Research On Influence To Bearing Characteristics Of Pipe Piles Group In Excavation

The catastrophic problem caused by excavation and unloading of foundation pit engineering has always been one of the hot spots of social concern.In recent years,engineering accidents such as pipe pile fracture in the pit and pile foundation tilting caused by foundation pit excavation have occurred.Unfortunately,so far,there are few theoretical and experimental studies on the bearing mechanism related to pipe piles.The relevant literature only focuses on the accident handling of pipe pile breakage and deflection.However,foundation pit excavation affects the pile foundation.Research on the influence of mechanical properties is only for cast-in-place piles.Based on this,this article mainly focuses on the impact of foundation pit excavation on static pressure piles,and carries out a series of tests through indoor model tests and numerical simulations.The main contents are as follows:(1)The residual strength of the pile-soil interface was experimentally studied from the three influencing factors: water content,compaction and preloading.The research results show that the residual strength increases with the increase of normal stress and compaction.The increase of moisture content shows a trend of first increase and then decrease,and reaches the maximum near the optimal moisture content.At the same time,the linear relationship between the residual strength,the residual strength after preloading and the normal stress is significant,and the shear failure of the interface conforms to the Mohr-Coulomb shear failure criterion.(2)Through indoor model tests,the changes in the mechanical properties of the model piles at different positions before and after unloading were compared and analyzed,and the impact of unloading on the mechanical properties of the two different pile-forming methods of buried piles and pressed piles was investigated.The study showed that:Under the pile top load,the settlement of the unloaded model pile,the bearing capacity and rigidity of the pile foundation have all been reduced to varying degrees.After unloading,the pile body appears tensile force,and the lower part of the shaft body appears negative friction resistance,both of which increase with the increase of the unloading level,and the neutral point position decreases with the increase of the unloading level.At the same time,it is found that the impact of the pile shaft force and side friction resistance by unloading is significantly greater than that of the corner pile.(3)ABAQUS was used to carry out three-dimensional numerical simulation of the mechanical characteristics of pile groups at the bottom of the pit after excavation,and the influence of pile length,pile spacing and other parameters on the mechanical characteristics of the pile body was analyzed.The research results showed that the axial force of the center pile after the excavation was completed.The largest,the side pile is slightly larger than the corner pile,but the two are obviously smaller than the center pile,and the lower part of each pile has different degrees of negative friction.The axial force of the pile body increases with the increase of the pile length,but the position of the neutral point is gradually blurred.As the pile spacing decreases,the number of piles increases,and the maximum value of the pile shaft axial force is significantly reduced.The 5D pile spacing is the most beneficial for the exertion of side friction resistance.The friction coefficient increases,the axial force and side friction resistance of the pile foundation increase;the excavation depth increases,the maximum axial force value of the pile foundation at the same position increases with the increase of the excavation depth,and the position of the maximum axial force gradually decreases.