Study On Vertical Bearing Characteristics Of Bored Piles Considering Hole Wall Stabilit

The instability of pile wall is a common phenomenon in engineering construction.The instability of pile wall is closely rel ated to the property of soil layer in which the pile hole is located.When the soil layer of pile hole side wall becomes unstable,the shape of pile hole wall will become irregular,and different degrees of diameter enlargement or neck contraction will occ ur in the depth direction of pile hole soil layer.If the soil layer of pile hole is enlarged and necked,it will lead to the appearance of variable section pile after the formation of pile,which makes the filling coefficient of concrete after the formati on of pile obviously larger.For this kind of pile with variable section formed due to the instability of pile hole wall,the instability mechanism of pile hole wall and the vertical bearing performance of pile with variable section formed after the instability of pile hole wall are the focus of scholars.Based on pile foundation project of Lanzhou Zhongchuan International Airport Phase III expansion project,this paper adopts statistical analysis method and combines theoretical analysis and numerical simul ation software ABAQUS to discuss the relationship between filling coefficient and soil layer properties in large-thickness layered foundation in Lanzhou New Area,borehole wall stability of bored pile,and vertical bearing characteristics of variable secti on pile formed after pile wall instability.Finally,the vertical bearing characteristics of the variable section pile formed after the instability of the symmetrical section pile and pile hole wall are compared and analyzed.The main contents are as follo ws:(1)The corresponding relationship between the filling coefficient and the soil layer is analyzed by statistical analysis method,and the factors affecting the filling coefficient as well as the diameter enlargement and neck shrinkage of the pile hole soil layer in the large-thickness layered foundation are analyzed statistically.(2)The borehole wall stability of bored cast-in-place pile is discussed,and based on the stress state of the borehole wall,a mechanical model of borehole wall stability is established,and the stress of soil layer around the side wall of the borehole wall is analyzed,and the discriminant formula is obtained when the borehole wall is in a stable state.The local instability and global instability of pile hole wall were calculated respectively,and the safety factor of soil layer in the stable state of pile hole side wall was obtained.The factors affecting the stability of bored pile hole wall were analyzed by using numerical simulation software ABAQUS.(3)The characteristics of the pile with variable section formed after the hole wall instability and the similarities and differences with ordinary equal diameter pile and variable section pile are analyzed.The bearing capacity calculation formula of the pile with variable section formed after the hole wall instability is derived based on the theory of hole expansion and elastic-plastic theory.Based on the layer-summation method,the settlement formula of pile with variable section is derived and the failure mode of pile with variable section is analyzed.(4)Based on engineering field tests,numerical simulation software ABAQUS was used to compare and analyze the bearing characteristics of ordinary equal diameter pile and pile with variable section formed by pile hole wall instability,and the following conclusions were obtained:1)The bearing performance of pile with variable section due to the instability of pile hole wall is improved to some extent compared with ordinary pile with equal diameter,and the settlement amount of pile foundation can be effectively reduced at the site with variable section.2)Under vertical load,axial force of equal-diameter piles decreases slowly along the pile depth direction.For pile with variable section formed by the instability of pi le hole wall,sudden change of axial force at the position of variable section is prone to stress concentration.The distribution of soil layer of pile hole is different,and the corresponding location of variable section is different,and the vertical bea ring performance of pile is different.The variable section pile formed by the instability of pile hole wall can effectively reduce the settlement and increase the vertical bearing capacity of pile foundation.