Characteristics Of Pile-soil Interface Of Cement-soil Compaction Pile In Loess Foundation

Cement-soil compaction pile is a typical friction pile,which is often used to treat collapsible loess foundation.The bearing capacity of friction pile mainly depends on the exertion of pile side friction resistance,which depends on the working characteristics of pile-soil interface.In order to explore the characteristics of pile-soil interface of loess-cement-soil compaction pile,the pile-soil interface of cement-soil pile in loess composite foundation was extracted on site,and the model of cement-soil interface was made by laser scanning and 3D printing.Then,the direct shear test of loess-cement-soil interface was carried out by using direct shear instrument,and the changing laws of shear strength,tangential stiffness,cohesion and internal friction angle of the interface were discussed,and the effects of moisture content and dry density of loess on the shear characteristics of the interface were analyzed.The nonlinear finite element method considering the parameters of pile-soil interface is used to simulate the single pile and composite foundation of cement-soil compaction pile.The effects of loess moisture content and dry density,pile length and pile diameter on the side friction resistance and load-settlement characteristics of single pile of cement-soil compaction pile are discussed,and the effects of loess moisture content and dry density and cushion thickness on the working characteristics of composite foundation are analyzed.The main conclusions of this paper are as follows:(1)Under the conditions of high water content and low dry density,the shear stress-shear displacement curve is mainly strain hardening type;The peak shear stress of contact surface is mainly related to dry density,and it increases with the increase of dry density.The tangential stiffness increases with the increase of dry density.The higher the dry density,the greater the change trend of tangential stiffness.The tangential stiffness of contact surface increases with the increase of dry density and normal stress,but when the dry density is high,the normal stress has a great influence on the tangential stiffness of contact surface,and the cohesion of contact surface decreases with the increase of water content,but it rises when the water content is 14%.The internal friction angle of contact surface increases with the increase of dry density and the decrease of water content.(2)The numerical simulation of a single pile shows that when the pile body is above 4m,the side friction resistance increases with the increase of dry density at the same moisture content.At the same dry density,the side friction resistance is the smallest at a moisture content of 12%,while below 4m,the opposite is true.However,when the dry density is the maximum dry density,the moisture content has almost no effect on the side friction resistance.The settlement of pile top decreases with the increase of dry density at the same moisture content.The settlement of pile top is the highest at a moisture content of 12% at the same dry density.However,when the dry density is the maximum dry density,the moisture content has almost no effect on the settlement of pile top.The longer the pile length,the larger the pile diameter,which can provide higher frictional resistance.The optimal pile length and pile diameter are 10.0m and 0.5m,respectively(3)The numerical simulation of composite foundation shows that the settlement and pile-soil stress ratio of composite foundation decrease with the increase of dry density at the same moisture content,and the settlement and pile-soil stress ratio of composite foundation with 12% moisture content are the largest at the same dry density,but the settlement and pile-soil stress ratio of composite foundation are very little affected by the moisture content when the dry density is large.Under the same moisture content,the negative friction section of pile side increases with the increase of dry density,while the positive friction section is hardly affected by dry density,and the moisture content has little effect on pile side friction.When the pile is over 3.2m,the vertical stress of the composite foundation pile increases with the increase of dry density at the same moisture content,but it is opposite when the pile is below 3.2m.When the dry density is the maximum dry density,the moisture content has the least influence on the vertical stress of the pile.Through the comparison of settlement and pile-soil stress ratio under different thickness cushion,300 mm is the best choice.