlement and axial force are derived at different stages of elasticity,hardening,and plasticity.Based on Terzaghi's one-dimensional consolidation theory,Mesri creep model and hyperbolic model,a calculation method for the negative friction is established by considering consolidation and creep effect in layered soils.Then the axial force and neutral point of pile foundation are solved by iterative method.In addition,based on the established calculation method of negative friction,the influences of the degree of consolidation,pile top load,the construction sequence of pile top load and surcharge load,pile stiffness,creep parameters and other factors on the negative friction are studied,and it is found that the consolidation and creep settlement will decrease the bearing capacity of the pile foundation and increase the settlement of pile foundation.It reveals the disease mechanism of the weakening of pile foundation bearing capacity in the soil consolidation site.(4)Based on the single pile negative friction field test in Wenzhou reclamation area,the negative friction,pile-soil settlement and neutral point variation with time were studied.It is found that the soil settlement,pile settlement,and drag load basically show a hyperbolic increase with time after surcharge load.They behave stabilized in about 3 months after surcharge load.The field tests reveals the mechanism and time effect of negative friction of pile foundation in coastal reclamation area.(5)Based on the improved solution of Boussinesq additional stress,the horizontal additional stress calculation formula and pile passive pressure calculation formula are deduced under different surcharge load distribution forms such as rectangular distributed load,strip distributed load,trapezoidal strip distributed load,then the remaining horizontal load is further deduced for passive row piles.By considering the long-term evolution of critical earth pressure and the long-term creep attenuation characteristics of soft clay modulus around pile,a two-stage analysis method of passive pile based on pressure method is established with nonlinear p-y curve model,and the differential equation of passive pile balance is solved by difference method.(6)Based on the unbalanced surcharge load filed test in the reclamation area of Wenzhou and Taizhou Bay,the variations of parameters such as pile-soil deformation,pile side soil pressure,pile axial force and pile bending moment over time were studied,and the problems and properties were discussed further,including passive pile cracking problem,passive pile negative friction,the flow around mechanism of the soil around passive pile,the shadow effect and the soil arching effect mechanism.The field tests reveal the bending and torsion coupling deformation mechanism and the long-term deformation disease mechanism of the passive pile under unbalance surcharge load.