Research On Soil Plug Effect And Construction Technology Optimization Of Pipe Piles In The Reconstruction And Expansion Of Expressways In Soft Soil Areas

In the expressway reconstruction and expansion project,the differential settlement control of new and old roadbed has always been a hot and difficult problem to ensure the quality control of the project.It is the key to solve the problem of differential settlement between new and old subgrade to improve the treatment effect of reconstruction and expansion foundation and reduce the post-construction settlement of newly-built roadbed.With the advantages of wide applicability,stable bearing performance,simple technological process and good economy,open pipe pile is widely used in the foundation treatment project of expressway reconstruction and expansion.With the engineering application and popularization of pipe pile,it is found that the open pipe pile will produce construction effect when it penetrates into the foundation,such as "soil plug effect" and "soil squeezing effect".Among them,the soil plug effect will directly affect the pile driving resistance and the bearing capacity of the pipe pile,and then significantly affect the overall bearing performance of the foundation,but at present,the evolution mechanism of the soil plug effect,the vertical bearing capacity of the pipe pile and the optimization of pile design parameters are not clear.It limits its engineering application,so it is urgent to carry out related research.For this reason,this paper uses the methods of laboratory test,field test and numerical simulation to study the factors affecting the soil plug effect and vertical bearing capacity of pipe pile,analyze the significance of each factor and optimize the design parameters of pile.so as to provide theoretical basis and engineering guidance for pipe pile design and construction.The main research contents and achievements of this paper include:（1）The model test and numerical simulation of soil plug effect in the process of pipe pile driving are carried out to explore the formation and evolution process of soil plug and its internal mechanism.Furthermore,the macro and micro effects of pipe pile wall thickness,inner diameter,inner wall roughness,soil compactness,pile sinking rate,foundation type and pile sinking mode on soil plug effect are analyzed.The results show that with the decrease of wall thickness,the increase of pipe diameter,the decrease of inner wall roughness,the increase of relative density of sand or the increase of pile sinking rate,the height of soil plug in the pile increases,the degree of soil plug decreases,and the soil plug effect weakens;the distribution of the upper hard and lower soft soil layer is easy to form the soil plug blocking phenomenon,while the upper soft and lower hard soil layer is easy to slide.Compared with the hammering method,the soil plug height of the static pressure method is smaller,the complete blocking phenomenon is more likely to occur,and the soil plug effect is more significant.（2）Carry out the research on the influence law of design parameters,pile sinking rate,pile sinking foundation type and pile sinking mode on the soil plug effect,load transfer and bearing characteristics of pipe pile,and analyze the significance of the influencing factors（i.e.weight）.The results show that the inner friction resistance of pile increases with the increase of pile wall friction coefficient,diameter-thickness ratio and pile sinking rate,and the inner friction resistance of pipe pile penetrating into upper hard and lower soft foundation is larger than that of pipe pile penetrating into upper soft and lower hard foundation.The inner friction resistance of pipe pile penetration by static pressure method is larger than that by hammering method.The weights of the influencing factors of soil plug effect value from large to small are pipe pile diameter-thickness ratio,pile-soil interface friction coefficient,pipe pile sinking mode,pipe pile sinking rate and pile foundation type.（3）Considering the influence weight of various factors on the soil plug effect value,the multi-objective optimization function based on pipe pile bearing capacity and engineering economy is established,the optimal selection of pipe pile parameters is put forward,and the design parameters are optimized.The results show that under the comprehensive consideration of the reinforcement effect and economy of the pipe pile,the optimal value of the inner diameter of the pipe pile is near 300 mm,and the optimal value of the friction coefficient of the pipe pile is about 1.0.（4）The field test of PTC pipe pile in the flooded area of the Yellow River is carried out.Through the statistics of the relevant data of soil plug in construction,combined with static penetration test and static load test,the relationship between soil plug rate and different parameters is established.The results show that the soil plug rate of pipe pile decreases and increases with the increase of total hammering times and average pile sinking rate,and there is a linear positive correlation between soil plug height and soil plug rate.The cumulative settlement and rebound of pile top increase with the increase of soil plug rate,but the resistance of pile tip increases with the decrease of soil plug rate.