Analysis Of Mechanical Properties Of Locking Catch Steel Pipe Pile Cofferdam Under Deep Silt Condition

Deep and thick silt layers exist widely in ocean engineering,with the characteristics of high water content,high compressibility,low shear strength and poor permeability,the conventional cofferdam type is difficult to meet the structural bearing capacity Claim.As a new type of cofferdam structure,the locking steel pipe pile cofferdam has been gradually applied in marine engineering.However,there are few studies on the new cofferdam structure type under the conditions of deep silty soil layer,and the structural stress characteristics are not yet clear.This paper relies on the project of the locking steel pipe pile cofferdam of the Lingdingyang Bridge in the Shenzhen to Zhongshan Cross-sea Channel,and carries out the theoretical research and actual measurement of the pile-soil interaction under the horizontal load in view of the mechanical characteristics of the locking steel pipe pile cofferdam under the condition of deep and thick silt data analysis.The specific research contents and conclusions are as follows:1.Based on the measured data,the variation laws of stress,bending moment and horizontal displacement of steel pipe pile along the depth and time history are studied,the causes and main influencing factors are summarized.The analysis results show that:(1)Under the condition of the deep and thick silt,the general steel pipe pile needs a deep penetration depth to reach the supporting layer of the soil body,with the increase of load,steel pipe pile stress and bending moment of pile along the depth change trend are showing a single deformation into two-way,appeared two zero point,namely the first zero point appeared in about 5D from mud surface,and the second zero point will near the pile bottom in a larger trend.Meanwhile,the maximum stress and bending moment of steel pipe pile appear at about 10D from the mud surface.(2)The steel pipe piles are deformed in the direction of the outside of the locking steel pipe pile cofferdam.The horizontal displacement of the pile body decreases with the increase of the depth of the steel pipe pile into the soil,and the displacement at the bottom of the pile tends to zero,which is almost fixed at the bottom of the pile.(3)With the passage of time,the change trend of the steel pipe pile stress,turning moment,and horizontal displacement of the pile body along the time history are gradually increasing to become stable.At this time,the pile-soil interaction system can be considered to be stable.2.The measured data show that the p-y curve of the soft clay is not suitable for the stress and deformation analysis of the locking steel pipe pile cofferdam under the condition of deep and thick silt.In this paper,with reference to the p-y curve of Matlock soft clay,the p-y curve of large steel pipe pile diameter loaded pile is modified under the condition of deep and thick silt.The analysis results show that:(1)Steel pipe piles use the soil resistance of the pile to bear the horizontal load.When the horizontal load is small,the soil resistance of the pile is borne by the surface soil.The soil deformation is mainly elastic deformation.As the horizontal load increases,the horizontal displacement of the pile body increases,and the surface soil gradually produces plastic yielding,so that the horizontal load is transmitted to the deeper soil layer,and the pile body stress and bending moment values also increase.(2)When the foundation soil contains a variety of soil layers,the direction and magnitude of the force exerted by the soil at the boundary of the soil layer.Therefore,the resistance of the soil on the side of the pile changes alternately along the depth,and the three zeros appear at about 3D-4D,6D-9D and 11D-14D respectively from the mud surface.(3)Due to the large lateral bearing capacity of large-diameter steel pipe piles,the pile foundation will not enter the yield stage under normal service conditions.Therefore,compared to the limit of soil resistance p,initial foundation counterforce modulus6)₄9)4))on silt soil layer under the influence of the large diameter steel pipe pile foundation bearing capacity and deformation of pile foundation is more important.The initial and silt soil coefficient of subgrade reaction6)₄9)4))gradually increase with the increase of the depth.3.Based on the m method,the finite element numerical simulation of the locking steel pipe pile cofferdam under the condition of deep and thick silt is analyzed,the mechanical characteristics of the cofferdam structure are analyzed,when considering the nonlinear action of soil,and the m method and the p-y curve method are respectively summarized differences and applicable conditions in the analysis and calculation of the cofferdam structure with steel pipe piles.The analysis and simulation results show that:(1)Because m method simplifies the action system between steel pipe pile and soil layer,it can't reflect the change rule of resistance of pile side soil along the nature of soil layer and calculated depth.Therefore,the finite element numerical simulation based on m method can be used as a qualitative reference for the static simplified calculation in the design of the locking steel pipe pile cofferdam.(2)The p-y curve method fully considers the nonlinearity of soil,and has good applicability in the case of small displacement or large displacement of the structure.It can reflect the interaction and the ultimate bearing capacity of soil between pile and soil more accurately.(3)The locking steel pipe pile cofferdam is restrained by C section locking steel pipe,I-type sheet pile and parallel steel wire hoop.Under the same external load,it can well limit the force and deformation of the pile body,making the structure overall the load-bearing performance is more balanced and stable,preventing localized excessive stress concentration.Therefore,it is more inclined to choose the structural form of the locking steel pipe pile cofferdam under deep and thick silt.