| With the rapid development of our country’s economy,the scale of infrastructure construction has further expanded,and the construction of pile foundation projects under complex geological conditions has become more and more common.In our country’s coastal areas,the middle and lower reaches of major rivers and lakes,such as Guangzhou,Shanghai,Wuhan,Fuzhou and other cities,there are widespread silt,silty soil and other soft soils.Due to the need to build a fast and convenient transportation system and to solve the problem of less people and more land in the central and southeast coastal areas of our country,many large-scale engineering projects,such as long-span bridges,super high-rise industrial and civil buildings,have to face deep silt Construction under this unfavorable geological condition.In order to meet the re Quirements of bearing capacity,the substructure of this kind of project usually adopts super-long pile foundation to penetrate the overlying silt soil layer,and is driven to the relatively solid rock and soil layer below.As a typical soil with poor engineering properties,silt has high water content,strong compressibility,low shear strength,and low horizontal resistance.When there is a thick silt covering layer on the upper part of the pile,the bearing performance of the pile foundation will be affected.It is lowered,and there is the possibility of instability and failure under the load of the superstructure.Therefore,it is necessary to analyze the force characteristics of the ultra-long pile foundation under this special geological condition.At this stage,there are few relevant studies on the bearing characteristics of the super-long pile foundation in deep silt soil.The understanding of the bearing behavior and the stability of the super-long pile foundation under this special geological condition is relatively vague,and the theoretical analysis is largely backward.In engineering practice.In view of this,in this paper,combined with specific engineering examples,the finite element method is used to analyze the bearing characteristics of ultra-long piles in deep silt soil,and the stability of the pile foundation is discussed based on the energy method.The main research contents are as follows:(1)By reading relevant domestic and foreign literature,understanding the research status,integrating and analyzing relevant research results,discussing the main research methods of pile-soil interaction,comparing the advantages and disadvantages of the classical theoretical analysis method and the finite element method;summarizing the failure of pile foundation Stability related theories,by comparing the existing main research methods of pile foundation stability,illustrate the applicability and superiority of the energy method for studying the instability of ultra-long pile foundations in deep silt soil.(2)Taking the construction of the pile foundation of Fankou Park Bridge as the research background,using the large-scale finite element software MIDAS/GTS,selecting the 14# bridge pier super-long pile foundation with the thickest overlying silt layer as the research object,and establishing a deep silt soil layer The finite element calculation model of the super-long pile learned the load mechanism of the super-long pile foundation under this special geological condition from two aspects:the distribution of the axial force of the pile body,the side friction resistance of the pile and the performance of the pile tip resistance.The results show that under the action of vertical load,the model pile foundation behaves as obvious friction pile behavior.Among them,the side friction resistance provided by the overlying silt layer is relatively small.The bearing capacity of the pile foundation is mainly provided by the pile side friction resistance of the non-silt layer soil in the middle and lower parts of the pile body;the side friction resistance is distributed along the pile body in a "hump" shape.The side friction resistance in the middle of the pile body exerts the highest degree.With the increase of the pile top load,the side friction resistance of the non-silt layer soil is exerted from top to bottom along the depth direction.(3)The settlement and compression characteristics and change laws of the pile body when the super-long pile is loaded in the deep silt soil layer are discussed.According to the load-displacement(Q-S)curve,the 14# bridge pier super-long pile foundation with the thickest overlying silt layer is determined The vertical ultimate bearing capacity of a single pile,and the influence of the thickness of the silt layer on the vertical ultimate bearing capacity,settlement and compression characteristics of a single pile is discussed.The results show that with the increase of the thickness of the silt layer,the pile top settlement and pile compression caused by the vertical load are increased,and the bearing capacity of the pile foundation is reduced;the elastic theory method is adopted.When calculating the pile compression of the super-long pile foundation with the overlying deep silt soil layer,the comprehensive coefficient of pile compression should be appropriately increased according to the thickness of the silt layer.(4)The energy method is used to deduce the critical load of pile foundation instability under the layered foundation.The influence of the half-wave number of the deflection function on the critical load of instability is discussed.Based on the engineering background,for the 14# bridge pier super-long pile foundation with the thickest overlying silt layer,the convergence half-wave number that meets the calculation accuracy is selected,the critical load of the pile foundation instability is calculated by MATLAB language programming,and the silt layer is discussed.The effect of thickness on the convergence half-wave number.The results show that the half-wave number of the deflection function has a great influence on the calculation results of the energy method.For the overly long pile foundation with a deep and thick silt soil layer,the greater the silt layer thickness,the smaller the convergence half-wave number of the deflection function,and the faster the convergence rate of the critical load of instability.(5)The two ways to consider the horizontal resistance of multi-layer foundation soils — the "layered calculation method" and the "standard conversion method" are compared with the calculation results of the energy method for the critical load of instability.When the proportional coefficient of the horizontal resistance of the foundation soil is simplified,the calculation error of the critical load of instability and its influencing factors.The results show that: for a layered foundation with a deep and thick silt layer,the critical load for pile foundation instability calculated by the "Code Conversion Algorithm" will be larger,and this calculation error will vary with the thickness of the silt layer and the foundation coefficient of each soil layer.The difference increases and expands.(6)The relationship between the thickness of the silt layer and the critical load of pile foundation instability is discussed,and the proportional coefficient of the horizontal resistance of each layer of soil,the constraint conditions of the pile top,the weight of the pile body,and the side of the pile are analyzed under different silt layer thicknesses.The influence of friction resistance,pile elastic modulus and foundation horizontal resistance mode on the stability of ultra-long piles in deep silt soil layer provides reference and guidance for the design and construction of the same type of pile foundation engineering.The results show that the thickness of the silt layer has a greater influence on the stability of the super-long pile foundation.With the increase of the thickness of the silt layer,the pile stability of the super-long pile continues to decrease,and when the thickness of the silt layer increases to a certain extent,the critical load for the instability of the pile foundation will decrease sharply.In the range of the thickness of the silt layer where the instability load drops sharply,the stability of the pile foundation is more sensitive to changes in the horizontal resistance of the silt soil layer,and the longer the pile length,the ratio of the instability load to the thickness of the silt layer(Qcr-ω)curve The smoother the pile,the lower the rate of decrease in the stability of the pile body is relatively small. |
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