Study On Negative Skin Friction Characteristics Of Double-Sleeve Pile Foundation Under Silt Soft Soil

The pile foundation structure built in the area of silt soft soil is usually faced with the problem of negative skin friction caused by large area and deep backfilling,which leads to the excessive settlement and damage of pile foundation.In order to solve the above problems,this paper studies the negative skin friction characteristics of double-sleeve pile foundation,and explores its application in reducing negative skin friction.Taking the construction project of Wenzhou wharf in Zhejiang Province as the background,the research is conducted on the characteristics of silt soil-steel interface,negative skin friction of pile foundation and reduction of negative skin friction on pile side through physical model test and numerical simulation calculation.Firstly,by the test of improved direct shear apparatus,the effects of different steel surface roughness,normal stress and soil water content on the relationship between shear strength and shear displacement are studied.Secondly,the self-designed experiment device is used to consider the influence of different pile top load and surcharge load on the axial force and negative skin friction of pile,and the change law of time effect on the pile’s mechanical characteristics is analyzed.Finally,based on the finite element numerical simulation,the test results are verified,and the effects of silt layer depth,loading grade,pile top load and pile tip elastic modulus on the axial force distribution of single pile in multi-layer soil are studied.And the effects of different outer tube diameter and length on the displacement and axial force distribution of double-sleeve pile are explored,and the stress characteristics of double-sleeve pile under side load are analyzed.The conclusions of the study are as follows: The hardeningsoftening behavior of soil-steel interface is significantly affected by normal stress and soil moisture content.The shear strength increases with the increase of steel surface roughness and normal stress at low soil water content.The shear strength decreases with the increase of soil water content.At high water content,the shear strength under the same normal stress is greatly affected by matrix suction and less affected by roughness.Under the action of pile top load,the axial force and negative skin friction of single pile and double-sleeve pile increase with the increase of loading grade,and increase with the accumulation of time,while the location of neutral plane decreases with the increase of loading grade and time.The axial force of single pile increases with the deepening of silt layer depth and the increase of surcharge,the location of neutral plane increases with the increase of pile top load,and the pile tip resistance increases with the increase of elastic modulus.In the double-sleeve pile,the diameter of the outer tube has little influence on the axial force of the pile,while the length of the outer tube has a significant influence on the axial force of the pile.Under the action of side load,the bending moment change of pile foundation shows "S" distribution.Compared with the single pile,the double-sleeve pile foundation can effectively isolate the settlement of the soil around the pile,reduce negative skin friction on the pile side and improve the bearing capacity of the pile.The research results of this paper enrich the theory of negative skin friction characteristics of doublesleeve piles in silt soft soil area,and have a certain reference value for the design and application of pile in area.