Study On Mechanical Mechanism Of Super Long Micro Pile In Underpinning Of High-rise Building

With the rapid development of social economy,the national civil construction is also increasing on a large scale.Micro pile underpinning is often used in survey,design,construction and other projects,such as building incline,building storey increase,foundation reinforcement and so on.Due to the increase of building volume and the poor quality of foundation soil,the application of super long micro pile in the underpinning of high-rise buildings is more and more,but at present,there is little research on the stress mechanism of super long micro pile.In this paper,through a 32 story residential building micro pile underpinning project,the field test is carried out to study the stress mechanism of super long micro pile,super long micro pile group,no with the same construction technology,the bearing capacity of super long micro pile and other contents,the main conclusions are as follows:(1)In the static load test of single pile,the rebound value of super long micro pile is very large,and the settlement of pile top is mainly caused by the compression of pile body;when the friction resistance of super long micro pile on the side of pile is insufficient,the vertical load can be transmitted along the pile body to the pile end,so that the resistance of pile end can be fully exerted;when the friction type super long micro pile reaches the ultimate load,the penetration or bending damage may occur.(2)In the initial stage of bearing force,the vertical load on the top of the super long micro pile is firstly borne by the upper part of the pile,and the resistance at the end of the pile is almost 0.With the increase of bearing force,the middle and lower parts of the pile are gradually stressed,and the resistance at the end of the pile is also gradually increasing;the axial force of the super long micro pile is large in the middle and upper part,and changes greatly;the shaft at the lower part of the pile body the small force and the small change indicate that the side friction of the middle and upper part of the pile is large;the super long micro pile is mainly borne by the side friction of the upper part at the initial stage of vertical loading,while the side friction of the middle and lower part is small.As the applied load continues to increase,the side friction of the upper part increases little or decreases,and the side friction of the middle and lower part gradually plays a role.The side friction of the super long micro pile is an asynchronous process.(3)When the side friction of the super long micro pile with only one pressure grouting or two pressure grouting reaches the limit value in the upper soil layer,the side friction of the pile body will be reduced,and the shear failure of the upper soil body will occur,so that the side friction of the pile body will be reduced.After reaching the limit value,the side friction of super long micro pile degenerates,but the decrease is very small,about 10%.(4)In the process of underpinning high-rise buildings with super long micro pile group foundation,the stress of the micro pile group is a slow process,and the load of the upper building will be redistributed;when the super long micro pile group foundation is subjected to the vertical force,it will transfer more downward along the pile body;when the super long micro pile group foundation does not have the rock stratum,increasing the pile length properly is conducive to improving the bearing capacity;when the bearing capacity of super long micro pile group is small,the axial force is transferred down along the pile body,and the side friction is very small;when the bearing capacity of super long micro pile group is large,the side friction of the upper part of the pile body begins to play,showing the trend of playing down from the upper part of the pile body in turn,the same soil layer changes little along the pile body,the axial force is transferred down along the pile body,and the pile end is also gradually stressed.(5)In the process of underpinning high-rise buildings with super long micro pile group foundation,the stress of micro pile group is a slow process,and the load of upper buildings will be redistributed;with the increase of the load of upper structure,the side friction of the upper part of pile body begins to play a role first,and then presents a downward trend from the upper part of pile body,and the axial force is transmitted downward along the pile body the pile end resistance is also gradually stressed.